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Earth's Layers
The earth is divided, from its surface to its core, into layers. The rocky surface of the Earth, called the crust, varies from three to 31 miles in depth. A thick layer of rocks, to a depth of around 1,800 miles, lies under the crust and is called the mantle. Together, the crust and the uppermost part of the mantle, its solid part, are referred to as the lithosphere, which averages about 60 miles in depth. The mantle immediately below the lithosphere is made up of partially melted rock and is called the asthenosphere; it is about 155 miles in depth.
Tectonics
The surface of the Earth is divided into a number of slabs, known as tectonic plates, that slowly move above the deeper, hotter asthenosphere in response to its changing conditions. When one plate crashes, or converges, into another plate, its edge slides beneath the edge of the other plate. This is called the subduction zone. Close to 80 percent of volcanoes are located along subduction zones. When the edge of the submerged plate descends about 70 miles into the mantle, the rock of which it is formed melts in the high temperature and pressure at that depth. This thick molten rock is called magma, which is composed of liquid "melt," solid minerals that crystallize from the melt, sold rock xenoliths (or inclusions) and dissolved gases.
Magma
Magma is less dense than the surrounding solid rock. Pressure from dissolved gas in the magma forces it upward through cracks and fissures, until it collects in reservoirs called magma chambers. As magma nears the Earth's surface, pressure on it is reduced and its dissolved gases escape. The release of these gases causes magma to become explosive, similar to a bottle of carbonated beverage being shaken, which separates the dissolved carbon dioxide gas from the beverage, and then opened quickly.
Volcanic Eruption
As a magma chamber fills, its pressure increases. Eventually, the pressure of the magma exceeds that of the rock overlying it and a volcanic eruption occurs. The degree of explosiveness of magma depends upon its viscosity. Low viscosity (a more fluid state) allows the dissolved gases to escape easily and results in oozing lava that flows out and over the volcanic surface or accumulates in a dome. High viscosity (a less fluid state) prevents the gases from escaping until enough pressure builds to eject the lava in a large explosive eruption.
After the Eruption
Magma formation, movement, storage and release is a continuous process. When pressure inside a magma chamber builds to the point of volcanic explosion, magma is ejected from the magma chamber until the pressure is equalized. Then the explosion ends until pressure again builds to an explosive level. Magma left in the volcano's conduit or vent cools and congeals into a volcanic plug. In some highly explosive eruptions, the magma chamber may be almost entirely emptied. Left unsupported, the roof of the chamber may then collapse. This forms a steep-walled circular basin called a caldera. Calderas range in diameter from one to 15 miles. They may fill over time with water from precipitation and become lakes.