Magma, which is essentially a mixture of liquid rock, crystals and dissolved gas, is a little bit like Mary Poppins's purse: No matter how much you pull out, it seems there's always some left inside. Unlike that enviable purse, though, the formation of magma isn't magical. Special circumstances have to occur for it to form. It helps to first picture the Earth's layers: there's the core at the center, followed by the mantle and finally the crust, atop which the oceanic and continental plates rest. The way the temperature inside the Earth changes in response to depth or pressure is called the geothermal gradient, and under normal conditions, this gradient is not high enough to melt the rocks within. However, when those aforementioned plates interact in certain ways, all bets are off.
One way magma may form is through spreading center volcanism. This happens when two plates separate, resulting in enough of a decrease in pressure that the mantle rock below can melt. Another way magma can form is when two plates collide and one gets pushed under the other. Known as subduction zone volcanism, this type of magma production forces the subducted plate into the mantle, where conditions force it to melt. The fact that these two processes occur where plates meet also helps explain why many volcanoes exist along plate boundaries.
In 2010, scientists discovered another potential source of magma -- this one located some 1,800 miles (2,900 km) beneath the Earth's crust. Based partly on an abrupt change in the speeds of seismic waves as they travel through certain parts of the Earth's interior, experts think the boundary region between the core and the mantle may be partially melted. This finding would help explain why some volcanoes, like the hot spot volcanoes found in Hawaii, also exist in areas not associated with plate boundaries [source: Handwerk].
So if for some odd reason the plates decide to stop doing business with one another, it seems the Earth has a backup plan.
Volcanoes have been blamed for past mass extinctions. (Frank Krahmer/Photodisc/Getty Images)
Magma is molten material in the Earth's mantle (the layer between the crust and the core). Although the mantle is usually solid, changes in pressure and temperature can cause it to shift and become fluid. The tectonic plates, which carry ocean floors and continents, rest on top of the Earth's mantle. Volcanoes usually rest on plate boundaries, where two tectonic plates are pushing against one another - - these areas are called convergent boundaries. Magma erupts from the volcanoes of convergent boundaries, and rises up through rift valleys, which are created at divergent boundaries, where two plates are moving apart. Once the magma has left the Earth's crust, it's called lava. When one tectonic plate slides underneath another one (in a process known as "subducting"), the lower plate's crust becomes part of the Earth's mantle. So there's essentially an endless cycle going on, and the Earth will never run out of magma.
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