Earthquakes and Plate Tectonics

Introduction
Earthquakes and Plate Tectonics are vitally connected. The movement of the Earth's crustal plates is the major cause of earthquakes, and volcanoes also.

The Earth's major plates are shown on page 5 of the Earth Science Reference Tables.

If you were to plot all of the earthquakes that occur on Earth, you would find that they follow a pattern.

This pattern follows fairly closely the plate boundaries indicated on the reference table.

Plotted Earthquakes and Volcanoes Diagram

[click here to see the diagram]

You would see a similar pattern if you plotted the volcanoes of the world.

Earthquakes

· An earthquake is a movement or shaking of the Earth's crust.

· Most earthquakes occur along a fault.

· A fault is a crack or break in the Earth's crust along which there has been some movement.

Earthquake Fault Diagram

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This picture shows the effect on the surface after the movement along such a fault.

Earthquake Focus Diagram

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· The exact location of the crustal movement is called the focus.

· Since we are usually concerned about effects on the surface, we often refer to the epicenter, which is the location on the surface directly above the focus.

· When an earthquake occurs, several kinds of seismic waves are produced, and travel outward from the focus.

Measuring Earthquakes
There are two different scales that are commonly used to measure the severity of an earthquake:

· The Richter Scale measures the amount of energy released by the earthquake. It is a logarithmic scale, meaning that a 6 is 10 times more powerful than a 5.

· The Mercalli Scale attempts to measure the severity of the earthquake by observing the damage that it causes. A simplified Mercalli Scale is shown below:

Mercalli Scale Table

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Earthquake Waves
Although earthquakes produce several different types of waves, we will focus (no pun intended) on two: P waves and S waves.

· Both waves are produced at the moment an earthquake occurs, but they have several different characteristics. It is important to understand the differences between these two waves.

P waves	S waves
Primary waves	Secondary waves
Travel faster, and at seismic stations first.	Travel slower, and arrive at seismic stations second.
Push-pull, or compression waves.	Side-to-side, or shear waves.
Travel through solids, liquids, and gases.	Travel only through solids.

The two pictures below illustrate the difference between the motion in a P wave (the top), and an S wave (the bottom).

P Wave Motion Diagram

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S Wave Motion Diagram

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Locating the Epicenter (handout)

The Earth's Interior

· seismic waves are our main source of information about the structure of the inside of the Earth.

· From the wave travel times, speeds, and refraction (bending), we can estimate the density and composition of the Earth's internal layers.

Here's what we've learned:

The Earth has several distinct layers, including the crust, mantle, outer core, and inner core.

Earth’s Interior Layers Diagram

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It is believed that the outer core is liquid, and that the other layers are essentially solid. This inference is based mainly on the fact that S waves can't penetrate the outer core. Since these waves can only travel through solids, the outer core is inferred to be of liquid

composition.

The failure of S waves to travel through the outer core, along with the bending of waves due to density differences, gives rise to certain shadow zones when seismic waves travel. These shadow zones are areas on Earth that receive no seismic waves.

Shadow Zones Diagram

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The structure of the Earth's interior is summarized on page 10 of the Earth Science Reference Tables:

Earth’s Interior Diagram from pg. 10 of Reference Tables

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· Notice that on the upper right hand side, there is important density information.

· Also, there is a graph showing how the pressure changes with depth. This is basically a direct relationship (as depth increases, pressure increases).

· Below this, there is a graph showing how the temperature changes with depth. This is also basically a direct relationship (as depth increases, temperature increases).

Plate Tectonics

Pangea Diagram

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Continental Drift
Around 1912, a German scientist named Alfred Wegener theorized that all of the Earth's continents were once joined together in a single, large landmass, referred to as Pangea. He further proposed that the continents have separated and collided as they have moved around over the last few million years. He called this theory continental drift. He provided several pieces of evidence to support his theory:

1) Continent Shapes- The continents appear to be shaped in such a way that they would fit together nicely, like a jigsaw puzzle.

2) Rock Formations- There are rock formations on different continents that match up beautifully when the continents are put back together.

3) Fossils- There are fossils found on different continents that would also match up nicely if the continents were all once together.

People of the time mostly thought Wegener was crazy!

New Evidence
In the 1950's, scientists discovered some surprising evidence in support of Wegener's theory. While mapping the ocean floor, scientists discovered two important, and unexpected things:

First, the age of the rocks that make up the ocean floor gets older as you move away from the ridges at the center. This meant that the youngest rocks were found near the ridges, and the oldest rocks near the continents.

Rock Ages Diagram

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Second, there are stripes of alternating magnetic polarity on each side of the ridge.

Magnetic Polarity Diagram

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These discoveries gave rise to the now respectable science of Plate Tectonics. This is the theory that the Earth's seemingly solid crust is actually made up of several pieces, or plates, that move around independently.

Plate Boundaries
The places where the different plates meet, called plate boundaries, are where the tectonic action really is. There are three basic types: convergent, divergent, and transform boundaries.

Convergent BoundaryDiagram

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Convergent Boundaries: This a when two plates are moving toward each other, as shown above.

If the two plates are of relatively low, and similar densities, the plates will form a Collision Boundary.

Collision Boundary Diagram

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In this scenario, the crust is forced upward by the collision, resulting in mountain building. The diagram above shows how this type of collision between India and China forced the formation of the Himalayan Mountains.

Subduction Diagram

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If one of the plates is more dense than the other, as happens when oceanic and continental crust meet, then the more dense plate will be forced under the less dense plate. This forms a trench, or deep valley, where the plates meet. This is called subduction, and is shown in the diagram above. This often results in a chain of volcanoes running parallel to the trench.

Divergent Boundary Diagram

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Divergent Boundaries: As you might expect, this is essentially the opposite of a convergent boundary. This occurs when two plates are moving away from one another, as shown above. This is seen at mid-ocean ridges and rifts.

Transform Boundary Diagram

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Transform Boundaries: This type of boundary forms when two plates are sliding past one another. The diagram above illustrates this motion. The most popular example of this is the San Andreas Fault in California.

All of the different boundaries and their locations are found on  page 5 of the Earth Science Reference Tables, shown below. Notice the key that shows the different boundaries and their symbols.

Tectonic Forces
The movement of the plates is driven by convection currents in the mantle. These currents cause the solid plates to float along on top of the semi-molten mantle material.

Convection Currents Diagram

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Sometimes, there is an opening in the middle of a plate that allows the molten material to flow through it. This is called a hot spot, and usually results in a chain of volcanic islands that form as the plate moves over the hot spot. The Hawaiian Islands are a great example of this.

Hot Spot Example Diagram

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Adapted from: Regents Exam Prep Center http://regentsprep.org/Regents/earthsci/earthsci.cfm

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