What is Magma?

Tricia Christensen
Tricia Christensen

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.

Pumice igneous rocks have a sponge-like appearance and a low density.
Pumice igneous rocks have a sponge-like appearance and a low density.

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.

Magma under the earth’s crust is between 1292 and 2372 degrees Fahrenheit.
Magma under the earth’s crust is between 1292 and 2372 degrees Fahrenheit.

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.

Igneous rock is formed from magma.
Igneous rock is formed from magma.
Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent wiseGEEK contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

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Discussion Comments


Molten rock is magma, right?


After millions or billions of years how does the magma continue to maintain it's heat. Why doesn't it burn out?

Even the sun, how does it continue to burn and relatively maintain a constant temperature without burning out? God in control or what?


@parmnparsley - You're on the right track. Metamorphic rocks can originate from igneous rocks (as well as sedimentary rocks). Metamorphic rocks are the result of immense heat and pressure applied to rocks as they are forced under, over, and into one another. Igneous rocks are often created in areas of volcanic activity because these are the areas where the earth’s tectonic plates shear, subduct, and converge. I guess that you could almost conclude that you cannot have metamorphic rocks without magma, and you cannot have magma without the melting of igneous, metamorphic, and sedimentary rock.


Aren't metamorphic rocks formed by cooling magma as well? I thought that metamorphic rocks were formed underground from super heated, melted rock. Why aren't metamorphic rocks considered igneous rocks formed from magma? I know that metamorphic rocks look similar to igneous rocks like granite. Can anybody tell me if they are formed from magma?


I would like to point out that a distinction between lava and magma must be made before we can fully describe igneous rocks. Magma only produces plutonic igneous rocks. Magma is considered lava once it reaches the surface. This distinction is important because the properties of igneous rocks are greatly influenced by crystallization as well as mineral and gas content. Obsidian, basalt, pumice, rhyolite, and andesite are all extrusive igneous rocks that are formed when lava cools on the earth’s surface. They are identifiable because of their small grain size and lack of crystallization. Magma forms igneous rocks like granite, diorite, and gabbros which are easily identifiable by their large crystalline structure. These plutonic rocks are formed by the slow cooling of magma within the earth's crust.

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