When you’re standing on solid earth, it’s hard to think of the planet as anything but very solid rock. Yet this is not the case. Several kilometers below earth’s crust, hard rock is replaced by softer rock, and then ultimately by liquid rock with gases and minerals, that occasionally erupt from volcanoes or cracks in the earth.
You certainly wouldn’t want to be near this molten rock, since it is extremely high in temperature. Though a few materials melt at temperatures of about 1100 degrees F (593.33 C), most magma under the earth’s crust maintains temperatures between 1292 and 2372 degrees F (700-1300 C). When molten rock erupts or flows onto the earth’s surface, it quickly loses heat energy, though it’s still far hotter than exposure would warrant.
Some scientists make the distinction between magma and lava, defining lava as molten rock that is on or above the earth’s surface. This can also be called extrusive. Another way you will see the vestiges of molten rock above the earth, aside from in lava flows, is when you examine igneous rock. All igneous rock is formed from magma, and some rocks resemble the once liquid flow that produced them. Obsidian, for instance, is shiny and brittle, and somewhat resembles the flow of certain types of magma. It’s actually a naturally occurring glass that gets its shine and smoothness from the fact that the magma didn’t crystallize as it cooled.
Another interesting igneous rock created by once melted rock is pumice, which is very light, so light that it will float in most fluids. Unlike the smooth shine of obsidian, pumice resembles natural sponges, with a number of pockmarks. These are caused when gases create bubbles in the rock, which doesn’t have time to reform before it cools. This results in the bubbled appearance of pumice and its low density.
What creates magma? Temperature, and pressure increase as you delve farther into the earth’s layers. It’s comparable to the way things are heated when you place them in a microwave oven. The hottest part is always the center and interior.
Under certain conditions, where heat becomes extreme, part of the rock forming beneath the crust begins to melt. As heated rock rises, it begins to cool again, creating some of the igneous rocks that make up portions of the earth’s crust. When magma comes to the surface, especially underneath an ocean, where it may be much closer to the crust, it pushes slowly cooled rock upwards, creating volcanic mountains, a potential source for the extrusion of lava.
Earth is not the only planet on which magma exists. Recent research on Mars in comparison with volcanoes in Hawaii suggests a molten rock flow beneath the crust. Studies in 2007 posit that volcanoes on Mars that were once thought extinct, may only be dormant.
