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. P a g e | 1

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 Mantle Inner Core

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. P a g e | 3

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. P a g e | 4

C C C H H C Subducting Plate H C H 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 Oceanic + Oceanic Convergence 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.

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 . Oceanic + Continental – When a convergent Convergence 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 P a g e | 8

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 P a g e | 10

. 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. Foot As tensional stress stretches the Hanging

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. Hanging

Wall Foot As compressional stress is applied to the crust, a diagonal Wall 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. P a g e | 12

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. P a g e | 13

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.