Earthquakes, Volcanoes, and Mountains Study Guide (page 2)
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.
Most of the world's volcanoes are found at the margins of geologic plates. An enormous amount of heat is created by the friction that is generated when the subducting slabs of the lithosphere descend into the mantle. Plumes of magma rise up toward the surface in these subduction zones, forming volcanoes at the edges of continents and arcs of volcanic islands in the ocean.
Important to the study of volcanoes are the terms magma and lava. Magma is molten rock under the earth's surface, and it is magma that rises up to form the mid-ocean ridges of seafloor spreading centers. Indeed, the mid-ocean ridges are underwater chains of volcanoes.
Magma also exists in chambers under the continents, in places where pressure from the hot, deep Earth's astheosphere below pushes up rock toward the surface. Even before it gets to the surface, the reduced pressure (from the reduced amount of overlying rock) allows the rock to melt. This often forms chambers of magma, awaiting release through volcanoes. When the magma reaches the surface and flows out, it becomes what is known as lava.
One of the most famous places for lava, because the public can view it in action, is the big island of Hawaii in the Hawaiian chain of islands (Maui, Kauai, and others). On the big island of Hawaii, in Volcanoes National Park, vents open up and release lava, which flows down, sometimes destroying property but also literally creating new land when the lava solidifies into rock. Two giant mountains on the island (Moana Loa and Moana Kea) have been built over many thousands of years of nearly continuous volcanic activity.
The Hawaiian Islands, of course, extend below the surface of the water all the way down to the bottom of the ocean. They have been formed by a conduit within the earth that brings magma up from the deep mantle. Remarkably, as the tectonic plate that covers a large portion of the Pacific Ocean has moved westward in its course over many millions of years, this conduit (called a hotspot) has remained essentially in the same place relative to the mantle below. That is why the youngest of the Hawaiian Islands, the big island of Hawaii, is in the eastern part of the chain. Further west about 50 miles, the island Maui grew and was volcanically active about a million years ago. Another 100 miles or so west, lies the even older island of Kauai, which was volcanically active five million years ago.
The Hawaiian Islands support the theory of plate tectonics. As the Pacific Plate slid from east to west over what geologists now call the Hawaiian hotspot, the Hawaiian Islands have been formed, one by one. Millions of years from now, new islands will have emerged even further to the east. Many hotspots exist all around the world, often forming chains of islands. These hotspots provide important clues to the directions and speeds of the tectonic plates.
The volcanoes of the Hawaiian Islands are called shield volcanoes. These do not explode in the way, for instance, that Mount Saint Helens did in its massive eruption of 1980. Instead, shield volcanoes are built gradually, as lava flows out, sometimes at the top, often at the sides, but through various vents. The slopes of the sides of shield volcanoes are relatively gentle, usually about 5° near the top and 10° on the lower sides, somewhat like an inverted kitchen saucer.
In contrast, stratovolcanoes are gently sloped on the lower sides, perhaps 8°, and become steeper and steeper toward the summit, which sometimes has a slope as steep as 30 to 40°. One famous stratovolcano is Mount Fuji in Japan, probably the world's most photogenic volcano. The United States has many stratovolcanoes, primarily located in the active range of volcanic mountains near the Pacific Northwest coast. Examples include Mount Rainer, Mount Baker, and the famous Mount Saint Helens.
The shield volcanoes of Hawaii tend to be nonexplosive, even though individual eruptions can be quite violent to our human eyes. But compare the slow outflow of lava in Hawaii to the 1980 blast from Mount Saint Helens, which within a minute, took off 500 meters of the mountain's top, sent a column of hot ash into the stratosphere, killed trees within a radius of 20 miles, and killed 63 people who were simply in the wrong place at the wrong time.
An even larger eruption of a stratovolcano occurred in 1883, on the island of Krakatau, killing 36,000 people, primarily from the tsunami that resulted. So much material (dust and sulfuric gases) was put into the atmosphere that the earth's climate was globally cooler by about 1° F for the next year, and sunsets became more intense for months all over the world.
At the tops of volcanoes, in particular stratovolcanoes, are depressions called calderas. Calderas form when material that has built up slowly over time plugs the volcano. Pressure builds up inside and the volcano explodes and forms a depression, like a cup, at the summit. Calderas can sometimes be huge. For example, the famous, picturesque Crater Lake in Oregon is a caldera that is now filled with water. That caldera resulted from a giant explosion more than 6,000 years ago.
If lava comes from a shield volcano, what is the material that explodes from a stratovolcano? In a stratovolcano, gases burst out, as do hot pieces of magma in various sizes that have been suddenly shot up into the atmosphere. The large ones can fall like bombs close to the volcano; the smallest ones become ash high in the atmosphere. These fragments of magma are called pyroclasts. When tremendous amounts of pyroclasts explode out and form a giant wall of material that rumbles down the sides ofthe volcano, we witness what is known as a pyroclastic flow. One famous disaster of volcanic poisonous gases and a pyroclastic flow buried the Italian coastal city of Pompeii and in the year A.D. 79 from an eruption of Mount Vesuvius.
Volcanologists study the chemistry of the material that comes from volcanoes and have discovered that significant variability exists in the key component called silica (a combination of the elements silicon and oxygen). Silica affects the viscosity, which is the fluidity. Lower amounts of silica are one factor that creates magmas with a lower viscosity. High temperatures, too, create magmas of lower viscosity. Viscosity can be thought of as the liquidity of a liquid. For example, water has a relatively low viscosity, compared to the high viscosity of molasses.
Magma with lower viscosity tends to result in eruptions of lava that are nonexplosive, creating volcanoes that are shield volcanoes, like the Hawaiian Islands. In contrast, higher viscosity magma, which also usually has slightly lower temperatures, tends to build up and then explode in eruptions of pyroclasts from stratovolcanoes.
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