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 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 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.
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.
Rocks also melt in plate subduction zones, which are places where the seafloor lithosphere is pulled down into the mantle. Melting temperatures are lowered in this environment because of all the water trapped in the seafloor rocks and in the clay sediment that is carried down with the plate.
What Happens to Magma?
Because magma is a hot liquid, it has natural buoyancy, which causes it to migrate upwards, somewhat like a rising hot air balloon.
It can also move in response to pressure differences, just like any other liquid. Once rocks melt, the magma begins to move.As magma migrates away from the melting zone, it slowly cools off. As it does, more and more solid mineral grains crystallize until eventually all of the melt is gone. Sometimes, magma will get close enough to the surface to feed volcanic eruptions.
But most of the time, the magma cools and crystallizes below ground, forming intrusive igneous rocks.Huge masses of igneous rocks are called ‘plutons.’ When plutons are uplifted during tectonic plate collisions, then eroded by wind and water and ice, they are revealed to us in the rugged topography of the world’s mountain ranges. So, even if you never see magma, you can easily see the rocks it forms.And now the answer you’ve been waiting for. So, why do magma and lava have different names? Likely because lava was named first, around 1750, before anyone realized that it originated far underground. Location is everything, as they say.
Let’s review. Magma is molten rock found below the earth’s surface. Magma on the earth’s surface is called lava.
Magma is primarily a very hot liquid, which is called the melt. The temperature at which a rock melts is affected by its composition. It’s also affected by water and pressure. For example, if water is present, rocks will melt at a lower temperature than they otherwise would. On the other hand, if the rocks are under greater pressure, they will require higher temperatures to melt.Melting of rocks typically occurs in the lower lithosphere or upper asthenosphere.
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.Rocks melt under mid-ocean ridges, which are underwater mountain ranges formed where tectonic plates are sliding away from one another. They also melt in plate subduction zones, which are places where the seafloor lithosphere is pulled down into the mantle, and above mantle plumes, or hot spots, which are places where rising hot mantle rocks heat the base of the lithosphere. Once formed, magma migrates upward due to its buoyancy and eventually either feeds volcanoes or cools and crystallizes into intrusive igneous rock.
Once you’ve completed this lesson, you’ll be able to:
- Define magma and lava and explain why they have different names
- Describe the factors that contribute to the temperature at which rocks melt
- Explain the environments where rocks melt
- Define geothermal gradient