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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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Crust
Lithosphere
Outer Core
Inner Core
Mantle
Asthenosphere
Mesosphere
Core
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.
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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 semisolid/liquid back into a solid state (e.g. asthenosphere to mesosphere) is attributed to a
high increase in pressure.
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C
C
C
C
H
H
H
H
C
Subducting Plate
C
Convection Cell
Divergence
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.
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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
Oceanic + Oceanic
Convergence
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.
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 + 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
Oceanic +
Continental
Convergence
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asthenosphere, magma will rise up to the surface and create a volcanic arc on
the continent.
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.

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.
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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.
As tensional stress stretches the
Hanging
Foot
crust, a diagonal fault plane will
Wall
Wall
form and the hanging wall will
drop.
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.
As compressional stress is
applied to the crust, a diagonal
fault plane will form and the
hanging wall will rise.
3. Strike-Slip Fault: A strike slip fault is a result of shear
stress. Two sections of the lithosphere move along a
horizontal plane.
Shear stress causes a vertical
fault plane to form and the two
blocks move in a horizontal
motion along that plane.
Hanging
Wall
Foot
Wall
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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.