GEOL 1340 FINAL EXAM REVIEW.txt

UNIT 1

INTRODUCTION

SCIENTIFIC STUDY OF EARTH - GEOLOGY

the branch of science concerned with the physical structure and substance of the earth, the processes which act on these, and the earth’s development since formation.

UNIT 2

MINERALS

HOW DO MINERALS FORM?

• solidification from melt
• precipitation from solution
• bioprecipitation
• solid-state diffusion
• direct precipitation from vapor

WHAT IS A MINERAL?

naturally occurring, inorganic, crystalline solid that has a specific chemical composition.

WHAT DOES IT MEAN TO BE CRYSTALLINE?

regular, repeating, and orderly.

EXAMPLES OF ORDERLY CRYSTAL STRUCTURE

we often use models to depict crystalline structures of minerals

• ball and stick
• geometric figures

TYPES OF BONDS

ionic (electrons transferred)

covalent (electrons shared)

metallic (van der Waal’s forces)

SPECIFIC CHEMICAL COMPOSITION

which elements? what proportion?

MOST ABUNDANT ELEMENTS IN EARTH’S CRUST

1. Oxygen (O) 47%
2. Silicon (Si) 28%
3. Aluminum (Al) 8%
4. Iron (Fe) 5%
5. Calcium (Ca) 4%
6. Sodium (Na) 3%
7. Potassium (K) 3%
8. Magnesium (Mg) 2%

these eight constitute 98% of the weight of the crust.

most minerals are silicates

• i.e. contain combinations of Si and O
• silicates total >90% of earth’s crust

silicates are most common minerals because Si and O are the most available elements.

SILICATE MINERALS - (SiO4)^-4 ← negative charge

strong bond within the tetrahedron (implication?)

bonding among SiO4 tetrahedra defines major structure types of silicates.

• how are the tetrahedra arranged? How many adjacent O do they share?

1. isolated
2. single-chain
3. double-chain
4. sheet
5. framework

OTHER MAJOR MINERAL CLASSES: NON-SILICATES

1. carbonates (CO3)
2. sulphides (S)
3. sulfates (SO4)
4. oxides (O, O2)
5. phosphates (PO4)
6. chlorides (Cl)

and others…

most are classified by their anion

also… native elements

CARBONATES - MAJOR EXAMPLES

calcite (CaCO3) the most common non-silicate mineral ; dolomite (CaMg(CO3)2)

• the rocks limestone, dolomite, and marble are made of calcite and dolomite.

SULPHIDES - MAJOR EXAMPLES

pyrite (FeS2) ; Galena (PbS) ; sphalerite (ZnS) ; Chalcopyrite (CuFeS2)

SULPHATES - A MAJOR EXAMPLE

Gypsum (CaSO4*2H2O)

CHLORIDES

halite (NaCl) ; sylvite (KCl)

NON-SILICATES: NATIVE ELEMENTS

silver ; copper ; sulphur ; diamond (C) ; graphite (C) ; gold

PHYSICAL PROPERTIES OF MINERALS

examples of physical properties

• colour
• streak
• hardness
• specific gravity
• crystal form
• cleavage (preferred directions of breakage)

AND MANY OTHERS

PHYSICAL PROPERTIES REFLECT…

identity of atoms (chemical composition)

arrangement of atoms

type of atomic bonds

UNIT 3

4 INTRO TO ROCKS, CRYSTALLIZATION FROM MAGMA

WHAT DEFINES A ROCK?

• crystalline grains - interlocking crystals
• clastic grains - pressure and natura cement precipitated from water bind rock fragments together
• grains of a rock - mineral crystal fragment or rock fragment

coherent

naturally occurring

aggregate of 1+ minerals

WHY ARE THERE SO MANY DIFFERENT TYPES OF ROCK?

rocks are a product of:

materials, processes, and environment

we name rocks based on their:

composition, texture, and structure

CLASSIFYING AND IDENTIFYING ROCKS

classifying and identifying rocks gives us a basis for understanding the environments in which they formed.

we classify rocks into three basic types:

IGNEOUS

solidify (crystallize) from molten rock (magma)

igneous rocks comprise two major groups:

• extrusive = volcanic rocks     - lava ; pyroclasts
• intrusive rocks

SEDIMENTARY

weathering

is important because this is how we get consolidated sediments, including soil and ions dissolved in water ; decomposition of rock at or near the surface of the earth

• influenced by temperature, water, pressure, etc.
• physical weathering
• chemical weathering

erosion

process where moving liquid water, ice, or wind loosens and moves material

notice the difference between weathering vs erosion

sediment - unconsolidated material (not a rock)

from what can sediment derive?

weathering and erosion can affect any type of rock:

• intrusive and extrusive igneous rock
• a sedimentary rock
• a metamorphic rock
• any kind ca be weathered and eroded

formation of clastic sedimentary rocks - lithification

compaction and cementation

when sediments accumulate, the pressure from the weight of overlying sediments compacts the material.

some mineral, usually calcite or quartz, precipitates from the groundwater to bind the clastic particles. this is the process of lithification.

1. clastic sedimentary rocks     1. physical sedimentary rocks, lithified (compacted and cemented)
2. chemical sedimentary rocks     1. precipitate or crystallize from water solutions
3. biochemical sedimentary rock     1. formed by the accumulation of shells or skeletons of organisms
4. organic sedimentary rock     1. formed from accumulation of carbon rich relics of organisms.

METAMORPHIC

pre-existing rocks crystallize and form new, stable mineral assemblages

solid state reactions

at depth, response to high P & T conditions

• increased T and P with burial
• increased T and P with tectonic forces
• minerals may no longer be in equilibrium

partial melting results in production of new magma

ROCK CYCLE

a concept that we use to visualize the interrelationship among these three major rock types.

HOW DO ROCKS FORMED AT DEPTH END UP ON EARTH’S SURFACE?

exhumation = uplift by tectonic forces + erosion of overlying rocks

which ones form at depth?

• intrusive igneous rocks
• physical sedimentary rocks
• organic sedimentary rocks
• metamorphic rocks

what’s formed at/near the surface?

• extrusive igneous rocks
• chemical sedimentary rocks
• unconsolidated sediments

“DYNAMIC EARTH”

surface is reduced

weathering

• breaks down rock

erosion

• lowers relief
• depletes soil

surface is renewed

tectonic uplift

• raises rock to surface

intrusion and extrusion of magma

• generates new rock

deposition of products of erosion and weathering

DESCRIBING INTRUSIVE BODIES

composition

depth of emplacement - (formed at depth or shallow)

grain size (crystal size), accessory mineral (indicate pressure)

size (large or small)

geometric shape?

follow layering in country rock?

concordant (corresponding in direction with the planes of adjacent or underlying strata → sill

tabular intrusive bodies that crosscut layering → dike (discordant)

INTRUSIONS FORMED AT DEPTH

intrusion formed at considerable depth, we call them plutons

pluton - “plutonic rocks”

• blob shaped, do not follow any layering in the country rock (discordant)

we describe plutons by their size (surface expression)

• stock: outcrop area <100km^2, discordant
• batholith: outcrop >100km^2, discordant pluton

PEGMATITE DYKES

very coarse-grained intrusive rocks

• individual crystals can be 1cm in size and often larger than that; even 1m in size
• slow cooling, low viscosity

most pegmatites are silica-rich

• particularly rich in volatiles; yielding a very low viscosity magma enriched in rare elements     - cesium, rubidium, and others
• magma in a very late stage of crystallization, all of those leftovers cool and crystallize.

WHY ARE SOME ERUPTIONS VERY EXPLOSIVE AND OTHERS RELATIVELY QUIET?

the more viscous and the greater volume of gas → more violent

gases help “fuel” explosion:

hot gases propel pyroclasts high into atmosphere

WHAT INFLUENCES VISCOSITY?

amount of volatiles dissolved in magma

more dissolved gasses → less viscous

composition: silica content

higher silica → more viscous

• which is more viscous: felsic or mafic magma?     - felsic magma is more viscous

temperature

higher t → less viscous

• which is more viscous: hot tar or cold tar?     - cold tar is more viscous

ex. felsic solidifies ~800degC

so let’s say the composition of our lava is felsic…

]if T ~850degC

if T ~1000degC

which would be more viscous?

cooler temperature

IGNEOUS ROCK NAMES… VOLCANIC FLOWS

composition and TEXTURE

reminder: extrusive rocks include lava flows and pyroclastic rocks.

TEXTURES OF FLOW ROCKS:

aphanitic (<1mm)

glassy (composition of obsidian is similar to rhyolite)

vesicular (has holes like Aero chocolate)

porphyritic (has two cooling stages: slow cooling [large crystals], faster cooling [fine grain])

STRUCTURES IN FLOW ROCKS

pillows (round, more prominent separations)

pahoehoe (ropy, more together)

Aa (ah-ah) (gravelly, rough looking)

columnar joints (very geometric, organized into columns)

IGNEOUS ROCK NAMES… PYROCLASTIC ROCKS

volcanic bombs

lithified tephra (unconsolidated pyroclastic material produced by a volcanic eruption)

tuff, volcanic breccia, pumice

PYROCLASTIC FALL DEPOSITS

pyroclastic material falls back to the ground after being erupted

“THE MILLER-UREY EXPERIMENT”

WHY IS VOLCANIC LIGHTNING SO COOL?

PYROCLASTIC FLOWS

dense mixture of hot pyroclasts and gas

thick deposits of pyroclastic material may form

MUDFLOWS MAY ACCOMPANY VOLCANISM

lahat

• pyroclastic debris (mostly ash) and lots of water

where does the water come from?

PARTS OF VOLCANO, LANDFORMS ASSOCIATED WITH VOLCANOES

volcano ; magma chamber ; vent ; crater ; flank eruption ; fissure eruption ; lava field ; caldera ; hot springs ; geyser ; fumarole

lava fountain ; along fissure

lave tunnels

lava field - large, mostly flat areas of surface or subaquatic lava flows. such features are generally composed of highly fluid basalt lava

volcanic crater

distinguish: crater, caldera (crater is less open, caldera is more open)

geyser, fumaroles, and hot springs

geyser - a rare kind of hot spring that is under pressure and erupts, sending jets of water and steam into the air

MAJOR TYPES OF VOLCANOES

shield ; stratovolcano ; cinder cone (differ in size, shape, and composition)

shield volcano - broad, gently sloping (2-10deg) - low viscosity basalt flows

mauna loa, kilauea

stratovolcano - intermediate slope - high viscosity - dominantly andesite (some rhyolite)

mt. fujiyama, mt. rainer, mt. st. helens, krakatau

italy and greece

cinder cone - 30deg slopes (limited by gravity to ~33deg - pyroclastic fragments ejected from a central vent (most material lands near the vent) - most common: mafic or intermediate lava

mt. capulin, mt. adziza, paricutin,

flood basalts - lava plateaux - very low viscosity lava - large volume, widespread

6 VOLCANOS AND THEIR IMPACT ON THE ENVIRONMENT AND SOCIETY

RECENTLY ACTIVE MAJOR VOLCANOES

active volcanoes, plate tectonics, and the “ring of fire” (between north america and asia)

WHERE DOES VOLCANIC ACTIVITY OCCUR?

divergent plate boundary

• 90% of world’s volcanism
• mid-ocean ridges (ex. Iceland)
• rift zones (ex. East African Rift)
• basaltic magma

convergent plate boundary

• flux melting
• mostly andesite
• circum-Pacific, Mediterranean, Caribbean

intraplate volcanism

• “hot spots”
• basalt: ex. Hawaii, Columbia Plateau, Deccan Plateau
• rhyolite: ex. Yellowstone Park
• ~5% of world’s active volcanoes

VOLCANISM IN CANADA

Volcanism in BC and Yukon

Cascade Mts., Pacific Northwest

SIGNIFICANT EFFECTS ON HUMANS: CATASTROPHIC

destruction of landforms

loss of life and injury ; damage to crops, waterways, fisheries, and forests

damage to man-made structures

tsunami

effect on climate: global cooling (crop failures, famine)

• ex. destruction of Pompeii, 79 AD Mt. Vesuvius, Italy
• ex. starvation, 92k casualties, 1815 Tambora, Indonesia
• Krakatoa, Indonesia, 1883

CONSTRUCTIVE EFFECTS OF VOLCANISM:

build landforms

earth’s hydrosphere and atmosphere

fertile soil

geothermal energy

UNIT 4

7 WEATHERING AND SOILS

WEATHERING

creates sediment, soil, and ions dissolved in water

leads to sediment and sedimentary rock

weathering is progressive ; weathering increases towards surface

weathering along joints (fractures) , rounded corners

greater surface area aids weathering (more cracks, more surface area, more weathering)

different materials weather at different rates

two types of weathering:

1. mechanical = physical          rock physically disintegrates into smaller pieces; but does not undergo chemical change.          major types:          - frost action         - freeze-thaw cycle         - ice is an unusual mineral, and one of its unusual properties is that it occupies a greater volume than its liquid state (water)         - not only are the solid pieces broken, but frost heaving tends to push up the rock and soil vertically             - roads             - farmer’s field     - pressure release         - exhumation due to unloading         - melting of glaciers                  the rock breaks apart along the sheet joints and exfoliates                  - it is common in mountainous areas, pressure release weathering can take place     - plant growth         - biological weathering                  plant roots create and enlarge passageways for water and air to get underground                  plant growth is an important factor in the development of soil              - burrowing animals         - biological weathering         - mechanically weather rock         - burrows allow for air and water to penetrate, aiding chemical weathering     - change in temperature : expansion and contraction         - rock surface heats up and expands         - rock surface cools and contracts         - joints form in the outer part of the rock         - small fragments break off the rock
2. chemical = “rock decomposition”          minerals in the rock react with water and gasses in the atmosphere to produce different compounds.          minerals + H2O & air (CO2) → different mineral species + ions in solution          - removal of chemical constituents from rock     - generation of new minerals                  ex. the coffee-maker model:                   what kind of conditions would promote chemical weathering?          - abundant liquid water     - warm                  ex. breakdown of granite, weathered monuments                   common reactions:          dissolution          minerals dissolve in water (ex. salts, carbonate minerals)          ex. solution of calcite:          ex. solution of halite:          - caves     - discolor and dissolve marble and limestone statues, monuments     - where do ions go?         - “hard water”         - why is seawater salty?             - products of weathering carried as dissolved ions by rivers into the ocean (4 billion tons per year)          hydrolysis          minerals react with water → break down, form other minerals (clay minerals are the products          ex. weathering of K-feldspar:          oxidation (reduction) redox reaction          may or may not involve oxygen          iron bearing mineral loses and electron and becomes a cation (oxidized)          oxygen or another element would gain the electron(s) and become an anion (reduced)          ex. rusting iron          ex. yellowish-reddish soils are rich in Fe oxides          - Limonite, goethite (yellow)     - hematite (red)     - aluminum oxide         - Laterite soils ; highly leached soil     - bauxite         - a major resource of aluminum and is an aluminum oxide produced by chemical weathering in tropical climates          hydration          H2O absorbed into the crystal structure          ex. limonite , aluminum oxide, gypsum, olivine + water → serpentine)          mineral stabilities - surficial conditions          fast weathering = least stable          slow weathering = most stable          → halite , calcite , olivine , Ca-plagioclase , pyroxene , amphibole , Na-plagioclase , biotite , K-feldspar - muscovite , clays , quartz , aluminum oxide , hematite          does this sequence look familiar?          what happens to quartz when weathered?          1. smaller particles of qtz+     2. dissolved in water as silica ions         - precipitate forms common cement in sedimentary rock          after complete chemical weathering, what is left?          solids: clays, quartz , iron oxides, aluminum oxide          ions: including Ca^2+, SiO2, HCO3-          precipitate → cement for sedimentary rocks          calcium and bicarbonate precipitate as calcite          silica ions precipitate as quartz     

SOIL

layer of weathered, unconsolidated material on top of bedrock

clays (holds water and ions are plant nutrients), quartz (sand grains provide drainage)

mature soil is layers (soil horizons)

• appearance
• chemical composition

soil development

• climate
• parent rock composition
• organisms
• relief
• drainage
• time

8 CLASTIC AND SEDIMENTARY STRUCTURES

TRANSPORTATIOMN OF SEDIMENTS

agents of transportation:

running water

glaciers

waves

wind

during transport, sediments bump and scrape against each other and against confining rocks, grinding away the sharp edges and corners of rock fragments. this is called rounding

angular sediments are close to their original source

rounded sediments imply that they have been transported

which transportation agents would be good or poor at rounding?

running water is a good sorting agent, so are waves and winds

glaciers are poor sorting agents

would the other transportation agent be good or poor?

maturity (notice what changes? roundness, sorting, and types of minerals)

less mature sediments = poorly sorted

more mature sediments = well sorted

DEPOSITION OF SEDIMENTS

transported materials settle

for river its gradient or its volume of water drops, it loses the energy to carry.

for waves and wind, when the wind is calm, their energy drops and transported materials settle.

glaciers lose energy by melting and their transported sediment settled.

PRESERVATION

not all sediments are preserved as sedimentary rocks… (why not? where would they be most easily preserved?)

sediments can be eroded transported to a new location and redeposited once the agent of transportation gains energy again or once a new agent of transportation is introduced.

marine sediments on the deep ocean floor are the most easily preserved, with shale being the most common.

LITHIFICATION

during compaction, the porosity decreases

the pore space will be filled with fluid and ground water - cement

common cements:

• quartz (silica)
• calcite (from ions in groundwater that crystallize in the pores)

CLASSES OF SEDIMENTARY ROCKS

1. fragments of rocks are buried and lithified (compacted and cemented)          = clastic sedimentary rocks          = physical sedimentary rocks     
2. precipitate (crystallize) from water solutions (seawater, saline lakes)          = chemical sedimentary rocks     
3. rock formed by accumulation of shells or skeletons of organisms          = biochemical sedimentary rock     
4. rock formed by accumulation of C-rich relicts of organisms          = organic sedimentary     

CLASTIC SEDIMENTARY ROCKS

• lithified sediment
• classified by:     1. grain size     2. texture (grain shape)
• largest volume of sedimentary rock types               | Clast Diameter (mm) | Sediment | Sedimentary Rock |     | --- | --- | --- |     | >2 | pebbles, cobbles, boulders | breccia (angular particles) |     | >2 | | conglomerate (rounded particles) |     | ≤2 | sand | sandstone |     | 1/16 | silt | siltstone |     | 1/256 | clay | shale |

COARSE GRAINED

sedimentary breccia ; conglomerate

SANDSTONE

quartz sandstone ; arkose ; greywacke

SILTSTONE

shale is made up of clay and consist of finer grains than siltstone but it is difficult to distinguish between them with the naked eye

SHALE

shale is formed from mud that is a mix of clay minerals and tiny fragments quartz and calcite.

SEDIMENTARY STRUCTURES

features in sedimentary rock formed before lithification

can help reconstruct paleo-environment

help understand deformed rocks

BEDDING

a single layer of sediment or sedimentary rock

graded bedding: grains settle out as energy recedes (larger sediments below finer sediments, layers of bedding with the gradient)

cross bedding: deposition in a current (water or wind) (currents create ripples or dunes)

FORMATION

a formally named rock unit

the surface between two formations or groups is called a contact.

sedimentary facies

9 CHEMICAL SEDIMENTARY ROCKS

CHEMICAL SEDIMENTARY ROCKS

precipitated directly from solution

• crystalline structure

solutions:

• oceans
• saline lakes
• groundwater
• hydrothermal systems (underground hot water systems)

major basis for classification is not grain size but what minerals are present.

LIMESTONE

the common rock limestone (made of calcite) can form different varieties:

• crystalline limestone     - precipitate from marine or fresh water
• aphanitic limestone     - precipitate in lagoons and lakes as well
• oolitic limestone     - precipitates in shallow warm seas

travertine is a limestone that precipitates in caves from groundwater in cold springs and hot springs.

DOLOSTONE

limestone altered by Mg-bearing waters; calcite is being replaced by dolomite.

soon after a limestone is formed or long after burial.

CHERT

cryptocrystalline quartz

inorganic

nodules

also with hydrothermal systems

high silica in hot water

also a biochemical origin

EVAPORITES

chemical salts deposits precipitate from saline water (restricted seas and saline lakes over saturated by evaporation of their water.

evaporites have a crystalline texture.

main evaporites:

Gypsum

Halite

other salts:

Potash

potash precipitated from shallow oceans is used as fertilizer

Sodium Sulphate

Magnesium sulphate

the chloride minerals that make up potash, phosphate, nitrate and borate salts are evaporites.

BIOCHEMICAL SEDIMENTARY ROCKS

derived from organisms

• shells, skeletons, algae, bacteria, plankton
• made from ions extracted from seawater
• creatures die: shells become sediment
• mostly limestone     - calcite > quartz

when these creatures die, they settle to the bottom of the seawater. the shells and skeletons accumulate as sediments. these rocks have bioclastic textures.

LIMESTONE

fossiliferous limestone

coquina

made of only weakly cemented shells

it looks like a granola bar made out of shells

chalk

calcite remains of submicroscopic nanoplankton

aphanitic limestone - made of calcitic mud

DOLOMITIC LIMESTONE

Manitoba’s Tyndall stone - dolomitic limestone

fossils are commonly preserved

a famous building stone product in Manitoba and across Canada

burrows in the mud provide pathways for magnesium rich waters to replace the calcite.

thalassinoides

DIATOMITE

diatoms - microscopic algae, silica exoskeleton

diatomite and chalk, very much resemble each other, but they are made of different minerals

diatomite = quartz

chalk = calcite

zooplankton wit siliceous hard parts

CHERT

SiO2 (cryptocrystalline)

accumulated on sea floor

also inorganic varieties

biochemical can form bed layers

10 RESOURCES AND SOCIETY

ORGANIC SEDIMENTARY ROCKS

All renewable energy sources that are so much in the news these days fit into that green slice.

oil, natural gas, coal, nuclear energy, hydroelectric, renewables

COAL

formed from incompletely decayed plant material

STAGES IN COAL FORMATION

1. surface accumulation of leaf litter, twigs, branches, and other fragments of vegetation is buried as swamp deposits, partly decayed, and compressed to peat.
2. shallow burial transforms peat to lignite
3. further burial under hundreds to thousands of meters of sediment transforms lignite to soft (bituminous) coal.
4. continued burial and structural deformation, plus heat, metamorphose soft coal to hard (anthracite) coal.

OIL AND NATURAL GAS

crude oil = petroleum

liquid hydrocarbons

distilled to make various petroleum products

natural gas

gaseous hydrocarbons

formation of an oil reserve

4 conditions must occur:

1. source rock
2. thermal maturity
3. reservoir rock
4. oil trap

“conventional” - drill a well → pump from the well

“unconventional” - need special extraction techniques

OIL TRAPS

anticline - most common trap (structural fold)

Athabasca Oil Sands

bitumen-cemented sand or sandstone deposits

petroleum separated from the sand with steam or naphtha

oil sands in northern Alberta

~64% of western Canada’s production (2018)

URANIUM

not an organic sedimentary rock

majority of Canadian deposits in clastic sedimentary rocks (derived from igneous source rocks)

uraninite (UO2), pitchblende (U2O5)

U used as fuel for nuclear power plants to produce electricity

nuclear reactors currently provide ~10% of world’s electrical generation

UNIT 5

11 METAMORPHISM

METAMORPHIC ROCKS

formed in earth’s interior

a rock undergoes solid-state change in response to the high P, T conditions at depth in Earth

process of metamorphism

WHAT CHANGES OCCUR IN THE ROCK DURING METAMORPHISM?

recrystallization (coarser grained)

reactions to form new minerals reflect equilibrium state - high P, T

atoms rearrange themselves

different crystalline structures

different mineral species

UNDER WHAT CONDITIONS DOES A ROCK METAMORPHOSE?

metamorphism can be the result of:

deep burial (inc P, inc T)

tectonic forces (inc P, inc T) (T may or may not be high)

high T (near a hot pluton)

why do we think these changes take place deep in the crust?

mineral assemblages can only form at high P and T found at depth

METAMORPHIC CHANGES INFLUENCED BY CHEMICAL COMPOSITION OF ROCK

Protolith

the original pre-metamorphic rock

the chemical composition of the protolith provides the ingredient

if the protolith is a limestone - you have calcium and carbonate

metamorphic rock have the same composition

METAMORPHIC CHANGES INFLUENCED BY TEMPERATURE

tectonism can influence the geothermal gradient

METAMORPHIC CHANGES INFLUENCED BY PRESSURE

high pressure packs the atoms into denser crystal structures

metamorphism takes place in the range of 3 - 12 kilobars of pressure

corresponds to 10 - 40 km depth in continental crust

confining pressure

differential stress

rocks can undergo

tension

compression

shear

METAMORPHIC CHANGES INFLUENCED BY FLUIDS

especially H2O

hydrothermal fluids are an agent of metamorphism

transport ions between mineral grains

facilitate metamorphic reactions

some of these fluids can be incorporated into the chemical compositions of some mineral species.

METAMORPHIC CHANGES INFLUENCED BY TIME

metamorphism is a process that takes place over millions of years, which cannot be duplicated in lab experiments

we know:

long periods of geologic time are involved to establish equilibrium in mineral assemblages in metamorphic rocks.

evidence

incomplete reactions preserved in many metamorphic rocks.

higher temperature and larger amounts of fluid speed reaction rates.

SUMMARY: METAMORPHISM IS INFLUENCED BY CHEMICAL COMPOSITION, FLUIDS, TEMPERATURE, PRESSURE, AND TIME.

CLASSIFYING METAMORPHIC ROCKS

1. TEXTURE          FOLIATED          - platy/elongated mineral aligned          FOLIATED METAMORPHIC ROCKS          Slate          progressive metamorphism: unmetamorphosed shale          increasing temperature and pressure, 1st metamorphic rock - slate (compacted crystallized clay)          Phyllite          with increasing temperature and pressure, instead of slate, phyllite would form          clay minerals metamorphosed → mica minerals          ‘satiny’ lustre reflects change in mineralogy          Schist          mica minerals are often still present, even dominant, but some other metamorphic minerals will crystallize as well (e.g. Garnet - silicate)          Gneiss          the iron magnesium minerals segregate from the more felsic minerals, creating bands of foliation.          Migmatite          at highest temperatures and pressures, partial melting takes place.          magma collects in layers where it solidifies as light colored felsic bands          hybrid rock between an igneous and a metamorphic rock.          NONFOLIATED          - minerals lack alignment     - suggests confining pressure conditions during metamorphism          FORMING NONFOLIATED ROCKS          non differential stress          confining pressure          equant minerals          same dimensions in all directions          do not align like needle like platy or flaky minerals          CLASSIFYING NONFOLIATED METAMORPHIC ROCKS          by mineralogy          Marble          calcite, many colours          Quartzite          quartz, many colours          Amphibolite          Hornfels          metamorphosed shale, limestone, and other rocks     
2. MINERALOGY     - the pressure, temperature, stability fields of the metamorphic minerals, reflect the conditions of metamorphism.

PROGRESSIVE METAMORPHISM

conditions of temperature and depth that yield low, intermediate, and high grade metamorphic rocks.

METAMORPHIC FACIES AND TYPES OF METAMORPHISM

assemblage of minerals that form under the same temperature conditions is said to belong to the same metamorphic facies.

the pressure-temperature relationship between metamorphic grade and metamorphic facies.

TYPES OF METAMORPHISM

SHOCK

 aka impact metamorphism

temperatures up to thousands of degrees Celsius and pressure in the gigapascal range

when a large meteorite strikes earth, the frictional heat and pressure from its impact may induce shock.

CONTACT

aka thermal metamorphism

adjacent to a hot igneous intrusion

the dominant factor here is he high temperature.

this is where the hornfels crystallize

the 1 line, conditions for contact metamorphism

with a relatively low confining pressure, well-developed rock developed in these circumstances.

BURIAL

rocks are buried beneath the sedimentary piles

see the turquoise strip between diagenesis of sedimentary rocks and the zeolite phases of metamorphism

REGIONAL

aka dynamothermal metamorphism

forms majority of metamorphic rock

occurs during the development of mountain belts

generally >5 km depth

T: 300-800degC

differential stress - foliated rocks

the big yellow arrow shows the general range of pressure temperature conditions under which regional metamorphism takes place.

HYDROTHERMAL PROCESSES

DYNAMIC (TEXTBOOK)

WHERE ARE METAMORPHIC ROCKS EXPOSED SO THAT WE CAN SEE THEM?

mountain ranges

continental shields

oldest exposed parts of continents

uplift and erosion of overlying material

12 HYDROTHERMAL PROCESSES

HYDROTHERMAL PROCESSES

ions precipitate from hot water solutions (cooler or lower pressure regime)

hydrothermally altered rock (source for many metallic ores)

water infiltrates into the ground

water heats up → cold water sinks into crust → hot water rises and reacts with rock.

hydrothermal minerals crystallize to fill in networks of fractures, forming mineralized veins

hydrothermal minerals crystallize, to fill pore spaces in the host rock, forming disseminated deposit

hydrothermal deposits are the most important source of base metal ores

large disseminated copper deposits often occur in porphyritic igneous rocks and so are referred to as porphyry deposits

hydrothermal fluids emanate from the sea floor

change in chemistry where the hydrothermal fluids interact with the cold sea water

induces precipitation of metal sulphide minerals

build up chimney shaped structures around those vent sites

minerals precipitate when the hot water comes into contact with cold sea water forming the chimneys.

hydrothermal vents have been found in hundreds of different locations around the world, including in the Arctic.

hydrothermal vents - 34% of the heat input to the earth’s oceans, 25% of earth’s total surficial heat. remainder being provided by the sun.

entire volume of the world’s oceans cycles through hydrothermal vent systems along the global spreading centers about every 10 to 20 million years or so.

ENVIRONMENT OF HYDROTHERMAL VENTS

These vents exist in extreme environments characterized by complete darkness at great ocean depths, extremely hot water discharge, intense fluid flows, and high concentrations of toxic metals.

this should be an extremely inhospitable environment for life.

One of the things that surprised scientists the most about these hydrothermal vents was that these fields of chimneys are swarming with rich, unusual organic communities.

Unusual varieties of crabs, mussels, shrimp and others all adapted to live in this dark, toxic environment in the deep sea.

When considering where life may have first evolved on Earth around 3.7 billion years ago, hydrothermal vent systems have become increasingly accepted as the setting where this may have taken place.

WHY ARE HYDROTHERMAL VENTS IMPORTANT?

major source of metals:

Cu, Zn, Pb, Co, Ag,

massive sulphide deposits

UNIT 6

13 MASS WASTING

MASS WASTING

Earth’s forces are driven by two heat engines (internal heat engine, external heat engine) and GRAVITY

external: circulates atmosphere & oceans

influences surficial processes

ex. weathering & erosion

MASS MOVEMENT ↔ GRAVITY

Role in rock cycle: 1st step in transportation of sediment

Role in landscape development: most rapid means of modifying shapes of slopes

CLASSIFYING MASS MOVEMENT

1. rate of movement
2. type of material (rock? regolith?)
3. nature of movement (coherent or chaotic? wet or dry?
4. environment (subaerial? submarine?)

SLOW (SOLIFLUCTION AND CREEP, SLUMPING)

Creep - the gradual, downslope movement of soil or rock on a slope (Google AI)

Slumping - a type of mass wasting that occurs when a mass of material moves down a slope, usually along a curve surface (Google AI)

Solifluction - the gradual movement of water-saturated soil down a slope due to freeze-thaw activity (Google AI)

FAST (LAHARS AND MUDFLOWS)

water-saturated FLOWS ; viscous to fluid motion ; moves as slurry (a watery mixture of insoluble matter (such as mud, lime, or plaster of paris). (Merriam-Webster)

Mud & debris flow ; lahar - mudflow with volcanic ash + meltwater

FASTEST (DEBRIS FLOWS, ROCKFALLS AND ROCKSLIDES [ROCK AVALANCHE])

Scar - exposed cliffs of limestone (Google)

Talus slope - a geological formation that occurs when loose rocks fall from a cliff or rock fact and accumulate at the base (Google AI)

Abyssal fan = submarine fan (Turbidity current)

FOR MASS MOVEMENT TO TAKE PLACE…

1. fracturing and weathering must have started     - fresh, intact rock too strong to undergo mass movement     - weathering weakens materials
2. unstable slopes          high local relief          thick debris/soil above bedrock          planes of weakness parallel to hillside          sparse vegetation          - different materials have different “angles of repose”         - fine sand (35 deg)         - coarse sand (40 deg)         - angular pebbles (45 deg)     - planes of weakness parallel to slope     - roots stabilize the potential failure plane
3. event to initiate motion          increase steepness of slope (river erosion, wave erosion, construction)          reduce the strength of slope (weathering, strip vegetation, add weight to the slope, saturate with water)          shocks and vibrations (earthquakes, construction, traffic)     

ROLE OF WATER

Critical factor

small amounts: inhibits mass movement

larger amounts: rate of movement increases (oversaturation)

Extremes increase instability:

long periods of droughts vs episodes of heavy precipitation

freeze & thaw cycles

IDENTIFYING RISK

Landslides occur where they have occurred before

Conditions present to initiate motion

Detect signs of early motion

PREVENTING AND MITIGATING MASS MOVEMENT

Change the grade of the slope

Prevent undercutting (ie. add riprap, provide a drainage diversion)

Improve drainage (ie. retaining walls, drainage pipes, ditches & culverts)

Re-vegetate (ie. plant more trees)

Safety structures (ie. rock bolting, soil nailing, shotcrete)

Debris Chute and Retention Structures

Controlled Blasting

UNIT 7

14 RIVERS

Stream runoff is an important geologic agent

Changing landscape along a stream

• streams drain the landscape of surface
• the land drained by the network of streams is its watershed

HOW DO STREAMS CHANGE FROM SOURCE TO MOUTH?

In general, the longitudinal profile of a stream (elevation change along its length) resembles a concave-up curve

Base level is the lowest point to which a stream can erode. Ultimate base level is sea level. A lake serves as a local (or temporary) base level

Base-level changes cause stream readjustments. Raising base level results in an increase in deposition. Lowering the base level accelerates erosion

DEPOSITIONAL LANDFORMS PRODUCED BY STREAMS

Alluvial fans are conical, fan-shaped structures that build at the base of a mountain front. Sediments drop out rapidly with a change in stream gradient. The coarsest material is found near the mouth of the canyon; sediments grow fine and thin with distance

Braided streams form where channels are choked by sediment. Flow is forced around sediment obstructions, and the diverging and converging flow creates sand and gravel bars.

Meandering streams. Channels form intricately looping meanders along the lower gradient portion of the longitudinal profile

• Meanders evolve over time, becoming more sinuous (by cut bank erosion and point bar growth) before eventually being chopped off.
• Meanders become more sinuous with time. The cut bank erodes, while the point bar accretes. Meander curves become more pronounced and elongate.
• Meandering streams occupy only a small part of the floodplain, which is typically bounded by eroded bluffs. During floods, the entire floodplain may be immersed. Natural levees are sand ridges that parallel the channel.
• Meander cutoffs occur when cut banks converge and a meander neck thins. During floods, high-velocity flow jumps out of the channel and erodes the meander neck away. The meander cutoff forms an oxbow lake, which eventually fills with sediment.

Deltas consist of sediment deposited at the mouth of a stream. When a stream enters standing water, the current slows, loses competence, and sediments drop out.

• On top of a delta, the stream divides into a fan of distributary channels.
• The Mississippi has a bird’s-foot delta.
• The has an arc-like delta
• The Nile Delta was so-named because the patch of sedimentary land deposited at its mouth is shaped like the Greek delta (triangle symbol).
• Deltas evolve. The main channel feeding a delta may jump to a new location, a process called avulsion.

THE EVOLUTION OF FLUVIAL LANDSCAPES

Beveling Topography. When base level drops, a new equilibrium is slowly established as streams cut into the former surface, valleys widen, and hills erode. Eventually, the landscape is eroded to the new base level.

Stream Piracy. A stream with vigorous headward erosion will eat through the divide separating watersheds and capture the flow of the neighboring stream. Downgradient of the point of capture, the old stream dries up.

Drainage Reversals. In the early Mesozoic, rivers drained westward, from the interior of Pangaea (see Figure 17.27b)

Stream rejuvenation is initiated by base-level fall. Meanders that initially developed on a gentle gradient will chainsaw downward when uplift raises the landscape. This creates incised meanders.

Superposed streams occur in deformed terrain, yet they ignore the underlying structure. Superposed streams initially develop in younger, flat-lying strata. As the stream incises downward into the older deformed rocks, it maintains the geometry developed at an earlier time.

Antecedent and Diverted streams. If erosion is faster than uplift, the stream will cut through the mountain range: an antecedent stream. If the rate of uplift exceeds erosion, the stream is deflected by the mountain range: a diverted stream.

15 FLOODING

RAGING WATERS: FLOODS

• Floodwaters are devastating to people and property. Discharge and velocity increase and flow spills out of the stream channel, immersing adjacent land. Waters scours floodplains, altering the landscape and destroying structures.

CAUSES OF FLOODING

Floods occur when:

• abrupt, heavy rains dump large volumes of water quickly.
• long, continuous rains have saturated soil pores.
• abrupt warm weather rapidly melts winter snow.
• a natural or artificial dam breaks, catastrophically releasing water.

RAGING WATERS: SEASONAL FLOODS

Seasonal floods take time, hours, or days, allowing for evacuation. Still, so many people live in floodplain and delta-plain settings that losses are still gigantic. Seasonal flood recur on an annual basis. Monsoons, the tropical rains of the Indian subcontinent, generate long periods of rain and severe flooding. In 1990, a monsoon killed 100,000 people in Bangladesh.

CASE HISTORY OF A SEASONAL FLOOD: THE MISSISSIPPI AND MISSOURI RIVERS, 1993

In the spring of 1993, the jet stream moved over the midwestern United States. This trapped moist humid air from the Gulf of Mexico and rain fell in great abundance. In July of 1993, floodwaters invaded large areas. Flooding lasted 79 days, covering 40,000 square miles. The toll was enormous: 50 people died, 55,000 homes were destroyed, and the damage totaled $12 billion.

RAGING WATERS: FLASH FLOODS - 1

Flash floodwaters rise so quickly that they may be impossible to escape. They are typified by a rapidly moving wall of debris-laden water. Flash floods occur from unusually intense rainfall, a dam collapse, or a levee failure. They strike with a little warning and may be very destructive.

RAGING WATERS: FLASH FLOODS - 2

In 1889, a flash flood from a dam failure claimed 2300 lives in Johnstown, Pennsylvania

FLASH FLOOD CASE HISTORY: THE BIG THOMPSON RIVER, ESTES PARK, COLORADO, 1976

At 7:00 P.M. on July 31, 1976, rising moist air drenched the Rockies with 19 cm (7.5 inches) of rain in an hour. Discharge in the Big Thompson River swelled to four times the largest recorded maximum. Rock and soil, stripped from the landscape, were added to the flow. Houses, bridges, and roads vanished, claiming 144 lives.

CATASTROPHIC ICE-AGE FLOODS

The Great Missoula Floods: When ice dams broke, glacial torrents from Glacial Lake Missoula scoured portions of the Columbia River plateau.

LIVING WITH FLOODS - 1

Flood control is expensive and sometimes futile. Dams hold water back from trunk streams. Artificial levees and flood walls increase channel volume and transmit intensified flooding downstream. Levees are sometimes overtopped or undermined.

LIVING WITH FLOODS - 2

People living in floodplains face hard choices: move or realize eventual catastrophic loss. Defining floodways, places designed to transmit floods, and removing people and structures from these places may mitigate the magnitude of catastrophic loss.

PUTTING GEOLOGY TO USE - RECURRENCE INTERVALS

Flood risks are calculated as probabilities. Discharges plotted on semi-logarithmic paper against recurrence intervals yield a straight line. The probability of a given discharge, as % chance of occurrence, can bee determined by graph inspection.

LIVING WITH FLOODS - 3

Flood risks are borne by homeowners, insurance companies, lenders, and government agencies.

Hydrologic data are used to produce flood risk maps, which allow regulatory agencies to better manage risks. Building in flood-prone settings is tightly regulated.

CHANNELIZATION

In many places, especially in arid settings, humans use so much river water for irrigation and industry that not enough is left to maintain hydrologic and ecologic functions.

EFFECTS OF URBAN DEVELOPMENT

Hydrographs, which record discharge as a function of time, show the effects of urbanization.

UNIT 8

16 GROUNDWATER

POROSITY (PRIMARY)

Groundwater resides in subsurface pore spaces, the open spaces within any sediment or rock. The total volume of open space is termed porosity. Porosity can be filled with water or air. Pores can also become filled with mineral cement and other fluids, like oil or natural gas.

SECONDARY POROSITY

Secondary porosity is new pore space created after the rock was first formed. Examples include fractures, fault breccia, and solution cavities.

PERMEABILITY

Permeability is the ease of water flow due to pore interconnectedness. High-permeability material allows water to flow readily. Water flows slowly through low-permeability material.

AQUIFERS AND AQUITARDS

An aquifer is a high-porosity, high-permeability rock that transmits water easily.

An aquitard is a lower-permeability rock that hinders water flow.

Aquifers and aquitards are commonly interlayered.

GROUNDWATER AND THE WATER TABLE

The water table is the top of the groundwater reservoir in the subsurface.

The water table separates the unsaturated (vadose) zone from the saturated (phreatic) zone. The capillary fringe forms at the boundary.

RECHARGE

Groundwater flows from recharge areas to discharge areas along curving flow paths.

Groundwater infiltrates at recharge areas, which are at higher elevations.

Groundwater exits the subsurface at discharge areas, which occur at lower elevations.

GROUNDWATER FLOW

Groundwater flow occurs on a variety of time and spatial scales. Some groundwater may flow hundreds of kilometers across sedimentary basins.

The transit time depends on the flow path. Deeper flow paths take longer.

TAPPING GROUNDWATER SUPPLIES

Springs are natural groundwater outlets.

Human use requires that groundwater be captured.

Wells are holes that are excavated or drilled to obtain water.

TAPPING GROUNDWATER SUPPLIES: WELLS

A modern ordinary well pulls water through a well screen with a submersible electric pump and pushes it through piping to the top of the well. Packed sand filters the water.

CONE OF DEPRESSION

Pumping groundwater affects the water table. If groundwater is extracted faster than it can be replaced, a cone of depression develops around the well.

Pumping by a large well may lower the water table enough to cause a nearby small well to go dry.

ARTESIAN WELLS AND THE POTENTIOMETRIC SURFACE

Artesian wells tap confined, tilted aquifers that are pressurized by upland recharge. Water rises in artesian wells to the potentiometric surface, which is an analog of the water table for a confined aquifer.

SPRINGS

Springs are locations of natural groundwater discharge that have been important resources for humans for millennia. A spring develops where the water table intersects the surface, most often in a valley. Springs are marked by wetland vegetation, perennial wetlands, saturated soils, nonfreezing ground, and stream flow.

GROUNDWATER PROBLEMS: DEPLETION AND FLOW REVERSAL

Severe water table decline can alter surface water flow. Before groundwater withdrawal, the water table is high, discharging to a swamp and permanent stream. After protracted groundwater withdrawal, the water table falls and no longer intercepts the surface at the swamp and stream, which dry up.

GROUNDWATER PROBLEMS: SALINE INTRUSION

Saltwater intrusion renders groundwater unpotable. Beneath coastal land, freshwater “floats” on saltwater. Pumping causes the fresh/saltwater boundary to rise, contaminating the well.

GROUNDWATER PROBLEMS: LAND SUBSIDENCE

the gradual settling or sudden sinking of the Earth’s surface (Google AI)

GROUNDWATER QUALITY: HUMAN-CAUSED CONTAMINATION

There are many human activities that are sources of groundwater contamination.

acid mine waste; fertilizer; septic tank; farm animal sewage; gas; landfill; salt; waste containers

HUMAN-CAUSED CONTAMINATION: CONTAMINANT PLUME

Groundwater transports pollutants away from a source of input and creates a contaminant plume. Contaminant plumes have high concentrations near the pollutant source. Concentrations decrease with distance.

KARST LANDSCAPES

Limestone dissolution creates unique karst landscapes.

Elements common in karst landscapes include disappearing streams, natural bridges, caves, speleothems, sinkholes, and springs.

Dissolution occurs near the water table in an uplifted sequence of limestone. Downcutting by an adjacent river lowers the water table. The caves drain and speleothems grow. With additional dissolution, roof collapse creates a landscape pockmarked with sinkholes.

CAVES AND KARST

Caves develop when weakly acidic groundwater dissolves limestone.

CO2 reacts with water to form carbonic acid.

H2O + CO2 → H2CO3

CAVES AND JOINTING

Joints, which are conduits for water, are dissolved by the flow, creating a network of caves and passageways. Soluble beds dissolve more rapidly.

UNIT 9

17 OCEANS

LANDSCAPES BENEATH THE SEA

oceans exist because of difference in lithosphere. Continental lithosphere “floats higher” on the mantle. Ocean lithosphere “floats deeper” in the mantle. Ocean basins collect water because they are lower.

CONTINENTAL SHELVES, SLOPES, AND RISES

• The sea floor exhibits highly varied bathymetry.
• Passive margins occur on both sides of the Atlantic.
• Active margins border the Caribbean and the western coast of South America.

ACTIVE MARGINS

• The active continental margin along the west coast of South America.

PASSIVE-MARGIN BASINS

• The formation of a passive-margin basin (PMB)     - the top surface of PMB is the continental shelf.

OCEAN WATER: SALINITY

• Regional variations in salinity reflect differences in freshwater addition versus elevated evaporation rates.
• Salinity changes with depth, governed by latitude-related evaporation versus freshwater input.

OCEAN WATER: TEMPERATURE

• Regional variations in sea surface temperature are clearly linked to the temperature variation between the tropics and the poles.

TIDES

• The larger (sublunar) tides bulge always faces the Moon. The smaller tidal bulge is always on the opposite side of the Earth from the sublunar bulge. Viewed from the side, the sublunar bulge does not align with the equator.

SPRING AND NEAP TIDES

• The gravitational pull of the Sun adds to, or subtracts from, the lunar pull. When the Sun is aligned with the Moon, stronger, higher tides result. These are called spring tides. When the Sun is at right angles to the Moon, weaker, lower tides result. These are called neap tides.

INTERTIDAL ZONE

• The tidal range is the vertical distance between high and low tide. The region between high and low tides is called the intertidal zone.

UPWELLING AND DOWNWELLING

• A surface flow that moves water away from the shore causes upwelling. Upwelling brings up nutrients and fosters the growth of algae near the coast.

THERMOHALINE CIRCULATION

• Thermohaline circulation results in a global-scale conveyor belt that circulates water throughout the entire ocean system.
• Because of this circulation, the ocean mixes entirely in a 1500-year period.

18 COASTLINES

COASTAL LANDFORMS

Coasts, the belts of land bordering the sea, vary dramatically in terms of topography and associated landforms around the globe.

i.e. uplifted terraces ; drowned river valleys (estuaries) ; glacial fjords ; coastal plains and offshore sandbars ; a swampy delta ; coral reefs off a mangrove swamp ; coastal sand dunes and a wide beach

SEDIMENT BUDGETS

The sediment budget determines much of the character of the coastline.

Sediment is brought into the system by rivers, erosion of cliffs, and by wind.

COASTAL LANDFORMS: BEACHES AND TIDAL FLATS

The beach comprises many different sub-environments, which occur in distinct zones.

The composition of beach sand depends on its source. Some consists of quartz, some of shell fragments, and some of basalt.

BEACH PROFILES

Beaches develop distinctive seasonal profiles.

In the winter, stormy weather mobilizes sand, which is moved to offshore shelves. Winter beaches are narrow and gravelly.

In the summer, more moderate wave energy brings the sand back, replenishing a broad beach.

COASTAL LANDFORMS: TIDAL FLATS

Tidal flats are intertidal regions that accumulate mud and silt to form thick, sticky mud deposits. Tidal flats display bioturbation, abundant sediment reworking by burrowing organisms.

WAVE-CUT NOTCHES

Exposed bedrock along rocky coasts is subjected to powerful, concentrated wave action. Rocky coasts develop unique landforms as a result.

WAVE-CUT BENCH

Wave-cut notches progress until the cliff collapses and the process resumes. Over time, the cliff retreats and is marked by a wave-cut bench (or platform), and erosional remnant of former cliffs often exposed at low tide.

COASTAL LANDFORMS: HEADLANDS AND EMBAYMENTS

There are many landforms that develop along the rocky shore. Beaches collect in embayments; erosion concentrates on headlands.

COASTAL LANDFORMS: ESTUARIES

River valleys that are flooded by sea-level rise are called estuaries. They develop as river canyons that cut into the continental shelves during glacial sea-level lows.

COASTAL LANDFORMS: FJORDS

Fjords landscapes form where relative sea-level rise drowns glacially carved valleys.

COASTAL LANDFORMS: ORGANIC COASTS - COASTAL WETLANDS

Organic coasts are those in which living organisms control landforms along the shore.

Vegetation in coastal wetlands is controlled by climate.

COASTAL LANDFORMS: ORGANIC COASTS - COASTAL REEFS

Coral reefs grow in tropical marine settings and create large rocky structures of cemented skeletons.

Coral reefs are among the most biologically productive ecosystems.

COASTAL PROBLEMS: MODERN SEA-LEVEL CHANGE

Future sea-level rise, due to melting polar ice caps, would flood many coastal cities.

COASTAL PROBLESM: BEACH DESTRUCTION AND PROTECTION

Storms (especially hurricanes) radically alter shorelines. Human development in coastal settings is often affected. Construction in coastal settings is increasingly regulated.

COASTAL STABILIZATION TECHNIQUES

Groins, jetties, and breakwaters arrest sediment transport. Usually this produces unintended consequences. Sediment deposition is enhanced in one place, but sediment erosion is accelerated in another.

SEA WALLS

A concrete or rock seawall can hasten erosion in extreme storms. Wave energy is concentrated, and erosion is enhanced at the base of the wall. Seawalls can then fail.

UNIT 10

19 DESSERTS AND WIND DEPOSITS

WHAT IS A DESERT?

the definition of desert is based on aridity, not temperature

deserts may be hot or cold

hot deserts characteristics

• low latitudes ;
• low elevations ;
• far from oceans ;
• high temperature exceeds 35 deg C

cold desert characteristics

• high latitudes ;
• high elevations ;
• near cold ocean currents ;
• temperature usually below 20 deg C

TYPES OF DESERTS

each desert has unique characteristics of landscape and vegetation. geologists group deserts into one of five classes

• subtropical deserts (Sahara, Arabian, Kalahari)
• Rain-shadow deserts (eastern Oregon)
• Coastal deserts (Atacama)
• Continental interiors (Gobi)
• Polar deserts (Antarctica)

MAKING A DESERT: WIND STORMS

sparsely vegetated ground is scoured by wind and fine sand and silt-sized sediment is lifted and moved. high winds can carry dust across entire oceans

surface load consists of grains moved by saltation (sand skipped and bounced off the land surface)

suspended load is finer sediment carried in the air.

MAKING A DESERT: LAG DEPOSITS

Coarse clasts cannot be lifted and moved by the wind. Lag deposits form when all finer sediment is blown away.

desert pavement is an erosion-resistant surface lag of stones that develops in stages. Dust that falls between pebbles and cobbles is protected from erosion and builds up as a soil. Stones crack into smaller bits and settle to form a mosaic.

MAKING A DESERT: VENTIFACTS AND YARDANGS

grains in wind often “sandblast” exposed surfaces.

ventifacts are stones that have been sandblasted by the wind.

yardangs are wind-sculpted bedrock knobs.

MAKING A DESERT: DEFLATION

deflation is a lowering of the desert land surface via wind erosion. concentrated wind erosion creates a depression in the desert surface called a blowout.

DESERT DEPOSITIONAL ENVIRONMENTS: TALUS APRONS

talus aprons, at the angle of repose, accumulate at the base of cliffs. rock blocks have shapes determined by jointing.

DESERT DEPOSITIONAL ENVIRONMENTS: ALLUIVAL FANS

alluvial fans are conical accumulations of sediment that accumulate where the water exiting a canyon spreads out and drops sediment.

DESERT DEPOSITIONAL ENVIRONMENTS: PLAYAS AND SALT LAKES

playas are desert lakes that have no outlet streams.

DESERT DEPOSITIONAL ENVIRONMENTS: LOESS

wind carries two types of sediment load: surface load made of the coarser (sand-sized) component and suspended load made of the finer-grained, silt-sized “dust.” sand cannot travel far and builds up as dunes within the desert boundaries. Dust, however, often is carried downwind of the desert accumulating as loess (”luss”).

DESERT LANDSCAPES: SAND DUNES

sand dunes are windblown accumulations of sand. dunes develop when sand carried by the wind accumulates around an obstacle. over time, a dune grows and begins to move downwind.

dune form depends on the direction and velocity of the wind and the abundance or scarcity of sediment.

forms of sand dunes: barchan (bottom of U towards the wind) ; parabolic (bottom of U away from the wind) ; transverse (perpendicular towards the wind) ; longitudinal (parallel to the wind); star (wind coming from more than one direction)

DESERTIFICATION

desertification is the aridification of non-desert areas as the result of human activity. the Sahel is the semiarid region along the southern edge of the Sahara. large parts have undergone desertification.

the Aral sea is a perfect case history of desertification.

desertification facilitates large dust storms that cross entire ocean basins and carry dangerous chemicals and disease organisms.

UNIT 11

20 GLACIERS

CONSEQUENCES OF CONTINENTAL GLACIATION: LOADING AND REBOUND

Ice sheets depress the lithosphere into the mantle. Slow crustal subsidence follows flow of asthenosphere. After ice melts, the depressed lithosphere rebounds and the land rises.

CONSEQUENCES OF CONTINENTAL GLACIATION: SEA LEVEL CHANGES

Uplifted beaches along the coast of Arctic Canada form terraces as the land undergoes post-glacial rebound.

Ice ages cause sea level to rise and fall because water is stored on land. Sea level was ~100m lower during the last ice age.

Deglaciation returns water to the oceans and sea level rises. If ice sheets melted, coastal regions would be flooded.

The sea level rise between 17,000 and 7,000 BCE was the result of deglaciation. Low sea level during the last ice age exposed continental shelves.

CONSEQUENCES OF CONTINENTAL GLACIATION: DRAINAGE REVERSALS

Glaciation changed the position of the divide between north-draining and south-draining river networks.

CONSEQUENCES OF CONTINENTAL GLACIATION: ICE DAMS

Glacial Lake Agassiz covered 250,000 km^2 and existed for 2,700 years. It drained abruptly when the ice dam failed.

CONSEQUENCES OF CONTINENTAL GLACIATION: PERIGLACIAL REGIONS

The distribution of periglacial environments in North America. These stone circles near Spitsbergen, Norway, were formed by repeated freezing and thawing, which separates gravel from silt.

THE PLEISTOCENE ICE AGE: GLACIERS

During the Pleistocene, several distinct ice sheets formed. In several places, neighboring sheets came into contact.

Erosion dominated northern and eastern Canada. Depostion dominated in the Great Plains. Erosion dominates beneath the interior of the glacier; deposition dominates along its margins.

THE PLEISTOCENE ICE AGE: LIFE AND CLIMATE

All climate and vegetation belts were shifted southward. Tundra covered parts of the United States, and southern states had forests like those in New England today. Cold-adapted, now extinct, large mammals roamed regions that are now temperate.

THE PLEISTOCENE ICE AGE: TIMING

Oxygen isotope ratios from marine sediments define 20 to 30 Pleistocene glaciations. Earth history has witnessed many ice ages.

CAUSES OF ICE AGE

Milankovic hypothesized that cyclic changes in orbital geometry predict climate variation over 100 to 300 Ka.

The shape (eccentricity) of Earth’s orbit around the Sun varies with a 100,000-year periodicity. Ice ages may result when cooling effects coincide.

LONG-TERM CENOZOIC COOLING

Geologists have reconstructed an approximate record of global climate for geologic time.

WILL THERE BE ANOTHER GLACIAL ADVANCE?

are we living in an interglacial? will ice return? very likely. interglacials last ~10,000 years. it has been ~11,000 years since the last one began. a cool period (1300 - 1850) resulted in the little ice age in Europe. today, a warming trend has caused glaciers to recede. through most of earth’s history, earth’s climate has changed due to natural causes.

UNIT 12

21 DEFORMATION

MOUNTAINS

mountains reflect the geologic processes of:

• uplift
• deformation
• metamorphism

they are evidence of tectonic activity.

constructive processes build mountains up.

destructive processes tear them down.

mountains occur in elongate, curvilinear belts, or orogens. mountain building is a process called orogenesis.

mountain building involved numerous geologic processes:

• deformation, jointing, faulting, folding, partial melting, foliation, metamorphism, glaciation, erosion, and sedimentation.

DEFORMATION

orogenesis applies force to rocks, causing deformation (bending, breaking, shortening, stretching, and shearing).

deformation changes the character of the rocks and is often easy.

undeformed (unstrained) rocks display horizontal beds, spherical sand grains, and no folds or faults.

deformed (strained) rocks show tilted beds, metamorphic alteration, folding, and faulting.

deformation results in displacement (change in location).

deformation results in rotation (change in spatial orientation).

deformation results in distortion (change in shape).

STRAIN

strain is the change in shape caused by deformation. there are several types of strain.

• shortening (contraction)
• stretching (elongation)
• unstrained

shear is a type of strain.

shear changes angular relationships.

BRITTLE DEFORMATION

brittle deformation occurs in the shallower crust; rocks break by fracturing.

there are two major deformation styles: brittle and ductile.

the type of deformation depends on temperature and pressure conditions.

DUCTILE DEFORMATION

a transition between the two types of deformation occurs at 10-15 km depth.

ductile deformation occurs at higher temperature and pressure conditions, which causes rock to deform by flowing and folding.

STRESS

strain is the result of deformation, which is caused by force acting on rock.

stress is the force applied across a unit area.

a small force per area, however, results in little deformation.

TYPES OF STRESS: COMPRESSION

compression takes place when an object is squeezed.

deformation shortens and thickens the material.

horizontal compression drives plate-tectonic collision and orogenesis.

TYPES OF STRESS: TENSION

tension occurs when the ends of an object is pulled apart, which stretches and thins the material.

horizontal tension drives crustal rifting.

TYPES OF STRESS: SHEAR

shear develops when surfaces slide past one another.

shear stress neither thickens nor thins the crust.

TYPES OF STRESS: PRESSURE

pressure occurs when an object feels the same stress on all sides.

ex. a scuba is exposed to equal stress on all sides: pressure

22 FAULTS AND FOLDS

JOINTS AND VEINS

joints are planar rock fractures without any offset that develop from tensile stress in brittle rock.

systematic joints occur in parallel sets.

joints often control the weathering of the rock in which they occur.

fractures filled with minerals are called veins.

groundwater often flows through fractures in the rock where dissolved minerals precipitate.

FAULTS

faults are planar fractures that show offset.

the amount of offset is called displacement.

faults are abundant in earth’s crust and occur at all scales.

FAULT ORIENTATION

on a dipping fault, the blocks are classified as the hanging-wall block above the fault, and the footwall block below the fault.

standing in a tunnel excavated along the fault, your head is near the hanging-wall block and your feet rest on the footwall block.

FAULT MOTION

faults are classified by their geometry (vertical, horizontal, or dipping) and relative motion.

dip-slip faults are characterized by blocks that move parallel to the dip of the fault.

strike-slip faults’ blocks move parallel to the fault plane strike.

oblique-slip faults have components of both dip-slip and strike-slip faults.

DIP-SLIP: NORMAL FAULTS

in a normal fault, the hanging wall moves down the fault slope.

normal faults are most common in regions experiencing crustal tension.

DIP-SLIP: REVERSE FAULTS

in a reverse fault, the hanging wall moves up the fault slope. a thrust fault is a special, low-angle type of reverse fault.

reverse faults are most common in regions experiencing horizontal compression.

DIP-SLIP: THRUST FAULTS

a thrust fault is a special type of reverse fault with a dip below 30 degrees.

NORMAL FAULTS

the hanging wall moves down relative to the footwall.

REVERSE AND THRUST FAULTS

the hanging wall moves up relative to the footwall shortening the crust. reverse faults have steeper dips (>30 degrees); thrust faults dip at lower angles (<30 degrees).

thrust faults act to shorten and thicken mountain belts.

thrust faults can transport sheets of rock hundreds of kilometers and are common features at the leading edge of orogenic deformation.

STRIKE-SLIP FAULTS

strike-slip faults have motion parallel to the strike of the fault.

these faults are classified by the relative sense of motion of the block on the far side of the fault from the observer.

• left lateral - opposite block moves to the observer’s left.
• right lateral - opposite block moves to the observer’s right.

FAULT RECOGNITION

THRUST-FAULT SYSTEMS

thrust-fault systems shingle fault slices one on top of one another. this process acts to shorten and thicken the crust due to horizontal compression.

NORMAL-FAULT SYSTEMS

in normal-faults systems, the fault blocks pull away and rotate to create half-graben basins that stretch and thin the crust. this is due to horizontal-extension (pull-apart) stress. fault dips often decrease with depth, joining a detachment.

FOLDS

layered rock may be deformed into complex folds by tectonic compression. folds occur in a variety of shapes, sizes, and geometries. orogenic settings produce large volumes of folded rock.

FOLD GEOMETRY

a hinge is a line along which curvature is greatest.

limbs are the less curved “sides” of a fold.

the axial plane connects hinges of successive layers.

not all folds look the same.

ANTICLINES

an anticline is a fold that looks like an arch. the limbs dip out and away from the hinge.

SYNCLINES

a syncline is a fold that opens upward like a trough. the limbs dip inward and toward the hinge.

FOLD SEVERITY

folds are described by the severity of folding.

an open fold has a large angle between fold limbs. a tight fold has a small angle between the limbs.

ANTICLINE GEOMETRY

folds are described by the geometry of the hinge.

a non-plunging fold has a horizontal hinge.

a plunging fold has a hinge that is tilted.

PLUNGING FOLDS

Sheep Mountain, Wyoming, is a plunging fold that creates a prominent landform.

Erosion-resistant sandstones form the highs; easily eroded shales are the lows.

DOMES AND BASINS

a dome is a fold with the appearance of a overturned bowl.

a dome exposes older rocks in the center.

a basin is a fold shaped like an upright bowl.

a basin exposes younger rocks in the center.

FLEXURAL-SLIP FOLDS

folds develop in two ways: flexural-slip and passive-flow.

for flexural-slip folds, a stack of layers bends, and slip takes place between the layers.

PASSIVE-FLOW FOLDS

passive-flow folds form in hot, soft, ductile rock at higher temperatures.

GENERATING FOLDS

horizontal compression causes rocks to buckle. shear causes rocks to smear out.

when layers move over step-shaped faults, they fold. deep faulting may create a monocline in overlying beds.

TECTONIC FOLIATION

foliation develops via compressional deformation as sand grains flatten and elongate and clays reorient. flattening develops perpendicular to shortening so that the foliation is parallel to the axial planes of folds.

FOLIATION

foliation can develop as ductile rock is sheared. this type of foliation is not perpendicular to compression.

23 MOUNTAIN BUILDING

MOUNTAIN BUILDING

orogenesis creates igneous and metamorphic rocks. regional metamorphic rocks are generated from the intense heat and pressure of compression and burial. contact metamorphic rocks are created by igneous intrusions. intrusive and extrusive igneous rocks are generated from subduction processes.

mountain uplift is driven by plate-tectonic processes at convergent plate boundaries, continental collision zones, and continental rifts. curvilinear-plate boundaries make curvilinear. mountain belts.

PUTTING GEOLOGY TO USE

if you took a road trip from Washington, D.C. to San Francisco, you would see a remarkable geologic record of the earth’s long history.

CONVERGENT TECTONIC BOUNDARIES

subduction-related volcanic arcs grow on the overriding plate. accretionary prisms (off-scraped sediment) thicken laterally and grow upward. compression shortens and uplifts the overriding plate. a fold-thrust belt develops landward of the orogen.

EXOTIC TERRANES

exotic terranes consist of island fragments of continental crust that had a separate geologic history before being sutured to the overriding plate at a convergent margin.

wester north America is littered with exotic terranes.

CONTINENTAL COLLISION

continental collision follows ocean-basin closure and complete subduction of oceanic lithosphere. this brings two blocks of continental lithosphere together. because continental crust is too buoyant to subduct, it shuts down the subduction zone.

continental collision creates a welt of crustal thickening due to thrust faulting and flow folding.

the center of the belt consist of high-grade metamorphic rocks.

fold-thrust belts extend outward on either side.

CONTINENTAL RIFTING

normal faulting in continental rifts creates fault-block mountains and basins.

thinning crust results in decompressional melting, which adds volcanic mountains, increases heat flow, and expands and uplifts rocks.

UPLIFT AND MOUNTAIN TOPOGRAPHY

the process by which the surface of the earth moves vertically from a lower to a higher elevation is uplift.

mountains require elevation changes on earth’s surface.

mount Everest is 8.85 km above sea level, and it is made of sediments.

WHY ARE MOUNTAINS HIGH?

convergent-margin horizontal compression causes horizontal shortening and vertical thickening.

these processes create a thick crustal root beneath mountain ranges.

adding igneous rock can thicken the crust.

volcanic material is added to the surface.

plutons are added at mid-crustal levels.

delamination removal of deep lithospheric mantle can cause uplift.

the Tibetan Plateau bears evidence of delamination.

WHAT GOES UP… …MUST COME DOWN

mountains reflect a balance between uplift and erosion.

mountains are steep and jagged due to high rates of erosion.

when tectonic uplifts slows or ceases, or rates of erosion exceed rates of uplift, mountains are reduced in elevation.

eventually, mountains may be eroded back to sea level.

orogenic collapse: there is a limit to mountain heights because the weight of mountains eventually overwhelms the strength of hot ductile rock in the lower crust.

CRATONS

a craton is continental crust that hasn’t been deformed in 1 Ga.

cratons are made of cool, strong, and stable continental crust.

there are two cratonic provinces:

• shields, which consist of Precambrian igneous rocks and metamorphic rocks.
• platforms, the sedimentary cover that is draped over shield rock.

cratonic platforms consist of sedimentary rocks covering Precambrian basement.

they exhibit domes and basins resulting from vertical crustal adjustments due to stresses transmitted from active margins to the cratonic interior.

OROGENIC CASE HISTORY: THE APPALACHIANS

the Appalachians consist of a complex mountain belt formed by three Phanerozoic orogenic pulses.

a giant orogenic belt existed before the Appalachians. the Grenville orogeny (1.1 Ga) formed a supercontinent (Rodinia). By 600 Ma, much of this orogenic belt had eroded away.

when the supercontinent rifted apart, a new ocean (the proto-Atlantic) formed. eastern north America developed as a passive margin and a thick pile of sediments accumulated. an east-dipping subduction zone built up an island arc.

subduction carried the margin into the island arc. the collision resulted in the Taconic orogeny (420 Ma). a doubly dipping subduction zone developed, and exotic blocks of continental crust were carried into the subduction zones and added to the margin during the Acadian orogeny (370 Ma).

east-dipping subduction continued to close the ocean during the late Paleozoic.

Alleghenian orogeny (270 Ma):

• Africa collided with north America, creating a huge fold-thrust belt and mountain range; assembled the supercontinent of Pangea.

Pangea began to rift apart about 180 Ma. Faulting and stretching thinned the lithosphere. this led to rifting, the eventual development of seafloor spreading, and the creating of the Atlantic ocean.

UNIT 13

24 EARTHQUAKES

EARTHQUAKES

earthquakes are usually caused when underground rock suddenly breaks and there is rapid motion along a fault. this sudden release of energy causes the seismic waves that make the ground shake.

EARTHQUAKE ORIGINS

the hypocenter (or focus) is the location where fault slip occurs. it is usually on a fault surface.

the epicenter is the land surface directly above the hypocenter. maps often portray the location of epicenters.

FAULTS: NORMAL FAULT

faults are planar breaks in the crust. most faults are sloping (vertical faults are rare). the type of fault depends on the relative motion of blocks.

• footwall (block below the fault)
• hanging wall (block above the fault)

on a normal fault, the hanging wall moves down relative to the footwall. it most often results from extension (pull-apart or stretching).

FAULTS: REVERSE FAULT

the slope (dip) of a reverse fault is steep. (≥60 degrees)

in a reverse fault, the hanging wall moves up relative to the footwall. it usually results from compression (squeezing or shortening).

FAULTS: THRUST FAULT

a thrust fault is a special kind of reverse fault that has a lower angle slope (dip). it’s a common fault type in compressional mountain belts. the slope (dip) of a thrust is less steep than a reverse fault. (≤30 degrees)

STRKE-SLIP FAULT

along a strike-slip fault, one block slides laterally past the other block. there is no vertical motion across the fault.

strike-slip faults tend to be close to vertical. the motion across the fault, however, is not vertical but horizontal.

STICK-SLIP BEHAVIOR

faults move in jumps. once movement starts, it quickly stops due to friction. over time, strain builds up again, leading to repeat failure.

this behavior is termed stick-slip behavior.

stick-friction prevents motion.

slip-friction briefly overwhelmed by movement.

ELASTIC STRAIN

elastic strain builds up in a stressed rock mass and that strain can be measured.

strain buildup can be measured as ground distortion using the InSAR (interferometric synthetic aperture radar) satellite. InSAR compares ground elevation changes over time and creates maps that display distortion as color bands.

DISPLACEMENT

fault slip is cumulative. faults can offset rocks by hundreds of kilometers given geologic time.

larger earthquakes have larger areas of slip.

displacement is greatest near the hypocenter. displacement diminishes with distance.

SEISMIC BODY WAVES: P-WAVES

p-waves travel by compressing and expanding the material parallel to the wave-travel direction. p-waves are the fastest seismic waves, and they travel through solids, liquids, and gases.

SEISMIC BODY WAVES: S-WAVES

s-waves travel by moving material back and fort, perpendicular to the wave-travel direction. s-waves are slower than p-waves, and they travel only through solids, never liquids or gases.

SEISMIC BODY WAVES: L-WAVES

surface waves travel along earth’s exterior. surface waves are the slowest and most destructive.

l-waves (love waves) are s-waves that intersect the land surface. they move the ground back and forth like a writhing snake.

SEISMIC BODY WAVES: R-WAVES

r-waves (Rayleigh waves) are p-waves that intersect the land surface. they cause the ground to ripple up and down like water.

SEISMOGRAPHS

modern seismographs use magnetic and electric coil. they record data digitally. they are able to detect very small ground motions, those that people cannot sense.

SEISMOGRAM

seismogram is the data record from a seismograph. it depicts earthquake wave behavior, particularly the arrival times of the different waves, which are used to determine the distance to the epicenter.

seismic waves arrive at a station in sequence.

• p-waves are first.
• s-waves are second.
• surface waves are last.

LOCATING THE EPICENTER

p-wave and s-wave arrival times can be graphed. a travel-time curve plots the increasing delay in arrivals. the time gap yields distance to the epicenter.

data from three or more stations pinpoints the epicenter. the distance is drawn as a radius from each station, creating a circle of equidistant points. circles around three or more stations will intersect at a point. the point of intersection is the epicenter.

MAGNITUDE AND INTENSITY

intensity (severity of damage observed in the field) and magnitude (amount of ground motion measured on a seismograph) are related in an approximate way, even though they are different measurements.

the modified Mercalli intensity scale is a subjective determination that assigns roman numerals to differing degrees of damage. Damage intensity decreases with distance from the epicenter.

EARTHQUAKES AT PLATE BOUNDARIES

shallow - divergent and transform boundaries

intermediate and deep - convergent boundaries

EARTHQUAKE DAMAGE

earthquakes kill people and destroy cities. damage can be widespread, horrific, and heartbreaking.

EARTHQUAKE HAZARDS: LIQUEFACTION

liquefaction causes soil to lose strength. land, and the structures on it, will slump and flow. buildings may founder and topple over intact.

EARTHQUAKE HAZARDS: FIRE

fire is a common result of earthquakes. shaking topples stoves, candles, and power lines, and breaks gas mains. important infrastructure may be destroyed (water, sewer, electricity, roads). firefighters are often powerless to combat fire without road access, no water, and too many hotspots. fire may greatly magnify the destruction and toll on human lives.

EARTHQUAKE HAZARDS: TSUNAMIS

destructive tsunamis occur frequently, about one per year.

tsunami means harbor wave in Japanese.

tsunamis result from displacement of the sea floor by an earthquake, submarine landslide, or volcanic explosion that displaces the entire volume of overlying water. sea-floor displacement creates a giant mound (or trough) on the sea surface. this feature may cover an enormous area (up to 10,000 mi^2). the feature collapses and creates waves that race rapidly away.

CAN WE PREDICT EARTHQUAKES?

yes and no.

they CAN be predicted in the long term (tens to thousands of years).

they CANNOT be predicted in the short term (hours and weeks).

hazards can be mapped to assess risk and develop building codes, implement land-use planning, and disaster response.

UNIT 14

25-26 INTERIOR OF THE EARTH

EARTHQUAKES (LAST UNIT)

seismic waves arrive at a station in sequence

• p-waves are first
• s-waves are second
• surface waves are last

p-wave and s-wave arrival times can be graphed. a travel-time curve plots the increasing delay in arrivals. the time gap yields distance to the epicenter.

SEISMIC BODY WAVES: S-WAVES

s-waves travel by moving material back and forth, perpendicular to the wave-travel direction. they are slower than p-waves and travel only through solids, never liquids or gases.

SEISMIC BODY WAVES: P-WAVES

p-waves travel by compressing and expanding the material parallel to the wave-travel direction. p-waves are the fastest seismic waves, and they travel through solids, liquids, and gases.

EARTH’S INTERIOR

earthquake (seismic) waves allowed geologists to refine the model of earth’s interior. the mantle and the core are subdivided.

DIFFERENTIATION

the other terrestrial planets and the moon all experienced differentiation early in their histories to form a layered interior of crust, mantle, and core. the proportion of these layers differs.

EARTH’S CRUST AND LITHOSPHERE

the lithosphere is the solid, outer part of earth. the lithosphere includes the brittle upper portion of the mantle and the crust, the outermost layers of earth’s structure.

ASTHENOSPHERE

the asthenosphere is the denser, weaker layer beneath the lithospheric mantle. the temperature and pressure of the asthenosphere are so high that rocks soften and partly melt, becoming semi-molten.

MOHOROVICIC DISCONTINUITY

Moho, is a discrete jump in seismic wave velocities - a seismic discontinuity - that has come to define the boundary between the crust and the mantle.

THE MANTLE AND CORE

the mantle is the largest layer of Earth and is composed of two layers (upper and lower mantle - separated by the transition zone).

the core is the densest layer. there is a liquid outer core and a solid inner core.

EARTH’S MAGNETIC FIELD

earth’s magnetic field is due to the flow of material in the liquid outer core. the magnetic axis is not parallel to the rotational axis.

PALEOMAGNETISM

iron materials in rocks archive the magnetic field at the time of formation.

magnetic inclination and declination can be determined in a laboratory for an oriented rock sample.

MAGNETIC REVERSALS

during reverse magnetic polarity, the magnetic dipole points in the opposite direction.

during these conditions, a compass needle would point to the south magnetic pole.

A CHRONOLOGY OF MAGNETIC REVERSALS

a magnetic reversal chronology for the last 4 million years (Ma) reveals differing durations of normal and reverse magnetic polarity.

EVIDENCE OF SEA-FLOOR SPREADING

magnetometers towed by ships moving perpendicular to the mid-ocean ridges recorded alternating strong and weak magnetic fields.

positive (strong) and negative (weak) magnetic anomalies lined up to form “stripes” on the ocean floor. these stripes are symmetric around the mid-ocean ridge.

MARINE MAGNETIC ANOMALIES

seafloor spreading predicts that magnetic anomalies should be mirror images across the mid-ocean ridge (and they are).

UNIT 15

27-28 PLATE TECTONICS

THE THREE TYPES OF PLATE BOUNDARIES

• Divergent boundary - two plates move away from each other
• Convergent boundary - two plates move towards each other
• Transform boundary - two plates slide past each other

AN ISLAND ARC ON A CONTINENTAL CRUSTAL FRAGMENT

• in some cases, a volcanic island arc develops on a small fragment of continental crust that has split away from the main continent.

TRANSFORM BOUNDARIES

• transform faults on the ocean floor connect and offset ridge segments.
• active faulting, as indicated by earthquakes, only occurs in the parts of the fracture zone between ridge axes.
• some transform boundaries cut continental crust. for example, across the San Andreas fault, the Pacific Plate moves northwest relative to the North American plate.

THE PROCESS OF RIFTING

• when continental lithosphere stretches and thins, the upper crust breaks by faulting. the upwelling asthenosphere initiates volcanism. rifting may split a continent in two.
• the Basin and Range province is a rift. faulting bounds the narrow north-south-trending mountains, which are separated by basins. the arrows indicate the direction of stretching.
• the East African Rift is an active rift.
• the Red Sea started as a rift, but it evolved into a narrow ocean basin.     - the rift axis became a mid-ocean ridge.     - these faults result from stretching and thinning of continental lithosphere.

CONTINENTAL COLLISION

• continental collision starts after subduction has consumed the oceanic plate that was once between two continents.
• continental crust is too buoyant to subduct; when two continents converge, rock undergoes compression and shearing, and a mountain range develops.

PLATE DRIVING FORCES: RIDGE PUSH

• plate driving forces: ridge push develops because of the gravitational energy associated with the topographic elevation of the mid-ocean ridge.

PLATE DRIVING FORCES: SLAB PULL

• plate driving forces: slab pull develops because old oceanic lithosphere is denser than the underlying asthenosphere, so it sinks.

THE MISCONCEPTION OF CONVECTION

• the conceptual models of convection cells have changed over time from the old model of a singular convection cell to the modern one with warmer and cooler areas.
• convection cells are no longer thought to drive plate motion alone.

THE CHANGING FACE OF EARTH

• as a result of plate tectonics, the map of earth’s surface changes slowly and continuously.

UNIT 16

29 GEOLOGIC TIME SCALE AND EARTH HISTORY 1

EARTH’S GEOLOGIC TIME SCALE

“Geologic Column”

• divided into:
• eons → eras → periods → epochs

Age of Earth: 4.56 Ga

• Meteorites: extra-terrestrial rocks
• Moon rocks: Apollo

Earth’s Eons

• Hadean Eon (4.6 -4.0 Ga): Planet Formation     - accretion of planetesimals     - formation of Moon     - high heat flow: radioactive decay, bombardment by meteorites                  Earth differentiated                  - molten iron core developed         - end of hadean: crust                  Rocks mostly not preserved                  - some minerals (zircon) as old as 4.4 Ga         - oldest known rocks on earth: 4.03 Ga (NWT)                  Volcanic degassing:                  - led to development of oceans (by 3.8 Ga)         - development of atmosphere (N2, CO2, SO2, H2O-rich) (denser than present day, not oxygenated)
• Archean Eon (4.0 - 2.5 Ga): Planet Evolution     - development of oceans by 3.8 Ga     - development of continental nuclei and origin of life     - magnetic field active ~3.5 Ga     - land surfaces: stark, unvegetated     - well developed hydrosphere         - large erosive rivers (weathering products → salty ocean)         - atmosphere: outgassing from volcano (reducing atmosphere)         - first life since at least 3.5 Ga     - cyanobacteria: mounds of stromatolites     - led to oxygenated atmosphere
• Proterozoic Eon (2.5 - 0.541 Ga)     - stabilization of continental platforms     - oxidization of atmosphere     - multicellular organisms abundant by end of eon
• Phanerozoic Eon (541 Ma - present)     - modern plate tectonic period     - expansion of life          Hydrosphere:          - shallow seas through much of time          Atmosphere:          - modern-day atmosphere          Biosphere:          - diversity of life     - transition from marine to terrestrial

Eons are divided into Eras

Eras of the Phanerozoic:

• Paleozoic (”old life”)          → >90% of species extinct     
• Mesozoic (”middle life”)          → ~75% of species extinct     
• Cenozoic (”new life”)

Periods of the Phanerozoic

periods are divided into epochs

30 GEOLOGIC TIME SCALE AND EARTH HISTORY 2

RELATIVE VS NUMERICAL TIME

• there are two ways of dating geological materials:     - qualitatively and quantitatively
• relative ages are based upon the order of formation and permit determination of older vs younger relationships.
• numerical ages are the actual number of years since an event occurred.

PHYSICAL PRINCIPLES: UNIFORMITARIANISM

• the principle of uniformitarianism states that the processes observed today were the same in the past.

PHYSICAL PRINCIPLES: ORIGINAL HORIZONTALITY

• the principle of original horizontality states that, because sediments settle out of a fluid by gravity, they tend to accumulate horizontally. sediment accumulation is not favored on a slope. hence, tilted sedimentary rocks must be deformed.

PHYSICAL PRINCIPLES: LATERAL CONTINUITY

• the principle of lateral continuity observes that strata often form in laterally extensive horizontal sheets. subsequent erosion dissects once-continuous layers.

PHYSICAL PRINCIPLES: CROSS-CUTTING RELATIONS

• the principle of cross-cutting relations holds that younger features truncate (cut across) older features.
• faults, dikes, erosion, etc., must be younger than the material that is faulted, intruded, or eroded (a volcano cannot intrude rocks that aren’t there yet).

RELATIVE DATING

• the principle of fossil succession describes the predictability of fossil distribution through time.
• index fossils are diagnostic of a particular geologic time.
• the fossil range describes the first and last appearance of a species.
• each fossil has a unique range.
• sometimes the ranges of unique organisms overlap.

NUMERICAL AGE: RADIOACTIVE DECAY

• isotopes are elements that have varying numbers of neutrons. isotopes of the same element have similar but different mass numbers.
• stable isotopes (i.e., Carbon-13 or ^13_C) never change. radioactive isotopes (i.e., ^14_C) spontaneously decay into other elements.
• the half-life is the time it takes for half of unstable nuclei to decay.

NUMERICAL AGE: RADIOACTIVE DATING

• the age of mineral is determined by measuring the ratio of parent-to-daughter isotopes.