EAPS 5 - Midterm 1

Differentiation of the Earth

The process when the planet separated into its distinct layers during the formation of the Earth 4.6 Ga

• Based on physical and chemical properties (like density and heat!)

• The heat in the middle of the planets caused elements to differentiate and separate → lighter

Accretion

When the gravitation pull of the early sun swept nebular dust → elements released into space (Fe, Ni) came together, or accrete

Planet building

Layers of the Earth

Inner Core

Outer Core

Mantle

Crust

Inner core

Solid most inner layer of the Earth

Made up of solid iron and nickel material (Fe, Ni)

Outer core

Made of liquid Fe and Ni that convects → creating Earth’s magnetic field

• Only liquid layer in Earth’s interior

Mantle

Thickest layer of the Earth (84% of the volume of our planet)

Made of dense rock material

Oceanic crust

Dense crust layer that subducts under less dense continental crust

Made up of basalt

Formed at mid-ocean ridges through seafloor spreading (where magma rises, cools, and forms new crust)

Continental crust

Less dense outer crust layer; thicker than oceanic crust

Made up of granite

Plate tectonic causes the crust to constantly move

Lithosphere

Includes the crust and mantle

Mechanical properties: Strong, made of cool rigid rock; supports all life on Earth

• Gets strength from it’s cool nature, making it rigid

• Cooling by conduction only

Asthenosphere

Underneath the lithosphere; divided by temperature property

Mechanical properties: Weak because of its warm nature, meaning it can flow

• Cooling by conduction and convection

*Tectonic plates from the lithosphere float on top of here

Conduction

The transfer of heat through direct physical contact

Convection

Transfer of heat through the movement of fluids (liquid or gas)

Compositional subdivisions of the Earth

Crust, mantle, core

Rheological subdivisions of the Earth

Asthenosphere, lithosphere

Rheo = greek word for flow or current

Oldest rock on Earth + How we know this

Acasta gneisses

• Known because of radiometric dating (U-Pb) of zircon crystals

Zircon

Good for radiometric dating: locks in uranium when it forms and does not reset when subjected to heat/pressure

Radiometric dating

Process that calculates the absolute age of rocks by measuring the decay of unstable radioactive isotopes (parent atoms) into stable isotopes (daughter atoms)

• Method that helped us figure out the age of the Earth (4.567 Ga)

Half life

Time it takes for half of a population of radioactive parent atoms to decay into daughter atoms

In one half life, ½ of atoms decay

• Ex: 256 → 128 → 64 → 32 → 16 → 8 → 4 → 2

Why is C-14 useless for dating rocks?

1. It’s half-life is too short for rocks (5,730 years too short for dating deep time)

2. Only useful for once living things: 14C is formed in the atmosphere and found in living things → when they die they stop taking in C and the 14C/12C decreases

Plate boundaries

When two lithospheric plates end up next to each other

Precision

How tightly a number can be specified

The uncertainty of a number is equal to half of its last digit

• Ex: 1.01 → (± 0.005) ; 2.2 → (± 0.05)

Accuracy

The degree to which a number is actually correct

Significant figures

The number of sig figs should have the same sig figs as the least significant number in the calculation

• Number of sig figs implies precision!

Mid-ocean ridge

Magma upwells from mantle → cools into basalt → creates new oceanic crust/ lithosphere

• Process known as seafloor spreading

Divergent boundaries

When tectonic plates move apart

• Occurs mostly in the ocean to build new oceanic crust through mid ocean ridges

• Creates mid-ocean ridges and volcanoes

Convergent boundaries

Creates volcanoes and mountain ranges

Oceanic-continental

Type of convergent boundary

Denser oceanic plate subducts beneath less dense continental plate

• Forms earthquakes and volcanic mountain ranges

• South America

Oceanic-oceanic

Type of convergent boundary

Two oceanic plates collide → older plate subducts beneath younger one

• Forms island arcs (chains of volcanic islands)

• Japan/Phillippines

Continental-continental

Type of convergent boundary

Both continental plates are too buoyant to sink → crumple upwards

• Creates mountain ranges (NO VOLANOES)

• Ex: the Himalayas, Alps

Transform boundary

When two plates slide past/against each other

Neither create/destroy crust, just cause friction

• Creates fault lines that trigger earthquakes

Oceanic crust

Thin and dense

• Basalt

• Younger than continental because of mid-ocean ridges

• Subducts beneath continental crust

Youngest oceanic crust found:

1. Near continental margins (close to continents)

2. Near mid-ocean ridges

Continental crust

Less dense but larger in size

Sits on top of oceanic crust because of density difference

• Granite

• Old!

Continental drift

Theory created by Alfred Wegener in 1912 to explain why continents appear to fit together

• Wegener proposed that all continents were once a supercontinent, Pangaea

Early evidence of continental drift

1. Shape of continents: look like they fit together

2. Age of rocks: Geological structures like the Appalachian mountains aligned perfected with age&rocks in Europe

3. Fossil correlation: Identical creatures were discovered widely spread, despite the fact that they don’t swim

4. Continental glaciation: Location of glaciation is the same today

5. Symmetry of magnetic reversal on ocean floor

Volcanic Arcs

AKA Island arcs; Chains of volcanoes formed during oceanic-oceanic convergent boundary

Formed at subduction zones

• Subduction of plate → heat/pressure make plate release water → partial melting creates magma → magma rises

Ex: Ring of Fire: Intense volcanic activity around the Pacific Ocean

Earthquakes

Release of energy in the Earth’s crust → seismic waves

• Caused by shifting plate tectonics when there’s underground friction OR human activity

Fault

Fracture in the Earth’s crust where the rock on either side has moved relative to the other

• Caused by tectonic movement (horizontal or vertical movement)

• Faults release energy → earthquakes

Epicenter

The point directly above the hypocenter (fracture) of an earthquake

How we map earthquakes

*Distance from epicenter to hypocenter determines the earthquakes depth

Focus

AKA Hypocenter

Exact underground location where the rock fractures during an earthquake

• Releases stored energy → seismic rupture

Seismic waves

Vibrations generated by earthquakes

Terms to describe a plane in 3D space

Strike

Dip

Dip direction

*How geologists define 3D orientation of tilted rock layers, faults, and fractures

Strike

Direction of horizontal line on a planar surface, such as a rock layer or fault

Dip

Angle of inclination of the layer, relative to horizontal

Dip direction

The down-dip direction (perpendicular to strike)

Hanging-wall

Block that sits above fault line (where you could hang a lantern)

Footwall

Block that sits below the fault (where you stand)

Rested on

Types of faults

1. Normal Fault

2. Reverse fault

3. Strike-slip fault

   1. Strike-slip fault, left lateral

   2. Strike-slip fault, right lateral

Dip-slip

Fractures in the Earth’s crust where the rock blocks are vertical, moving up or down

Ex: Normal faults and reverse faults

Reverse fault

Hanging-wall moves up-dip, footwall moves down-dip

Kind of dip-slip

Normal fault

Hanging-wall moves down-dip, footwall moves up-dip

Strike-slip

Fractures in the Earth’s crust where the rock blocks move horizontally past each other, parallel to the fault line

Ex: Right-lateral, left-lateral

Strike-slip fault, right lateral

Far side moves parallel to strike and rightwards with respect to near side

Strike-slip fault, left lateral

Far side moves parallel to strike and leftward with respect to near side

What kind of faulting do you expect from each plate boundary?

1. Divergent = Normal faulting

2. Convergent = Reverse faulting

3. Transform = Strike-slip faulting

Elastic rebound theory

Explains how tectonic forces create earthquakes

Stress & friction → rock along fault to bend & warp → stores elastic energy

When stress > frictional resistance → rock snaps → releasing stored energy as seismic waves

Two types of crustal deformations (earthquakes)

1. Stress: forces imposed on rock; the cause

2. Strain: change in rock’s shape in response to stress; physical deformation that happens to the rock as a result of stress (the effect)

Two kinds of earthquake strain

1. Elastic deformation = Happens under low stress; rock reverts to former shape when stress is removed

2. Plastic deformation = Rocks permanently change shape/flow; high stress conditions

Causes earthquakes

Foreshock

Smaller earthquakes that precede a larger earthquake

Smaller slips as fault begins to fail

Aftershock

Smaller earthquakes that follow it

Slips as fault continues to adjust after main shock

Offset

AKA displacement

Physical distance that the ground on either side of the fault shifted relative to one another

*How we measure the size of an earthquake

Surface rupture length

Total distance of the ground break created by an earthquake

*How we measure the size of an earthquake

Liquefaction

Caused by an earthquake

When loosely packed soils temporarily lose their strength/stiffness → soil loses ability to support weight → behaves like liquid

• Causes sandblows

Ex: 1976 Earthquake in Guatemala was triggered by intense liquefaction → destroyed homes

What can earthquakes trigger?

1. Sand blows

2. Expensive property damage

3. Landslides

4. Fire

Sandblows

When pressurized water and sand erupt into the surface

Liquefaction occurs → during earthquake, intense pressure squeezes water and sand through weak spots in the crust → erupts on surface

Caused by soil liquefaction

Terminology for waves

1. Wavelength: Distance between successive crests/troughs

2. Amplitude = ½ the difference in height between crests ands troughs

3. Velocity = Speed at which a crest/trough travels

4. Frequency = # of crest/troughs per unit of time

5. Period = Time interval between successive crests/troughs