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What Makes a Volcano Erupt

Describes how volcanoes erupt.

One of the most spectacular sights in nature is an erupting volcano, but what makes a volcano erupt? That is a question that has troubled mankind for millennia. Like most other things they couldn't understand early man believed that the Gods were punishing them for something they did to anger the Gods. A later theory was that there was a coal mine on fire in the bowels of the earth. Although the later theory was slightly closer to the truth it still failed to explain an erupting volcano.

There are more then one type of a volcanic eruption caused by different mechanisms. There is the rift eruption that occurs when a continental sized plate splits. A hotspot eruption occurs when a plume of magma comes up out of the earth causing a volcano or volcanoes to appear in the center of a plate. Both of these types of eruption are relatively rare. The most common type of eruption is caused by volcanoes that are inland from a subduction zone.

There is a fourth type of eruption that hasn't occurred for more then 20 million years this is the kimberlite eruption originating over 150 miles below the surface of the earth. These are extremely violent eruptions that blast a carrot shaped pipe into the earth. The explosion is so violent that scientists theorize that when the gas and magma reach the surface of the earth it is erupted freezing cold because of the sudden decompression of the gases. Kimberlites and lamproites, an associated eruption, are the bearers of all the world's diamonds.

The rift eruption is associated in a place where the surface of the earth bulges up in the middle of a continental plate eventually developing three cracks in a star shaped pattern. Two of the cracks go on to form a new ocean basin, the third fails do develop and is termed a failed arm of the sea or an aulocogene that is filled with lava and sediments. Lake Nipigon in western Ontario is located in an aulocogene, the Connecticut River Valley is another.

Rifts are associated with vast outpourings of basaltic lava often tens of thousands of feet thick. One related to the Lake Nipigon magmas is estimated to be over ten miles thick. However it came out in many flows to reach this thickness. You can find exposures of this on the Keweenah Peninsula of Upper Michigan.

The hotspot eruption occurs when a hot plume of magma develops under a plate and eventually breaks through the surface. Good examples of this today are the Hawaiian Islands, and Yellowstone Park. These illustrate two different types of eruption that can occur with a hotspot. The Hawaiian example is eruptions of a basaltic magma that is a relatively quite eruption. An eruption like the Yellowstone type is extremely violent causing continent wide damage. The magma erupted is rhyolite the erupted equivalent of granite. The magma erupted in this kind of eruption is rich in quartz, it is thick and pasty and loaded with gases. An eruption of this type can go on for days as a continuous explosion. It has been estimated that the last time Yellowstone erupted it ejected over 23,000 cubic kilometers of ejecta.

The most common type of volcano is one that is associated with a subduction zone where one plate is diving under another. In this type of volcano sea crust that is made of basaltic magma dives under another plate at an angle. As the plate is being subducted it also is taking along with it a load of sediment as well as a vast amount of seawater. As it dives deeper into the earth's mantle it heats up. Some of this heat is part of the thermal gradient where the deeper you go the hotter it gets. The other part of the heat is caused by friction where the subducted plate rubs against the upper plate.

This added heat of friction along with the seawater that acts as a flux lowering the melting point of the rock causes some of it to melt. This melted rock is lighter then the rocks confining it, and it starts to wiggle up through cracks and fissures in the rock until it finally reaches the surface of the earth giving birth to a new volcano.

Volcanoes of this kind are the so-called ring of fire surrounding the Pacific Ocean. They are also found in the South Atlantic as the Scotia Islands and the Caribbean Sea where they form the Virgin Islands and the rest of the Lesser Antilles reaching from Trinidad to Puerto Rico.

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