Earthquakes, Volcanoes, and Mountains Study Guide
The key to the workings of earthquakes, volcanoes, and mountains is the theory of plate tectonics. Most of the action occurs at the edges of plates, the margins at which they slide, bump, and collide. What causes earthquakes and volcanoes? How are mountains made?
Almost a million people died in China during an earthquake in 1556. An earthquake in Iran in 1990 killed 50,000. As the entire world recently witnessed in helpless dismay, more than 100,000 died in Indonesia and India from a tsunami started by the huge earthquake of December 2004. All told, about a million people have been killed just in the last 100 years from sudden shifts in the earth's plates.
With earthquakes, it's not a matter of if but when, because Earth's geological plates are in motion. In fact, hundreds of small earthquakes occur every day, but most are measurable only by seismographs. Less frequently occur medium-size ones; and very infrequently, the big ones strike.
Why do earthquakes take place where they do? The key to the answer lies along the margins of geologic plates. For example, the San Andreas Fault, a crack along the transform margin between two plates that happen to meet in California. These plates slide past each other, causing earthquakes. In 1906, a large earthquake along the San Andreas Fault destroyed most of San Francisco.
Earthquakes occur along the other types of margins, too. We don't worry too much about earthquakes from a diverging margin of a mid-ocean ridge, because they occur deep under water and too far away and they are smaller. But dangerous earthquakes can occur along convergent margins, when plates are colliding or one is subducting under the other.
Long ago, before the theory of plate tectonics presented a global explanation for earthquakes, geologists recognized a Pacific Ring of Fire. Roughly, a ring of fire exists around the outer edge of the Pacific Ocean, a ring where huge numbers of earthquakes and volcanoes occur. Why? The Pacific Ring of Fire occurs because the Pacific Ocean is ringed by many plate edges, and because earthquakes and volcanoes tend to occur at the boundaries between two plates. Japan, for example, sits on the Ring of Fire and has a boundary between two plates passing right through the country. Plate boundaries around the Pacific Ocean occur at the edge of North America, the Kamchatka Peninsula of Russia, Japan, Indonesia, and New Zealand.
The main concept behind the dynamics of earthquakes is the following. As plates move against each other (either along a transform margin or along a convergent margin), the friction between them often locks the edges together, even though the main bulk of the plate has shifted its position ever so slightly.
For example, press down with your fingers on the table with enough friction so that as you also try and slide them across the table, the friction holds your fingers in place. But as you apply more and more horizontal force, eventually your fingers jump ahead, often in little leaping steps.
Earthquakes are caused when plates that have been locked together suddenly overcome the tension (or compression) that has built up between them and they jerk ahead in whatever direction was being forced by plate tectonics. If the jerking steps are small, the earthquakes are small. If the steps are large, the earthquakes are, too. Earthquakes are thus releases of stored energy (like a spring) from the rocks at the edges of continental plates. On Earth's surface, these plate boundaries form faults, or fractures, in the rock. That is why the line along which earthquakes occur in California is called the San Andreas Fault.
The standard scale to measure the strength of earthquakes is the Richter scale. The Richter scale converts the readings that seismographs make of seismic waves into a number representing a measure of the amount of energy released by the earthquake. Each number grade in the Richter scale, from 1 to 10, represents an earthquake that is 30 times larger in terms of energy than the preceding number represented.
Rock has limits in how much energy can be stored in the locked plate edges before the friction gives way and the energy is released in the movement that creates an earthquake. These limits might be the reason why the largest earthquakes measured are about 8.6 on the Richter scale. This is the energy of 10,000 Hiroshima-size nuclear bombs and is about the magnitude of the Indonesian earthquake of December 2004.
When earthquakes occur in the shallow ocean or near the coast, the heaving of the crust can create a tsunami in the ocean, a giant wave. This giant wave is really a wall of water, very unlike a regular ocean wave, that travels across the ocean and can devastate coastlines when it breaks on shore.
Much of the danger that earthquakes pose come from tsunamis and from buildings that collapse on people, who usually have no warning. It is obviously necessary to have strong but flexible architecture. For example, earthquake-prone Japan has strict laws for designing buildings, to make them resistant to earthquakes. The need to predict earthquakes is the main reason for the global network of seismographs and seismologists, as an increase in activity of small earthquakes often precedes a large earthquake. This, however, is not always the case.
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