Background Information

Basic Concepts in Geology for the Non-Geologist

All information compiled by Michelle Vanegas.

Sources: United States Geological Survey, and

Grotzinger, John, Frank Press, Thomas Jordan, and Raymond Siever. Understanding

Earth. 5. New York: W H Freeman & Co, 2007. Print.

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Table of Contents

Composition of the Earth …………………………………………………………………… 2

Heat Convection ………….…………………………………………………………………… 4

Tectonic Plates ………………...………………………………………………………………… 5

Plate Boundaries ………….………………………………………………………………….. 6

Faults ……………...……………...………………………………………………………………… 11

Earthquakes …………..…….………………………………………………………………….. 12

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Composition of the Earth

The composition of the earth can be considered in two ways: chemically and

mechanically. To look at the earth’s chemical composition is to focus on what each layer

of the earth is made of. To look at the earth in the mechanical sense is to focus on how

each layer behaves based on its composition and depth. Inside the earth, pressure and

temperature increase as depth increases.

CHEMICAL

1. Crust – The crust is the outermost major layer of the earth. It is a rigid, solid

layer, ranging from about 10 to 65 km in thickness worldwide. The crust can be

divided into two subsets: Continental and Oceanic.

Crust

Mantle

Core

Lithosphere

Asthenosphere

Mesosphere

Outer Core

Inner Core

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a. Continental crust is primarily composed of felsic rock, made of light

minerals (silica, potassium, sodium, aluminum). The average density of

continental crust is 2.7 grams/cubic centimeter.

b. Oceanic crust is made of mafic rock, composed of denser minerals

(magnesium, iron). The average density of oceanic crust is 3.0

grams/cubic centimeter.

2. Mantle – The mantle is the middle layer of the earth’s interior and is roughly

2,900km thick. It is composed primarily of iron, magnesium, silica, and oxygen.

3. Core – The core is the innermost layer of the earth, and is roughly 3,500km in

thickness. It is composed largely of iron and nickel.

Mechanical

1. Lithosphere – The lithosphere encompasses the crust, as well as the uppermost

layer of the mantle, and it is roughly 10-200km in thickness. The uppermost

portion of the mantle that is included as part of the lithosphere is also a brittle

solid.

2. Asthenosphere –The asthenosphere is made of very viscous, ductile, semi-solid

material on which the lithosphere moves. It is a solid that can behave like a

liquid, and it is about 440km thick.

3. Mesosphere –The mesosphere is another rigid layer in the earth and it is roughly

2,200km in thickness.

4. Outer Core – The outermost layer of the core is liquid, and it is roughly 2,200km

thick.

5. Inner Core – The inner core is made of solid iron and nickel, roughly 1,300km

thick.

The transition from solid to semi-solid state or liquid state in the layers (e.g. lithosphere to

asthenosphere) is attributed to a high increase in temperature. The transition from semi-

solid/liquid back into a solid state (e.g. asthenosphere to mesosphere) is attributed to a

high increase in pressure.

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Convection Cells

In the interior of the earth, heat creates convection cells. Heat created by radioactive

decay escapes from the earth’s core. As it rises through the mantle, hot material

(magma) from the asthenosphere rises up under the lithosphere and can break the

surface. What doesn’t break the surface cools and spreads out in either direction under

the lithosphere. As the material continues to spread and cool, it sinks, taking with it part

of the lithosphere, which is reheated into magma in the asthenosphere.

H

H

H H

C

C

C

C

C

C

Convection Cell

Subducting Plate

Divergence

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Tectonic Plates

The lithosphere of the earth is broken into rigid slabs called tectonic plates. The plates are

composed of continental as well as oceanic crust, and vary in sizes from hundreds to

thousands of kilometers across. Because these lithospheric plates are “floating” on the

asthenosphere, they are constantly moving relative to one another – this movement being a

result of the heat convection in the interior of the earth. Convection cells are also

responsible for forming different types of boundaries between the tectonic plates.

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Plate Boundaries

There are three main types of plate boundaries that can exist between tectonic plates:

divergent, convergent, and transform. Depending on the type of crust that is involved, the

plates existing within these boundaries will behave differently.

Divergent – A divergent plate boundary forms in areas where the lithosphere is

spreading as a result of heat convection in the interior of the earth. These types of

boundaries can occur underneath both oceanic and continental crust. As the

lithosphere begins to separate, magma from the asthenosphere rises up to the

surface to fill in the empty space and create new lithosphere and oceanic crust.

Divergent Plate Boundary

Situated between South America

and Africa, the Mid-Atlantic Ridge is

an example of a divergent plate

boundary.

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Convergent – Convergent plate boundaries form when two tectonic plates come

together and collide with each other. These boundaries can have different results

depending on whether they form in continental crust or oceanic crust.

Oceanic + Oceanic – When a convergent plate

boundary forms between two pieces of oceanic

crust, one will subduct underneath the other

because of the high density of oceanic crust. As

one slab of lithosphere is reheated in the

asthenosphere, some of the material rises back

up through the lithosphere to create a chain of

volcanic islands known as an island arc.

Oceanic + Continental – When a convergent

plate boundary forms between oceanic and

continental crust, the oceanic crust will

subduct because it is made of denser material.

As that slab of lithosphere reheats in the

The Aleutian Islands are part of an

island arc that is a result of the

Pacific Plate subducting

underneath the North American

Plate off the coast of Alaska.

Oceanic + Oceanic Convergence

Oceanic + Continental

Convergence

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asthenosphere, magma will rise up to the surface and create a volcanic arc on

the continent.

Continental + Continental – When two pieces of continental crust come

together at a convergent plate boundary, neither one of them will subduct.

Their light density makes them

too buoyant to subduct into the

asthenosphere, so instead, they

rise up to create a mountain

range.

The Himalayan Mountain range is a

direct result of continental-continental

convergence between the Indian Plate

and the Eurasian Plate.

As the Nazca Plate collides with the South

American Plate, oceanic crust is subducted

into the Peru-Chile trench, and a volcanic arc

is formed on the west coast of South America.

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Transform – At a transform plate boundary, tectonic plates move horizontally past

each other. In this case, lithosphere and crust are neither created nor destroyed.

Transform plate boundaries can exist in both oceanic and continental crust.

Mid-Ocean Ridge Transform Fault – These types of transform faults offset the

spreading centers of mid-ocean ridges.

As the South American and

African plate are separated

by the Mid-Atlantic Ridge,

sections of the ridge are

offset by transform faults.

Transform Fault

Spreading Zone

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Continental Transform Faults – These faults are responsible for the

horizontal offset of continental crust.

The San Andreas Fault in California is arguably

the most well-known transform fault

boundary. It is the boundary between the

Pacific Plate and the North American Plate, and

it spans a distance of over 800 miles –

stretching from the Gulf of California up

through Point Delgada, CA.

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Faults

The different types of movement associated with the various plate boundaries create faults

in the earth’s crust. This happens because of the type of stress involved with the movement

of the tectonic plates. Faults are characterized based on the movement between the

hanging wall and the foot wall. There are three major types

of faults:

1. Normal Fault: A normal fault is formed around areas

of divergence, and is a result of tensional stress – the

stress used to pull an object apart.

2. Reverse Fault: Reverse faults are indicative of areas of

convergence and are a result of compressional stress –

the stress used in pushing two objects together.

3. Strike-Slip Fault: A strike slip fault is a result of shear

stress. Two sections of the lithosphere move along a

horizontal plane.

As tensional stress stretches the

crust, a diagonal fault plane will

form and the hanging wall will

drop.

Hanging

Wall

Foot

Wall

Hanging

Wall Foot

Wall

As compressional stress is

applied to the crust, a diagonal

fault plane will form and the

hanging wall will rise.

Shear stress causes a vertical

fault plane to form and the two

blocks move in a horizontal

motion along that plane.

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Earthquakes

Earthquakes are a release of energy that forms as a result of movement of lithosphere

along a tectonic plate boundary or fault plane.

As the lithosphere moves

along a fault plane, the edges

lock together and create

friction.

As movement continues

underneath, the crust above

is deformed and more

friction is created.

The edges of the plates

remain locked together,

causing deformation of the

crust to become more severe.

Eventually there will be enough stress

from the plate movement to

overcome the friction between the

two slabs of lithosphere. As the plates

“snap back” or rebound, energy is

released in the form of waves, which

are felt as an earthquake.

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Fault scarp: A fault scarp is a feature on

the surface of the earth that looks like a

step. It is caused by slip on a fault.

Fault trace: A fault trace is the intersection

of a fault with the ground surface.

Epicenter: The epicenter is the point on

the earth's surface vertically above the

hypocenter.

Focus: The focus – or hypocenter – is the

point within the earth where an

earthquake rupture starts.