Physical Geology

Hypothesis- an explanation with minimal evidence

Theory- an explanation supported by significant evidence

Law- statement that summarizes relationships with

Solar System Formation

Nebular Hypothesis
Dust and gases(nebula) started to gravitationally collapse(~5 Ga)
Forms a rotating disk, heated by conversion of gravitational to thermal energy; Formation of protosun

Repeated collisions caused dust particles to coalesce into asteroid-sized bodies, eventually forming planets

The outer planets, Jupiter, Saturn, Neptune, Uranus, were the first to form

Earth us 4.54 billion(Ga) years old

How do we know? Radiometric dating

In the first ~150 million years Earth was:

Heavily bombarded by comets and meteorites
Covered by magma ocean, no surface water
Toxic atmosphere no oxygen or ozone layer, intense UV radiation

Collisions and radioactivity kept the early Earth very hot
Hot enough to melt heavy elements like iron and nickel
These heavy elements then sank down toward the center of the planet
Lighter elements(oxygen, aluminum, silicon) floated toward the surface
This process is known as chemical differentiation

Chemical differentiation produced Earth's

Chemical Layers

Crust-outermost layer-mainly silicon and oxygen

Oceanic- 3-15 km thick, density ~3.0g/cm3 it is black with (less si and o slightly more fe and mh)

Continental- crust 20-70 km thick, density 2.7g/cm3 (more si and o, slightly less fe and mg)

Mantle 82% of eaths volume, fe, o, si, mg

Density 3.3-11 g/cm3

Core- iron(mostly) and nickel

Density ~11g/cm3

Physical layers

Layers can be defined by chemical composition or physical properties
Lithosphere- crust and uppermost, solid mantle
Asthenosphere- soft portion of the upper-mantle(due to high temperatures) playdough
Lower mantle- rigid due to increased pressure

Outer core- liquid due to huge increase in temperature
Inner core- solid due to increase in pressure

Earths surface is ocean and continents

Average ocean depth 3.8 km(2.4 mi)

Average continent elevation: 0.8 km (0.5 mi)

Elevation difference related to density and thickness of the crust

Continental margin: portion of the seafloor next to landmasses
Continental shelf: continental crust below sea level
Continental slope: steep drop off to the deep ocean (3 degrees slope)
Continental Rise: wedge of sediment, like sand

Deep oceans have:

Abyssal plains: very flat deep seafloor
Trenches: exceptionally deep, narrow parts of the seafloor
Seamounts: small, underwater mountains formed by volcanic activity

Mid-ocean ridges: linear chain of uplifted seafloorMountain Belt: Area of uplifted continental crust younger mountains tend to be along coast

Cratons: older, stable interior of continents

Can be covered by younger rocks; flat

Shields: part of craton exposed at the surface

Rocks- Three types

Igneous- rocks created from cooking magma/lava

Sedimentary- rocks created from pieces of pre-existing rock

Metamorphic- pre-existing rocks altered by heat and pressure

Rocks are made of minerals

Quartz, feldspar, calcite, etc.

The size, shape, and arrangement of minerals, is the texture

Mineral composition and texture are determined by the geologic processes that created the rock

The rock cycle shows the relationship between rock types and processes that create them

A map is to a geologist as a hammer is to a carpenter

Geologists primarily use:

Topographic maps
Digital elevation models(DEMs)
Geologic maps

Topographic maps show elevation with contour lines

Lines represent areas of equal elevation

DEMs are the basis for topographic maps

On geologic maps:

Colors represent different rocks
Letters represent age and name of the rock

Ex: Enchanted rock is pCtm

pC= Precambrian time period
Tm= town mountain granite

Geophysics is the study of the physical properties of the Earth

Earth's magnetic field

Gravity

Heat flow

Earths magnetic field is created by the movement of iron in the outer core

Protects us from harmful solar radiation

Gravity is stronger(red) in some areas of earth's surface and weaker(blue) in others

Due to elevation and non-uniform composition

Heat Flows through convection and conduction to earths surface

Seismic waves are vibrations generated by earthquakes or other energetic sources

Waves reflect and refract when they encounter different rocks

Tomography is a way of using seismic waves to see what the mantle looks like

Blue is cooler than expected, red is warmer

Ground-penetrating radar(GPR) can be used to find unmarked graves

A lot of earthquakes occur in regions with higher elevations

Continental Drift

Is the idea that the continents are moving across the surface of the eath

First proposed by Alfred Wegener in 1915

Continents drifted to their present positions

200 mya all continents were together in one supercontinent called Pangea

Ever look at a map and wonder if the continents fit together like puzzle pieces

Wegner's 4 lines of evidence for continental drift

Geometric fit of the continents
Matching fossils
Matching geology
Paleoclimatic evidence

The Problem: Wegener could not explain why or how the continents were moving

He died before having his theory proven

Marie Tharp and Bruce Heezen were first to expand on Wegener's ideas

Discovered mid ocean ridges based on earthquake locations in the 1950s

Mid-ocean ridges are linear mountains in the middle of oceans

Harry Hess, 1962:

Seafloor spreading

New oceanic crust is formed at mid-ocean ridges

Trenches are located all around the pacific ocean, among other places in the world

Seafloor (oceanic crust) forms at mid-ocean ridges
Seafloor recycled at trenched
Process driven by convection in the mantle

The lithosphere floats on top of the asthenosphere

Layers move independently, not "attached"

Tectonic plates

Plates are in constant motion

Most geologic activity occurs along the boundaries

Plate Boundaries

Divergent plates move away from each other

New oceanic crust created

Convergent plates move toward each other

Old oceanic crust recycled into the mantle

Transform- plates slide past each other

All divergent plate boundaries start in the middle of continents

Tensional stress caused by upwelling asthenosphere from below
Crust stretches and thins, forming a rift valley
1. Volcanoes form as magma rises through cracks(

Example: East African Rift Valley)

Spreading continues, oceanic crust is formed, narrow sea is formed
Narrow sea turns into a wide ocean with well-developed mid-ocean ridge

Ocean-Continent and Ocean-Ocean convergent boundaries involve subduction

One plate descends into the

Oceanic-Continent Convergent:(Ex: Pacific Northwest U.S.(Cascade Range))

Oceanic crust is denser so it always subducts

Melting occurs in the asthenosphere about 100 km

Creates a continental volcanic arc

Ocean-Ocean Convergent

Subduction creates a volcanic island arc(example: (western Aleutian islands, Alaska)

The older one subducts, denser

Subduction leads to continent-continent convergent boundary

Continental lithosphere doesn't subduct

Transform boundary

Plates slide past each other, horizontal movement

Forces drive plate motion

Mantle Convection- the movement of hot rock in the mantle
Slab Pull= as oceanic crust sinks it has a gravitational pull on the rest of the connected slab
Ridge push- gravitational push on oceanic slabs because the ridge is at a higher elevation

Plates move a few centimeters per year

Mineral Definition

5 key points to the definition

Natural occurring- Not artificial
Generally Inorganic- Not living, but can be made through organic processes(ex: a shell could be a mineral)
Solid
Definite chemical composition
Orderly atomic structure(crystal

Rocks are made of minerals(Ex: Granite-A rock is made up of a blend of minerals like quartz hornblende, and feldspar)

Minerals are composed of elements

A substance that can't be decomposed into a simpler substance
Basic building blocks of minerals
118 elements are known(94 naturally occurring)

Elements are composed of atoms

Smallest unit of matter that can't be divided

Only focusing on first 2 columns and last 6 columns

Electrons(- charge) surround the nucleus
Located in energy levels called shells
Outermost shell: valence electrons
Model atom has no net charge
# of protons=# of electrons

Number of electrons corresponds to columns in the periodic table

Ex: first column has 1 electron, 6th column has 6 electrons

Chemical bonding

A strong attractive force that links atoms together in a compound

A compound is two or more elements

Octet rule- atoms gain, lose, or share electrons in order to obtain the stable electron configuration of the noble gasses(8 valence electrons)

Ions are charged atoms(+ or -) due to loss or gain of an electron

Cations:(+) charged atoms due to the loss of electron

Anions:(-) charged atoms due to gain of electron

Ionic bonding: atoms gain or lose outermost(valence) electrons to form ions

Covalent bonding: atoms share electrons

Generally stronger than ionic bonds

notes: both ionic and covalent bonds can occur

The elements in columns I and II typically bond ionically with the elements in columns VI and VII

Elements in columns III and IV

Na and Cl form an ionic bond

Na donates an electron to cl

Polymorphs- minerals with the same composition but different crystalline structures

Ex: diamond and graphite are both made of carbon

Crystallization from cooling magma/lava

Types of minerals that form are dependent on temperature and chemistry of the magma/lava

Precipitation out of a solution

Water that is saturated in a substance will precipitate these ions in solid form(e.g. salt)

Simple tests or observations to identify common minerals

Common physical properties include:

Color
- Generally not diagnostic for mineral identification
- Often high variable due to slight changes in mineral chemistry
- Exotic colorations of certain minerals produce gemstones
Luster
- Appearance of a mineral reflected in light
- Metallic
- Non-metallic
- Other descriptive terms exist
Streak
- Color of a mineral in powdered form
- Only good for identifying metallic minerals
Hardness
- Resistance of a mineral to abrasion or scratching
- Mohs scale of hardness

Breakage

Cleavage

Tendency to break along planes of weak bonding

Produces flat, shiny surfaces

Described by resulting geometric shape, number of planes, angles between adjacent planes

Fracture

Uneven or curved breakage across strong bonds

Top and bottom are same planes

Sides are on the same plane

Back

Fracture- Minerals do not break along bonds of weakness

Very irregular, often curvy(conchoidal)

Other properties

Density

Crystal habit

Magnetism

Reaction to acid

Malleability

Double refraction

The most abundant elements in the earth's crust are oxygen and silicon, will aluminum and iron following

Silicate minerals are the most abundant

Feldspar being the most abundant

Large % of silicon and oxygen in the crust

92% of minerals in Earth's crust are silicates

Oxides

Economic uses: iron ore, gems, pigments, abrasives

Native element minerals: made of a single element

The basic building block of the silicate group is the silicon-oxygen tetrahedron

A complex ion

Tetrahedra link together to create different silicate structures

5 basic structures

Independent: no shared oxygens

Single chain: two shared oxygens

Double chain: 2 or 3 shared oxygens

Sheet 3 shared oxygens

Framework: all oxygens shared

Independent silicates don’t have cleavage

Bonds are the weak in all directions

Single chain silicares have 2 cleavage planes at ~90 degrees

Cations are fe,mg, and ca

Examples: pyroxenes

Double chain silicates have 2 cleavage planes at 60 degrees x ~120 degrees

Cat

Minerals that are critical to the united states

Ex: gadolinium, praseodymium, cerium, samarium, lanthanum, neodymium

Most critical minerals are located in the mountains

Most are not actively mined

Environmental effects

Expensive

U.S. is heavily reliant on importing critical minerals

China is the largest supplier of critical minerals

Magma/lava is completely or partially melted material

Melt- Liquid portion

Solids- Minerals present in magma

Volatiles- Dissolved gasses in magma

Substances that easily evaporate

Intrusive rocks cool inside the earth

Extrusive rocks cool on earth's surface

Crystallization: Formation of mineral grains as magma/lava cools

As magma cools, ions slow down and arrange themselves into orderly crystalline structures

Minerals

Grains are interlocking

Texture refers to how a rock looks

In igneous rocks, texture refers to the mineral grain size

Igneous textures are controlled by:

Rate at which magma cools

The amount of silica present

The amount of dissolved gases present

Cooling rates can be slow or fast

These two rocks have the same minerals but the size of the mineral grins are very different(different texture)

Slow cooling (thousands to millions of years)

Few areas of mineral generation

Large minerals that can be seen

The slower the cooling, the larger the minerals

Intrusive(plutonic) igneous rock

Fast cooling(days to years)

Many areas of mineral generation

Many small minerals that can only be seen with magnification

The faster the cooling the smaller the minerals

Extrusive (volcanic) igneous rocks

Chemical composition of igneous rocks are controlled by the chemistry of the magma

Broadly defined by relative abundances of light (felsic) and dark (mafic) minerals

Compositions:

Ultramafic

Mafic

Intermediate

Felsic

Magma is created when rocks melt

Different rocks melt at different temperatures

Partial melting- minerals with lower melting points will be the first to start melting

Silica-rich minerals like quartz and feldspars, begin melting at lower temperatures than Fe- and Mg-

Partial Melting- Minerals with the lowest melting points(more felsic) will be first to melt as you apply heat to a rock

Partial melting of:

Ultramafic(mantle_ Mafic(oceanic crust)

Temperature increases with depth, called the geothermal gradient

How do you get a rock to melt

Increase temperature

Decrease pressure

Increase volatiles

Compounds/elements with low boiling points(water)

Can the temperature increase?

Nope- geothermal gradient is set

Lower pressure?

Yes- decreasing pressure will lower the melting point of hot rocks

Called decompression melting

Add volatiles?

Yes- Water added to

Gray color is indicative of intermediate

Can the temperature increase?

Nope- geothermal gradient is set

Lower Pressure?

Yes - decreasing pressure will lower the

melting point of hot rocks

Called decompression melting

Add volatiles?

Yes - Water added to rocks in the earth will lower the rocks’ melting point

Called flux melting

Volatiles are substances with low boiling points, like water and carbon dioxide

Magma Formation

Decompression melting- lowering the pressure on rock while maintaining high temperature

Happens at divergent plate boundaries

Thinner lithosphere at a mid-ocean ridge reduces pressure on the hot asthenosphere below

Reduced pressure lowers the melting point of the asthenosphere(ultramafic), so it partially melts

That magma rises to the surface, cools, and solidifies forming new ocean crust(mafic)

Flux melting- adding water to the rock lowers the melting point

Happens in subduction zones

Water from a subducting slab is released into the asthenosphere above it

Lowers the melting point of the asthenosphere and causes partial melting(mafic magma)

Mafic magma rises and partially melts the lithosphere, changing to an intermediate or felsic magma (continental crust composition)

If the magma reaches the surface, it creates a volcano

What happens as magma begins to cool and solidify?

Minerals crystallize in a predictable way based on the chemistry and temperature of the magma

Bowen’s Reaction Series

Minerals that form at the same temperatures are found in rocks together

When magma is forced to cut through or in between rocks, it’s called an intrusion

Dikes are vertical sheet intrusions that cut through layers of rock

Sills are horizontal sheet intrusions in-between layers

Mafic not explosive magma/eruptions

Felsic explosive magma/eruptions