Earthquakes, volcanoes and mountain ranges tend to happen in similar areas.

The map above shows frequent earthquake activity as the bands of dots.

Most of the world's active volcanoes (triangles) are along the edges of tectonic plates (the lines).

Types of Plate Boundaries

Convergent- plates collide into each other.

Where an oceanic and a continental plate collide, the denser oceanic plate will be forced under (subduction) the other.

Divergent

• plates spread apart
• caused by magma upwelling from deep in the Earth
• usually found in the oceans along with mid-ocean ridges

Speed Links

Continental Drift

Seismic Waves

Time Math

P & S Wave Chart

EQ Strength

EQ Waves as X-Rays

Transform- two plates slide past each other.

In this aerial view of the San Andreas Fault (transform) the trees in the orchard (dots) have been offset by the slipping of the plates. The Pacific Plate is to the left and the North American to the right.

This picture shows how things like fences, roads, rivers and buildings can be offset by the sliding of the plates.

The exception to this arrangement are "Hot Spots" which are plumes of hot material (rather than belts) in the middle of plates. These spots stay stationary while the plate moves above it. The spot melts through the plate like a blow torch and produces a volcano above it. As the plate moves, the spot melts through another spot producing a chain of volcanic islands. Hawaii is an example of a hot spot island chain.

The movement of the plates is caused by convection currents deep within the Earth. The force that moves the plates around the earth are convection currents inside the mantle.

The different types of plate boundaries are caused by a combination of the direction of convection as well as they type of crust: continental or oceanic.

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Evidence of Continental Drift

Puzzle Fit- if the continents were cut out of a map, most of the landmasses will fit together to form a larger supercontinent, which is called Pangaea.

Fossil Evidence- in the picture above, fossils of many land-living have been found on opposite shores. When Pangaea is re-assembled, the fossils match up.

Glacial Evidence- when Pangaea is re-assembled, there is evidence of a single ice sheet (at least for this episode) affecting many of the southern continents. When viewed this way, this sheet leaves consistent evidence of a single glacier. When viewed on the current continents, it is inconsistent and even highly improbable. For example, India, which is north or the Equator, has glacial evidence coming from the south!

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Earthquakes

An earthquake is an event where two pieces of crust shift against each other. The rumbling felt is from the rocks slipping, sticking and breaking. The vibrations are called seismic waves. There are different types of seismic waves that vibrate in different ways.

The focus is the spot within the earth where the earthquake began.

The epicenter is the spot on Earth's surface closest to the focus.

A fault is a crack along which the rocks slide.

Seismic Waves- during an earthquake, several types of waves are generated. The vibrations felt are actually called seismic waves that are traveling through the Earth.

P-Waves

S-Waves

Youtube Video demonstrating P-Waves and S-Waves.

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How to Use the P-Wave and S-Wave Travel Time Chart

P-Wave & S-Wave Chart (opens a new window so your pop-up blocker may stop it)

The P-line shows how much time it take a P-wave to travel a certain distance. So if you need to know how much time it takes the p-wave to travel 2,000km, it is just over 4 minutes (about 4:05 ). The S-wave works the same way: for 2,000km it takes 7:20 .

To find the distance to epicenter:

You are in charge of watching the seismic station tonight when the seismograph detects an earthquake. The earthquake didn’t happen where you are- you can’t even feel it. As a result, you don’t know what distance or direction the earthquake happened. The P-wave and S-wave are separated by 4:05 (4 minutes, 5 seconds). You need to find a spot on the graph where the P-line and the S-line are separated by 4:05 .

To Find The Time That The Earthquake Occurred

When a seismograph detects an earthquake that happened at some distance, (2,600km for example) you know that the earthquake happened some time in the past and it took time for the waves to reach your station. But how long ago? All you need to do is answer the question “how long does it take a P-wave to travel 2,600km?

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Earthquake Strength

The intensity or strength of an earthquake is measured in two main ways:

Seismic waves as “x-rays”

• P-Waves travel through solid and liquid
• S-Waves travel only through solids
• Seismic waves travel faster through denser material.
• Because of this, the path traveled by a seismic wave is bent towards the surface.

Shadow Zone diagram photoshop drawing by Phil Medina

Properties of the material (such as density and pressure) that the waves pass through can be inferred by the speed and angle that the waves travel.
The layers of the earth are determined by the jumps in velocity and “echoes” of seismic waves.

The MOHO is a boundary between the crust and the upper mantle where the velocity of waves jumps up sharply. This sharp increase in velocity is called a discontinuity.

A shadow zone occurs on the opposite side of the earth from an earthquake because of the liquid outer core. S-Waves are stopped all together while the P-Waves are refracted (bent) to create a zone where no waves are picked up at all. This zone is between 102° and 143°around the earth from the earthquake.

Lab research and studies of meteorites suggest that the core is made of Iron and Nickel (FeNi).

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