Geology And Geophysics

Causes of Volcanic Eruptions



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Volcanoes occur all over the world. They can destroy entire cities, shape the land, and even erupt under the oceans. In fact, somewhere in the world, on average, 11-17 volcanoes are erupting, many of then under the surface of the ocean. An interesting question to explore is what cause volcanic eruptions.

The earth has an interior of mostly molten rock, covered by a thin layer of cooled and hardened crust. If the earth were an apple, the crust would correspond to the skin, the flesh to the mantle of elastic rock, and the apple core to the earth's core.

Much of the interior is in a melted elastic state, not unlike toffee, except immensely hotter. This material is under pressure, since the closer to the extremely hot core, the greater the gravitational forces are that are pressing on it.

The molten rock, magma, contains gasses as well as rock. However, the gasses are suspended in solution by the intense pressures. Still, it is constantly trying to find ways to escape to the less dense pressures above. When cracks are found in the crust, an eruption occurs, since the magma will flow to areas that have less pressure. This is similar to shaking a soda, then uncapping it. The soda sprays out because the pressure inside the can or bottle is greater than the pressure outside the container.

This explains the mechanism, but not the cause of the eruption.

Hot spots

Hot spots are actually the secondary cause of volcanic eruptions, however in some places, the pressure is greater and the crust is thinner than in others. Weaknesses in the cooled rocks above allow the magma to spurt to the surface in an eruption. A well-known example is Hawaii. The chain is formed over a hot spot, but since geologic plates move due to plate tectonics, the pressure and weakness also moves. In the case of Hawaii, this has formed a series of islands rising from deep beneath the ocean. From base to summit, the highest peak on the islands of Hawaii is the tallest mountain on Earth, eclipsing Mount Everest.

Another hot spot occurs under Yellowstone National Park, in the US. In this location, the crust is much thicker, so rather than simply bursting through all at once, it creates an enormous magma chamber beneath the surface. It is still under pressure; so, it percolates up, heating ground water to form geysers, mud pots, and the other familiar features of the park. A volcano will eventually erupt, with great likelihood, in this area, since pressures continue to increase and many scientists believe that the magma pocket continues to fill and to expand.

Subduction and tectonic movement

The major cause of volcanoes is subduction. As tectonic plates move, one can slip under another, as occurs in the Pacific along the west coast of the US. Denser rock slides underneath lighter rock. As it does this, the rock is heated, both by the friction, and by the heat rising from below. This puts molten rock relatively close to the surface, and again it will look for weaknesses in order to escape.

When it finds a weakness, an eruption may occur. Often, a magma pocket will be created, until the 'cork is removed from the bottle', so to speak. This is what happened with Mount St. Helens, In Washington State. In this case, the magma pocket was there, as it is under several other mountains of the Cascade Range. As magma moves, it can cause what would normally be considered a minor earth quake.

In this case, though, the magma was close to the surface, and the quake it caused let loose tons of rocks and dirt from the face of the mountain. This uncorked the bottle, and the result was an enormous release of molten rock and gas. A volcanic eruption was borne, releasing enormous pressure.

The US Park Service believes that the same thing was the cause of the much more powerful eruption of Mount Mazama, which left a very deep crater or caldera. A huge magma pocket built up, with the pressure increasing, until the mountain couldn't contain it, and it found a weakness at or near the summit.

The caldera was formed when the top of the mountain collapsed into the nearly empty magma chamber. The caldera then eventually filled with water from rain and snowfall, and the result is the deep blue Crater Lake in southern Oregon.

The eruption was powerful enough to send ash and light stones as far as Boise, Idaho. Some places in the national park are covered in close to a thousand feet of volcanic pumice, from the eruption. This gives some idea of how much pressure is being created and the amount of material thrown out by the melting of one plate as it slips under another. The Cascade Mountain range was formed from this sort of volcanic eruption cause. The same is true of the Sierra Mountains in California, though they are older.

The process continues, as does the hot spot process. In human terms, it takes a long time before the weaknesses are found. Geologically, it is a short amount of time. Both will probably continue as long as the earth is tectonically active. Defining the cause of volcanic eruptions isn't especially difficult, with our current knowledge. Predicting when and were the next volcano will erupt is another matter.

Sources:

United States National Park Service, special thanks to Crater Lake National Park and Yellowstone National Park

US Forest Service

US Geological Survey

The Science Channel

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