Igneous Rocks and Volcanism

Intrusive vs extrusive rocks

Igneous rocks have 2 major categories based on where they cool

Extrusive = lava cools on the surface

• aka volcanic

• low ambient temperature, cool fast

• chill too fast to make big crystals

Intrusive = rocks cool at depth

• aka plutonic

• high ambient temperature, cool slowly

• crystals grow slowly and large

commonly occur together

• magma chambers feed overlying volcanoes

• magma chambers may cool to become plutons

• many ingenious geometries are possible

• they are linked in space in time → both form at subduction and at rift zones during continental collision and at hot spots

Felsic vs mafic magmas

Erupting magma through Earth’s surface is called lava

• Magmas vary widely in composition but are mostly made of Si and O which are referred to as silicate magmas

Felsic magmas

• higher Si, Na, K

• melt at lower temperatures (700ºC)

Mafic magmas

• lower S

• higher Ca, Fe, Mg

• melt at higher temperatures (up to 1200ºC)

• 

  • lava formed the first surface rocks

    • our geological record goes back to ~4.3 Ga

    • these original rocks were mostly lost to our observation, weathered or covered by later rocks, cycled through tectonic processes

Dikes vs Sills

• always younger than the rocks they intrude

• tend top have uniform thickness that can be traced laterally

dike: cuts across rock layerings, causes rocks to spread sideways

sill: injected parallel to rock layering, creates vertical uplift

tabular intrusions

• dikes and sills modify the invaded wall rock (aka country rock)

  • cause it to expand and inflate

• they also thermally alter the wall rock

• cut across rock layering and sometimes occur in swarms

• common in extensional settings

• sills are injected parallel to layering

• below - basalt (dark rock) intruded light-coloured sandstones in Antarctica

• The intrusion lifted the entire overlying landscape

Magma chamber processes:

fractional crystallization

partial melting

assimilation

fractional crystallization

• the composition of the residual magma changes as it cools and crystallizes

• high melting point minerals (Fe, Mg, Ca → mafic silicates) crystallize early and settle to base of magma chamber due to gravity

• the remaining magma is enriched in Na, K, Al → felsic silicates & becomes more felsic

• while cooling, the magma evolves in composition bc different minerals crystallize at different temperatures and have different melting points

• 

  • the solid minerals are denser than the liquid, so they settle to the base of the magma chamber where a cumulate rock forms

  • at any point, an eruption of the liquid and suspended crystals may occur → the same magma chamber can make mafic or felsic lava

partial melting

• rocks rarely melt uniformly

• most magmas form from partial melting of existing rock → either in upper mantle or crust ← Si rich minerals melt first and Si poor minerals melt last

• partial melting therefore yields a silica rich, felsic magma, and removing the partial melt from its source (eg by eruption) creates a mafic residual magma with high melting temperature

• 

assimilation

• magma incorporates the wall rock it passes through

• blocks of wall rock (xenoliths) break off and fall into the magma

• assimilation (melting) of these rocks alters the magma composition

• Mafic xenoliths in granite. The one below has partially dissolved.

  

• all 3 processes can occur together → fractional crystallization, partial melting, assimilation

  • partial melting of wall rock produces new magma that mixes with magma from below

  • blocks of rock fall into magma and dissolve → assimilation’

  • deep magma rises

Bowen’s reaction series

• N.L. Bowen - conducted melt cooling experiments in the 1920s and discovered that minerals solidify in a specific order

• continuous: plagioclase changes gradually from Ca rich to Na rich with decreasing temperature

• discontinuous: minerals crystallize over a particular temperature range and then stop and then another mineral starts

  • olivine, pyroxene, amphibole, biotite

Decompression and Flux Melting

• Tectonic setting

rock melting:

• magma is not everywhere below the earth’s crust → only forms under particular conditions

• melting is caused by pressure release (decompression melting), addition of volatiles (flux melting), and heat transfer

decompression melting

• decrease in pressure (P) = decompression

• the base of the crust is not hot enough to melt mantle rock but due to the high pressure the rock doesn’t melt

• melting will occur if pressure is decreased

• pressure drops when hot rock migrates to shallower depths

• decompression melting occurs infantile plumes or beneath oceanic/continental rifts

• rifts and hotspots

  • upward convection of the mantle at spreading centres leads to melting in response to decreasing pressure (temp changes little)

  • 

flux melting

• caused by the addition of volatiles which greatly reduces the melting temperature

• volatiles lower the melting point of rocks

• common volatiles incl H2O and CO2

• subduction carries water into the mantle thus melting the rock

• 

• the addition of volatiles decreases the melting point of a dry rock

• 

  heat transfer melting

• magma from partial melting of mantle rocks rises up through the mantle

• may pool at the base of the crust or rise through the crust

• inc temp in nearby crustal rock to melting point

Properties of pyroclastic flows

pyroclastic flows:

• Greatest Volcanic Hazard

• Extreme temperature & velocity

• Made of ash and gases

Volcanic hazards

• pyroclastic flows = very hot and fast moving clouds of gas and ash

  • reach up to 700 km/h and 1000ºC

  • low density → can flow over topographic highs or small bodies of water

  • flows that glow red at night are called nuees ardentes = burning clouds

  • MOST DEADLY of all volcanic hazards

Properties of mafic and felsic lavas

• basaltic lavas - low silica contents have low viscosity and tend to form thin, effusive lava flows

• rhyolitic lavas - high silica content have high viscosity and form thick, slow moving and explosive volcanic eruptions

• 

  effect of viscosity

• mafic lava

  • low silica - so low polymerization (process in which monomers combine to become polymers) in the melt

  • low viscosity = gases can escape

  • reduces pressure

  • usually has a quiet, effusive flow

  • like an open bottle of cocacola that has gone flat

• felsic

  • high silica = high viscosity

  • too viscous for gas to easily escape so they tend to have higher volatile contents due to tectonic setting

  • higher pressure builds up

  • leads to explosive eruptions

  • like a closed bottle of cocacola shaken to release the gases

most volcanic glass (obsidian, pumice) is felsic but scoria is more mafic and characteristic of cinder cones

What eruptions are the most explosive

How does the addition of volatiles to a melt change its viscosity, eruption explosively, mineralogy

Sedimentary Rocks and Processes

Sedimentary rock formation steps: weathering → erosion → transport → deposition → lithification / diagenesis (burial, compaction, cementation)

Rock Types

• rocks are classified into 3 groups - igneous, sedimentary, metamorphic

• they are connected through the rock cycle

• 

sedimentary rocks

• made of sediments, these rocks are found only on the earth’s surface and cover about 75% of the total land surface

• only sed rocks contain fossils, from which we can deduce earth’s history

• all oil and natural gas reserves are found in sed rocks

• most of the worlds caves are found in the sed rock limestone

• form layers

  • layers are called beds, they record a history of ancient environments

  • sed rocks cover the underlying basement rock

types of sed rock

• clastic (detrital) = rocks made of parts of other rocks, plants, or shells (eg mudstone, conglomerate, sandstone)

• bioclastic = clastic rocks made from living things, organisms (eg coal, coquina, chalk)

• chemical = rocks precipitated from seawater (eg limestone, chert, banded iron formations, saltstone)

rock features such as grain size, sphericity, and sorting tell us about the energy of the transport medium and the distance the fragments have travelled from their source

• immature sediments have angular fragments, poor sorting and high feldspar contents

• mature sediments have rounded, well sorted class, high quartz and clay contents

clastic sed rocks are classed based on grain size: clay/silt sone → sandstone → breccia/conglomerate

Weathering is enhanced by increased surface area (eg along joints)

steps in the formation of a clastic rock

1. weathering - generation of detritus by breaking up rocks

2. erosion - removal of sediment grains from weathering site

3. transportation - dispersal by gravity, wind, water, and ice

4. deposition - settling out of the transport medium eg due to a decrease in current energy

5. lithification - transformation of sediments into solid rock

weathering

• physical - mechanical fragmentation of rock, cold and or dry climates

• chemical - dissolution of minerals in water, hot/humid climates

• weathering products are transported to sites of deposition by rivers, winds, and glaciers where they form sediments

  surface area and weathering

• fewer cracks, less SA → more cracks, more SA

• 

joints and weathering

• as rocks are exposed by uplift and weathering, the pressure from overlying rock is removed and expansion occurs

  • expansion leads to vertical and horizontal cracks called joints

  • these planes of weakness add more SA for weathering

  • 

  • joints are opened up by plants and ice

What rocks or minerals are most or least resistant to erosion

• carbonate rocks (limestones) are less resistant to weathering than silicate rocks

• degree of weathering depends on SA

Immature vs mature sediments

• with increasing distance from their source, sediments become rounder, better sorted and finer, with a mineralogy more resistant to weathering

• what minerals are present in the most mature sediments

Depositional environments: what sediments and rock types correspond to each and why

locations where sediment accumulates

• differ in

  • physical, chemical, bio characteristics

  • sediment delivery, transport, and depositional conditions

  • energy regime

• environments include

  • terrestrial

  • coastal

  • marine

terrestrial

• glacial sed = formed due to glacier movement

  • glaciers carry sed carved out of glacial valleys, sed are dumped during melting, creates glacial till (poorly sorted Gravel, sand, silt, and clay)

  • immature sediments due to short transport distances

• rivers = immature or mature seds

  • maturity depends on distance travelled → longer distances offer more time to break down into smaller debris → size of particles deposited depend on stream’s energy, low energy streams will drop out larger particles and carry finer ones

  • high energy rivers = large clasts during floods, poorly sorted coarse conglomerate is characteristic, cobbles and boulders are immobile

  • low energy rivers: channelized sed transport, sand and gravel fill conc up channels, find sand, silt, and clay are deposited on nearby flood plains

  • immature river sed: steep high energy mountain streams can transport coarse, unsorted debris

  • mature river sed: broad, slow moving rivers have lower energy and transport finer particles

channel sediments

• as rivers meander across a floodpain, the channel will migrate

• shales and siltstones - younger floodplain deposits

• sandstone - channel fill

• shales and siltstones - older floodplain deposits

• a single conc down channel + younger and older floodplain deposits above and below that are related to channels outside of the image

alluvial fans

• sediments that pile up at a mountain front due to seasonal streams

• rapid drop in stream velocity creates a cone shaped wedge

• sed become conglomerate and arkose

• immature - coarse, poorly sorted, high in feldspar

deltas

• sed are dep near or at sea level

• deltaic seds accumulates where a river enters the sea

• sed carried by the river is dumped when velocity drops

• deltas grow over time, building out into the basin

• 

beaches

• beach sands are deposited from seawater

• sed are constantly being processed by wave action, filtering out fine, silty particles

• common result is well sorted and well rounded sand

• beach ripple marks are often preserved in sed rocks

deposition from currents

• water or wind flowing over sed creates bedrooms - rhythmic sed features formed at the sed fluid interface

• ripple marks - cm scale ridges & troughs

  • dev perpendicular to flow

  • ripple marks are freq preserved in sandy sediments

  • found in modern beaches

  • found on bedding surfaces of ancient sed rocks

  dunes - bigger ripples

  • windblown, well sorted sand

  • faster sed transport velocity causes ripples to flatten out

  • occur in deserts or beach regions

  • often preserve large internal cross beds

  • the direction the beds are dipping indicates paleocurrent

cross beds - created by ripple and dune migration

1. sediment moves up to the gentle side of a ripple or dune

2. sediment piles up and then slips down the steep face

   • slip face continually moves down current

   • added sed forms sloping cross beds

turbidity currents and graded beds

• sediment moves on underwater slopes as a pulse of turbulent water

• as the pulse wanes, water loses velocity and grains settle

• coarsest material settles first, med next and then fines

  • process forms graded bedding in turbidite deposits

reefs

• shallow water carbonate rich envs

  • most sed are carbonates - shells of organisms, CaCO3

  • warm clear marine water → red free of clastic sed

  • protected lagoons accumulate mud

  • wave tossed reefs are made of coral and reef debris

  • main source of limestones

shallow marine environments

• shallow marine clastic deposits - finer sands, silts, muds

• fine seds deposited offshore where energy is low

• silts and muds turn into siltstones and mud or shales

• usually supports an active biotic community

• —> organic material —> black shales and fossils

deep marine envs

• fine sediments settle out far from lan

• skeletons of planktonic organisms accumulate as a calcareous mud on the seafloor and make chalk or chert

• fine silt and clay lithifies into shale

In summary:

sed rock types and features are closely linked to their depositional envs

- terrestrial

• glacial, streams/river, alluvial fans → river envs can be high energy or low energy & this relates to the size of the sediments transported

- coastal

• beaches, river deltas, reefs

• sands, shale / clay stone, limestone

- marine

• shallow and deep water

• shale, chalk, chert

bedding can be graded or cross bedded, and can have ripple marks if deposited from currents

Sedimentary basins: types and formations

basins form where tectonic activity creates space

• topographically low areas that accumulate sediment

• thick layers of sediments up to 10-20 km

foreland basins

• edge of collisional mountain belt

• flexure of the crust from loading creates a downwarp

• fills w debris eroded from mountains

• fluvial, deltaic, and lake sediments fill foreland basins

rift basins

• occur at divergent (pull apart) plate boundaries

• curst thins by stretching and rotational normal faulting

• thinned crust subsides

• sediment fills the down dropped troughs

• subsidence = sinking of the land caused by tectonic activity and sediment loading

sed basins

• intercontinental basins - plate interiors far from margins

  • may be linked to failed crustal rifts

  • continue to subside for millions of years after formation

• passive margins - continental edge far from plate boundary

  • underlain by crust thinned by prev rifting

  • thinned crust subsides as it cools

  • subsiding basin fills w sediment from rivers entering sea

Water and Ice: Glaciers

Alpine vs continental glaciers

• what is the difference

• how do they grow/shrink and flow

• what part of the glacier moves the fastest?

alpine

• mountain glaciers on peaks and in valleys

• flow from high to low elevation

continental

• ice sheets covering large areas of land

• flow outward from the thickest central part

glaciers flow by…

1. basal sliding - meltwater forms at the base of the glacier dec friction and allowing ice to slide

2. plastic deformation - glaciers deform under their own weight and are pushed by gravity

• flow rates vary widely - 10-300m / yr

• flow rate is controlled by:

  • steepness of slope angle

  • amt of basal water

  • location w/i glacier

  • greater velocity in centre of ice

  • friction slows the ice at the margins

glaciers are like bank accounts for water

• zone of accumulation - area of net snow addition

  • colder temps prevent meltong’

  • snow remains in summer months

• one of ablation

  • area of net ice loss

• accumulation>ablation - glacial toe advances downslope

• accumulation<ablation - glacial toe retreats upslope

Glacial landforms: how are they created?

• ice ages and interglacials repeat cyclically due to changes in earth’s orbit and rotation - milankovitch cycles

• alpine glaciers carve bedrock into cirques, aretes, and horns as well asu shaped and hanging valleys

• erosion polishes bedrock and wears down mountains thus producing rock flour and striations in rock surfaces

• glacial till is unsorted sediment left behind by retreating glaciers to form lateral, median and end moraines

• glaciers move due to basal meltwater and plastic deformation

Glacial till and moraines

moraines are comprised of glacial till - extremely poorly sorted sediment

erratics are boulders dropped by glacial ice - they’ve often been carried long distances are different from the local bedrock

Water and Ice: Streams

Streams evolve along their length and as they age

• graded (mature) streams are steepest at the headwater and the gradient gradually decreases toward the mouth

• ungraded (immature) streams have steep sections at various points and can have rapids and waterfalls

Meandering streams

• how do they develop and evolve over time

  meanders evolved during floods or times of higher discharge

  • faster current on outer curve causes erosion - cuts into the bank

  • slower water on inner curve deposits point bars - stream moves towards centre of channel

  • migrate downstream with time

Immature streams have higher energy due to the steep gradients and are able to move larger clasts

Water and Ice: Groundwater

How is groundwater recharge and discharge related to the location of the water table?

the water table is a subsurface boundary that defines the depth below which the ground is saturated with water

• not flat- generally deeper under hills and shallower in valleys

• may coincide with the ground surface → swamps, ponds, rivers

• in the saturated zone - beneath the water table, all pores are full of water

• in the unsaturated zone, above the water table, pores contain air

the water table is changeable

• water table position changes with the degree of recharge

• during seasonally rainy periods, the water table rises

• during prolonged droughts, the water table falls

• 

• ponds and rivers dry up if the water table falls below their bases

How does groundwater flow relate to the hydraulic gradient?

groundwater flow is controlled by the hydraulic gradient and rock permeability. groundwater storage is controlled by porosity

• groundwater flows slowly under the influence of gravity

• flow in the unsaturated zone is straight down

• in the saturated zone, flow follows curved paths governed by gravity and pressure

• from high to low hydraulic head, even if upward

  • this is the gravitational potential energy that drives fluid flow, is a measure of the pressure exerted by weight of the water column

  • h = z + P/pg

    • h is a function of the elevation of the measurement point z, fluid pressure P, water density p and Gravitational acceleration g

• hydraulic head is proportional to the height of the water column

• the difference between the water level’s surface elevation and a reference point

• hydraulic head at point a > point b - groundwater flows from high to low hydraulic head, or down the hydraulic gradient

What are aquifers, aquitards, and artesian wells?

• aquifers - sed or rock w high porosity and permeability that easily transmits water

  • unconfined aquifer - intersects the surface in contact w the atmosphere - easily contained

  • confined aquifer - beneath aquitard, isolated from the surface, less susceptible to pollution

  • perched aquifers- lens shaped aquitards may exist in the unsaturated zone, preventing downward infiltration to the regional water table → lead to perched water tables above and typically exist only after a period of recharge

• aquitard - impermeable or low permeability sed or rock that hinders water flow

• artesian wells - flow without pumping, tap confined, tilted aquifers that contain groundwater under pressure from upslope recharge

  • unconfined aquifers - well water rises to water table, confined aquifers - well water rises to a higher level, the potentiometric surface, due to higher pressure, if ground surface is below this level, a flowing or artesian well occurs

Water and Ice: Limestone Caves

How are limestone caves formed

caused by the dissolution of dense, jointed limestone by groundwater

• over millions of years, acidic groundwater or underground rivers slowly dissolve away the limestone through weathering

  mainly happens in hot, humid climates like reef environments

What are sinkholes and how are they formed

if a growing limestone cave extends close enough to the surface, the top collapses and produces depressions = sinkholes

Features of limestone caves and karst terrane

• karst topography is terrain characterized by highly eroded limestone bedrock or cliffs with caves, sinkholes, and underground rivers

• caused by the dissolution of dense, jointed limestone by groundwarer

• solution or karst caves are the worlds most common cave type

• 

Calcite: CaCO3 + 2H+ = Ca2+ CO2

Water and Ice: Costal Erosion

What events can lead to increased coastal erosion?

destructive waves - high energy or storm waves, stronger backwash than swash —> seaside arches and caves

constructive waves - low energy and quiet, stronger swash than backwash —> seaside beaches

What are the processes involved in the formation of coastal caves, arches, and stacks?

• cracks start to form in seaside cliffs due to wave action and abrasion, these cracks become caves and then arches, stacks, and stumps

  cliff erosion process

  1. hydraulic action - rock is broken by force of waves/compression of air into cracks in rock → esp joints in igneous rock

  2. abrasion - sand carried by the waves wears down the rock like sandpaper

  3. corrosion (dissolution) - the rock face is dissolved by water → esp limestone

  4. attribution - eroded fragments are broken into smaller pieces by collisions when carried by the waves

• climate change causes this to speed up

Longshore drift, development of spits and bars

• spits form where coastline changes direction or where the coast has a bay

• sand moved along the coast by longshore drift is deposited in the water at the headland or across the mouth of the bay

• a headland spit will curve inward when winds change direction’

• a spit that closes off a bay is a bar and the bay becomes a lagoon

longshore drift creates a current that carries sand along the shoreline & creates spits, sand bars and lagoons

Metamorphic rocks and processes

What are metamorphic foliations and how are they developed?

the solid state recrystallization of a parent rock that exists already or a photolithography, to a rock w a new mineralogy and/or texture (shape, size, arrangement of grains) in a process called metamorphism

• protoliths can be igneous, sedimentary, or metamorphic

What metamorphic rocks develop without a foliation (eg due to contact metamorphism?)

• foliation develops when new minerals are formed and are forced to grow perpendicular to the direction of greatest stress - esp elongate or platy minerals

• phyllite display crenelated - wrinkled - layers and a satin sheen due to aligned micas and chlorite —> they are fine grained

• similarly, the platy and elongate minerals in a schist are aligned into planes, but the minerals are med grained and can be seen with the naked eye, creating a scaly appearance

• the gneissic banding, the felsic & mafic minerals have sep into layers, minerals are coarse grained

non foliated meta rocks

• heat comes from magmatic intrusions emplaced in the crust

• eg quartzite, hornfels and marble - their minerals are equant and don’t show alignment

  • develop in contact metamorphic zones around intrusions

Types of metamorphic rocks and their protoliths

1. burial - several kms deep in sed basins

   • low P, low T, zeolite facies

2. contact - baked zone around an intrusion

   • low p, high t (path 1)

   • produces hornfels and other non foliated rocks like marble and quartzite

3. regional - convergent + divergent boundaries

   • collision - moderate p-t (path 3)

   • subduction

     • in subducting slab, high p, low t (path 5)

     • in overlying continent, moderate p-t (path 2)

   • spreading centres - low p, high t (path 1)

Changes in rocks as they are metamorphose to higher grades:

• grain size increases to decrease the energy at grain surface contacts

• minerals become unstable and recrystallize to form new minerals that are stable at the new conditions (e.g. clay → mica → amphibole (with increasing water content))

Faults, Folds, Orogenesis

Stress type: extension, compression

the degree of deformation of brittle or ductile nature of the rocks response to stress depends on rocks composition and strength, the pressure, the temp, and the rate of application of the stress

• the result of the stress = strain

• extensional stress = stretching strain

• compressional stress = shortening strain

Dip slip

dip slip is measured along dip

• normal faults

• special class = horses and grabens

• reverse faults

• special class = thrust faults

• 

faults showing displacement (slip) in the vertical direction = dip slip faults

they may be either normal faults produced by extension or reverse faults resulting from compression

• the sip in a dip slip fault is up or down the dipping plane

• normal → stress: extension, strain: stretching

  reverse → stress: compression, strain: shortening

Brittle vs ductile deformation

the 2 major types of deformation → brittle and ductile

• faults = structures that occur bc of brittle response to differential stress causing them to fracture

• folds are the ductile response to this stress

Know your dip-slip faults

• what are the differences

• are they caused by compression or extension

• in what tectonic setting are they found

normal (extension) or reverse (compression) → faults showing vertical displacement (slip) in the vertical direction are dipslip faults

marker beds determine whether dip slip faults are normal or reverse faults → marker bed has a distinctive appearance and can be recognized on both sides of the fault

normal faults - the hanging wall moves down relative to the footwall → accommodate pulling apart aka crustal extension and the below fault shows displacement and drag folding

Horst and graben fault blocks

created by pairs of normal faults w opposite dip directions

reverse vs thrust faults

• in reverse faults, the hanging wall moves up relative to the footwall

• reverse faults = fault dip is steeper than 45 degrees

• thrust faults = special class of reverse with a dip less than 45 degrees

• reverse faults accommodate crustal shortening (compression)

thrust faults = low angle faults that can be kms to 100s of kms long and may dip at just a few degrees, making it hard to identify

during continental collison, mountain building occurs as a result of the propagation of sets of thrust faulrs

• the thrusting often displaces soft sed rocks over a basement of hard igneous and metamorphic rocks

• the thrust slices are detached by faulting away from the basement

• 

What are the parts of a fold?

folds occur in many shapes, sizes, and geometries

commonly formed in orogenic settings (Orogeny is a mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin. An orogenic belt or orogen develops as the compressed plate crumples and is uplifted to form one or more mountain ranges.) and may record many events of deformation

• anticlines - fold limbs dip outwards

• synclines - fold limbs dip inwards

• anticlines have an ant hill shape

• synclines have a sink shape

• 

  anatomy of a fold

• hinge line - a line along which the curvature of the fold is greatest

• limbs - the less curved sides of a fold

• axial plane - connects hinges of successive layers

• 

How are folds classified?

fold classification

• based on shape and orientation

1. plunge angle of hinge - plunging or non plunging

2. dip of axial plane - upright (symmetric), inclined (asymmetric)

3. inter limb angle - gentle, open, close

   

plunging and non plunging folds

• the hinge lines of anticlines and synclines may be horizontal or plunge (analogous to the dip of a plane)

  • horizontal folds (eroded) will be seen on Maos as the rep of beds

  • in plunging folds the fold intersects the horizontal as a U

  • 

How are mountains formed and distributed around the planet

mountains frequently occur in elongate linear belts

• these belts correspond to past or present subduction zones and continent collision zones where tectonic forces cause the process of orogenesis (mountain building)

• continental collision

  • follows ocean basin closure

  • complete subduction of oceanic lithosphere

  • brings 2 blocks of continental lithosphere together

  • 

• in mountain building related to continent - continent collision, sets of thrust faults are present on either side of the mountain range

• mountains form via continent collision, subduction and related volcanic arcs, and rifting

How does the continental crust under mountains differ from that in the plains

continental collision follows ocean basin closure

• complete subduction of oceanic lithosphere

• brings 2 blocks of continental lithosphere together

• buoyant continental curst shuts down subduction

How do mountains formed from compression differ from those in extensional settings

leads to shortening and recumbent folds

• convergent margin horizontal compression causes horizontal shortening & vertical thickening

• these processes can double crustal thickness

• thick crustal root develops beneath mountain ranges

What rock types are found in the mountains

Why do mountains have a maximum height

mountains have a finite lifespan

• young mountains are high steep and still growing up

• older mountains are lowered by erosion

• 

• topographic elevation is caused by a balance between the effects of gravity and buoyancy on the vertical position of the lithosphere - isostasy

mountains are so high bc of igneous rocks thickening the crust - removal of lithospheric mantle due to density inc can cause uplift —> mountains reflect a balance btw uplift and erosion - they are steep and jagged due to erosion which is controlled by rock characteristics - resistant layers cause cliffs and easily eroded rocks form slopes

Geologic Time

Geological principles for relative age dating

• uniformitatianism

  • This principle holds that the geological processes operating today have been operating for most of Earth history

  • This means that our observations of current geologic processes can be applied to the same types of systems preserved in the rock record, e.g., sedimentation, rivers and glaciers, volcanism

• superposition

  • The Principle of Superposition applies to sedimentary and volcanic rocks and is based on the idea that younger rocks lie on top of older rocks

• original horizontality

  • The Principle of Original Horizontality holds that although sedimentary and volcanic rocks may not be horizonal at present because of deformation, they were deposited horizontally

• original continuity

  • The Principle of Original Continuity states that although erosion may locally remove sedimentary strata, they were once continuous and therefore can be correlated from one locality to the next

• inclusions

  • If a rock contains inclusions of another rock type, the inclusions must have formed earlier

• cross cutting

  • If an intrusive rock like a dike crosscuts the layering in the country rock, the layering must have been there first

• faunal succession

  • is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, reliable order that can be identified over wide horizontal distances (Wikipedia).

Index fossils

• may be used to correlate stratigraphies

• must be distinctive, abundant and widespread, limited to a particular time and contain hard parts that preserve well

How are fossils formed, what are they made of?

species dies, the flesh rots and leaves the bones behind, the bones are covered with sediment which cuts them off from the air and prevents further decay, groundwater fills in spaces in the organism and precipitates minerals like chert / calcite to preserve fine structure, tectonic uplift and erosion expose the fossils at the surface

Are fossils more common in terrestrial or marine rocks

marine rock since they are formed in rivers, etc and also there are more aquatic creatures that have been preserved through fossil

Types of fossils: body, cast and mould, trace

mold fossils - a fossilized impression made in the sed, a negative image of the organism

cast fossils - formed when a mold is filled in

trace fossils - fossilized burrows, footprints, etc

Do all fossils represent extinct species

Although fossils represent dead remains of the organism, it does not always mean that the organism in question has become extinct. Fossils of organisms that can even be found today means that the organisms have been on the earth since a very long time.

Bones are buried to stop decay and are replaced by minerals precipitated from groundwater

What are some events that might lead to mass extinction

mass extinctions occur periodically in response to major changes in global ecosystems

• may be caused by:

  • start / end of glaciation

  • flood basalts erupting

  • global warming from co2

  • ocean acidification

  • ocean anoxia

  • meteorite impact

  • shock waves and impact winters

What effect might a massive volcanic eruption have on the planet’s climate

would heat up the planet’s climate - could lead to mass extinction if hot enough

What about a meteorite impact

meteorite hitting earth would kill us all

Do glacial cycles cause extinctions

yes because we would either freeze or burn to death depending on how much water we have and where we are in the glacial cycle

Fossil Fuels

What circumstances lead to the formation of coal

coal has been used for fuel since pre historic times

it is composed of mostly carbon & hydrogen w some O, N, S, H2O and impurities

coal layers = seams

origin of coal

• preservation of terrestrial plant material in a low oxygen, peat accumulating env with stagnant water = peat swamps

• coal seams are formed in situ (in place)

• the organic matter is attacked by anaerobic bacteria, causing partial decomposition

• over time, bacterial decay splits off impurities and concentrates the carbon

petrification

• 

  coalification

• burial and compression cause peat to change in comp and become coal

• follows bacterial decomposition in peat swamps

grades

• burial, compaction and heating

• squeezes out water and gases

  • methane

  • hydrogen sulphide

  • carbon dioxide

• increases c content

• inc the coal grade and energy content

How does coal formation relate to sedimentary diagenetic processes

Coal formation is intricately tied to sedimentary diagenetic processes, as coal itself is a sedimentary rock formed from the accumulation and alteration of organic matter over millions of years. Here's how coal formation relates to sedimentary diagenetic processes:

1. Organic Matter Accumulation: Coal forms from the accumulation of plant debris in ancient swamps and marshes. Over time, this organic material accumulates in layers, forming what is known as peat.

2. Burial and Compaction: As more layers of organic material accumulate, the weight of overlying sediments increases. This leads to compaction of the organic matter below, reducing pore spaces and increasing pressure.

3. Diagenesis: Diagenesis refers to the physical and chemical changes that occur in sedimentary rocks due to increased pressure and temperature over time. In the case of coal formation, diagenetic processes play a crucial role in transforming peat into coal. The heat and pressure from overlying sediments cause chemical changes in the organic matter, driving off volatile components like water, methane, and carbon dioxide. This process, known as coalification, gradually transforms the peat into lignite, then sub-bituminous coal, bituminous coal, and finally anthracite coal, with each stage characterized by increasing carbon content and energy density.

4. Hydrothermal Alteration: In some cases, hydrothermal fluids can infiltrate coal-bearing formations, leading to additional chemical alterations. These fluids can introduce new minerals or modify existing ones, affecting the quality and composition of the coal.

5. Structural Deformation: Tectonic forces can deform sedimentary rocks, including coal seams. Structural deformation may lead to folding, faulting, or fracturing of coal beds, influencing their thickness, continuity, and accessibility.

Overall, coal formation is a complex process influenced by various sedimentary diagenetic processes acting over geological timescales. Understanding these processes is essential for both the exploration and exploitation of coal resources (ChatGPT).

What are the 3 essential rock components of a petroleum system and what are their properties? What types of rock might work for each

1 - source rock

• must be organic rich to generate the oil/gas

• must contain sufficient organic matter to produce oil / gas (1-10 percent)

• must exist at the appropriate depth and temp conditions to produce oil and gas

2 - reservoir rock

• a - porosity, to hold the hydrocarbons

• b - permeability, to allow fluid flow

  • sufficient porosity to hold economic quantities of oil and gas

  • sufficient permeability to enable extraction of commercial amounts of oil / gas

• types of reservoirs = 1. siliciclastic, 2. carbonate, 3. fractured

3 - cap rock (seal)

• reservoir must be sealed by an impermeable rock to prevent escape of the petroleum

• must have v low permeability to prevent the escape of oil / gas

• must be located above the reservoir rock in a physical config that will trap the oil / gas

• types of cap rock = 1. shale, clay (fine sed), 2. salt, anhydrite (evaporites), 3. unfractured carbonate or granite

if any of these are absent the petroleum system will fail

What else is necessary for a petroleum system to be successful

other essential ingredients in petroleum system:

1. production & preservation

   • sufficient heating in source rock to form oil and gas

   • no loss, biodegradation or over maturation

2. migration of hydrocarbons out of the source rock into more permeable reservoir rock

3. trapping by impermeable cap rock above the source

4. timing of trap formation - must occur before hydrocarbon migration

   

How are oil and gas trapped in the subsurface

petroleum traps = any barrier that impeded the upward movement of oil / gas and allows either / both to accumulate

• incl a low permeability cap rock to prevent hydrocarbon flow

• no trap is completely impermeable over geological time → can leak or be breached

• traps can e structural (anticline, fault) or lithological (salt dome, stratigraphic trap)

anticlinal trap

• anticline = folded rock seq w an upward facing dome

• permeable and impermeable rock layers

• permeable layer contains water, oil, gas

• oil and gas rose above water into dome of anticline

• further migration of oil and gas is prevented by impermeable cap rock

fault trap

• seq of permeable and impermeable rock layers offset by faulting

• oil and gas want to move upward along perm reservoir rock layer BUT further migration of oil / gas is prevented by cap rock which cuts off the left side of the reservoir rock due to fault movement

salt dome trap

• halite (salt), NaCl is an impermeable mineral and is soft enough to flow underground and make sat domes

• intrusions of impermeable salt block off underground avenues of escape for oil / gas