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Magma: Definition & Formation

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  • 0:07 Magma
  • 0:38 Magma Composition
  • 1:32 Magma Temperatures
  • 2:34 Where Does Magma Form?
  • 5:15 What Happens to Magma?
  • 6:36 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Magma is molten rock found below the earth's surface. The temperature at which a rock melts is affected by its composition, pressure and water. Learn how magma forms and how it either feeds volcanoes or cools and crystallizes into igneous rock.

Magma

Magma is defined as molten rock found below the earth's surface. It's a Greek word meaning 'thick ointment.' Why a geologist in the year 1859, which was the first known geological use of the word, decided that magma and ointment were similar is unknown. It would be one hot ointment. Here's another puzzle: magma on the earth's surface changes names and is called lava. Why? You'll have to watch the video until the end to find out!

Magma Composition

Magma is primarily a very hot liquid, which is called a 'melt.' It is formed from the melting of rocks in the earth's lithosphere, which is the outermost shell of the earth made of the earth's crust and upper part of the mantle, and the asthenosphere, which is the layer below the lithosphere. It is composed of whatever elements made up the minerals in the source rocks.

But most magma also has other things mixed in. For example, it usually contains bits and pieces of minerals that have not yet melted or have solidified (or crystallized) from the molten state as the magma cools. There are also many different gases dissolved in magma. Water vapor, carbon dioxide and hydrogen sulfide are common. There can even be gaseous forms of hydrochloric or sulfuric acid. Sometimes gas bubbles will form in the melt.

Magma Temperatures

As you might expect, it has to be pretty hot for rocks to melt. Magma temperatures usually fall somewhere in the range of 700-1300 degrees Celsius, which is about 1200-2400 degrees Fahrenheit.

How hot is that? Well, hot enough to completely vaporize your pizza, which bakes at about 425 degrees Fahrenheit, into atoms, but not quite hot enough to melt the oven's stainless steel cooking rack, which would require about 2750 degrees - assuming, of course, your oven could get that hot to begin with.

The composition of a rock - in other words, what minerals it is made of - determines its melting point. Different minerals melt at different temperatures, so when a rock begins to melt, its lowest-melting-point minerals melt first. That's significant because it produces magma with a different overall chemical makeup than the original rock. It's one reason why magma compositions vary so much.

Where Does Magma Form?

Where on Earth is it hot enough to melt rocks? You might guess a volcano. But, the magma and lava in volcanoes does not melt there. It comes from much deeper in the earth.

You might think that rocks would need to be a long way below the surface to be hot enough to melt. But in reality, thanks to the geothermal gradient, the earth gets hot pretty quickly as you dig down from the earth's surface. With the geothermal gradient, we see that the temperature increases with depth by an average of 25 degrees Celsius every kilometer. So, to get to 1000 degrees, you only have to go down about 40 kilometers!

Of course, there are other factors at work besides temperature. For example, if water is present, rocks will melt at a lower temperature than they otherwise would. On the other hand, the deeper rocks are in the earth, the hotter it has to be to melt them because greater pressure keeps them in a solid state longer.

On average, melting happens at depths below about 50 kilometers but above a few hundred kilometers. That's near the base of the lithosphere or at the top of the asthenosphere. Although rocks can melt anywhere, there are three environments in which melting is common due to mechanisms that help to increase the temperature of the rocks.

Melting happens below mid-ocean ridges, which are underwater mountain ranges formed where tectonic plates are sliding away from one another. At those places, hot mantle rocks rise in slow-moving currents. Melting happens at relatively shallow depths, less than the typical 50 kilometers, for two reasons. First, the lower lithosphere rocks melt because they are heated by the hotter rocks below. Second, the rising mantle rocks melt because the pressure on them decreases as they move upward.

Melting also occurs at places where rising hot mantle rocks heat the base of the lithosphere. These places are called mantle plumes, or hot spots. The lithosphere does not rift or gap as it did with the mid-ocean ridges but moves over the stationary hot spot. One well-known example is where the Pacific plate slides over a hot spot and the resulting volcanoes form the Hawaiian Islands.

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