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Earthquakes, Volcanoes, and Mountains Study Guide (page 2)

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Updated on Sep 25, 2011

Volcanoes

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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