What is Magmatism? - Definition, Process & Types

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  • 0:03 Definition of Magmatism
  • 1:11 The Formation of Magma
  • 1:44 Location of Magma Formation
  • 3:29 Rocks Formed By Magmatism
  • 3:54 Lesson Summary
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Lesson Transcript
Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

When you see smoke rising from a volcano or you touch a piece of granite, you are experiencing the effects of magmatism, which is the formation and motion of magma under the surface of the earth's crust. In this lesson, learn all about magmatism and its effects on the earth.

Definition of Magmatism

Let's say you're standing at the base of a big mountain when suddenly the ground starts to shake and you see smoke and ash rising from the top of the mountain. What's happening? The mountain must be a volcano that's erupting! Suddenly, you see something coming out of the mountain. It's red and hot. As it slowly moves down the side of the mountain, you realize that this must be lava coming up from deep beneath the ground. As you run away to avoid getting covered in hot lava, you wonder how all that lava got inside that mountain.

Volcanic eruptions are common throughout the world. Under the volcano, there is a lot of very hot, molten rock called magma. The formation and movement of magma under the earth's crust is a process known as magmatism. The word 'magma' was originally a Greek word used to refer to a kind of thick, oozing ointment, and this same word was first used in 1859 to describe the thick, hot, molten rock that forms just under the earth's surface. When magma comes to the surface of the earth, as it does during a volcanic eruption, it's called lava.

The Formation of Magma

Everything we can see on the surface of the earth makes up Earth's crust. A few kilometers under the surface, the structure of the rocks present changes as you cross into the mantle. The mantle is very hot and dense but is mostly semi-solid. However, right at the boundary between the crust and the mantle, conditions are sometimes just right for liquid magma to form. In order for rock to liquefy into magma, the temperature has to be very high and the pressure must also be relatively low compared with other areas in the mantle.

Location of Magma Formation

The crust of the earth is divided into many large sheaths of rock called tectonic plates, and these plates are always in motion relative to each other. Whenever tectonic plates come into contact with each other, it creates conditions that commonly result in the formation of magma. Magma usually forms at the boundaries between these tectonic plates.

In areas where two plates are pulling away from each other, known as divergent boundaries, the pressure is reduced between the plates, causing magma to form. As the magma is pulled up to the surface, it solidifies to form new crust, filling in the space between the separating plates. One place this is happening right now is at the Mid-Atlantic Ridge that runs the length of the Atlantic Ocean. At the Mid-Atlantic Ridge, the North American and South American plates are moving slowly west, while the Eurasian and African plates are moving east. Where the plates separate, there's a series of mostly underwater volcanoes, where magma rises to the surface as lava and turns into new crust. At the Mid-Atlantic ridge, these two plates move apart at a rate of about 2.5 cm every single year!

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