How Volcanoes Work

Magma can also push up under the middle of a lithosphere plate, though this is much less common than magma production around plate boundaries. Thisinterplatevolcanic activity is caused by unusually hot mantle material forming in the lower mantle and pushing up into the upper mantle. The mantle material, which forms a plume shape that is from 500 to 1000 km wide, wells up to create ahot spotunder a particular point on the earth. Because of the unusual heat of this mantle material, it melts, forming magma just under the earth's crust. The hot spot itself is stationary; but as a continental plate moves over the spot, the magma will create a string of volcanoes, which die out once they move past the hot spot. The Hawaii volcanoes were created by such a hot spot, which appears to be at least 70 million years old.

So what happens to the magma formed by these processes? We saw that the magma produced at ocean ridges just hardens to form new crust material, and so doesn't produce spewing land volcanoes. There are a few continental ridge areas, where the magma does spew out onto land; but most land volcanoes are produced by subduction zone volcanism and hot spot volcanism.

When the solid rock changes form to a more liquid rock material, it becomes less dense than the surrounding solid rock. Because of this difference in density, the magma pushes upward with great force (for the same reason thehelium in a balloonpushes up through the denser surrounding air and oil pushes upward through denser surrounding water). As it pushes up, its intense heat melts some more rock, adding to the magma mixture.

The magma keeps moving through the crust unless its upward pressure is exceeded by the downward pressure of the surrounding solid rock. At this point, the magma collects inmagma chambersbelow the surface of the earth. If the magma pressure rises to a high enough level, or a crack opens up in the crust, the molten rock will spew out at the earth's surface.

如果发生这种情况,流动的岩浆(now calledlava) forms a volcano. The structure of the volcano, and the intensity of the volcanic eruption, is dependent on a number of factors, primarily the composition of the magma. In the next section, we'll look at some different magma types and see how they erupt.