There to change rocks, not marshmallows. In

There are two types of metamorphism involved in the creation of metamorphic rocks. In this lesson, you’ll learn the definitions of and differences between contact metamorphism and regional metamorphism.

Metamorphism

If you ever roasted a marshmallow over a hot campfire and ended up with a lump of something black and charred, rather than something gooey and delicious, what you actually did was performed a mini-experiment in contact metamorphism. Of course with true contact metamorphism we are using heat to change rocks, not marshmallows.

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In this lesson, we will learn more about contact metamorphism and how it differs from regional metamorphism, which relies more on pressure to cause rock minerals to change.

Metamorphic Rocks

Now, before we go too far into our terminology, it’s important to understand that when geologists are talking about something going through a process of metamorphism, they are most likely talking about metamorphic rocks. These rocks are kind of neat because they were previously-formed rocks that have been transformed by exposure to heat and/or pressure into new rocks. So, at one time these rocks may have been igneous, sedimentary or even other metamorphic rocks, but due to forces placed upon them they were transformed, or metamorphosed, into different rocks.

Contact Metamorphism

Now, as we mentioned earlier, there are two types of metamorphism: contact metamorphism and regional metamorphism. Let’s start by discussing contact metamorphism, which is the one we introduced with the marshmallow experiment. Contact metamorphism is a type of metamorphism where rock minerals and texture are changed, mainly by heat, due to contact with magma.

This is an easy name to recall if you remember that these rocks change by actually coming in contact with something very hot, like magma.Image you were looking at a cross-section of the Earth where you can see the Earth’s surface, as well as some deep layers of rock underground. Now imagine that an intrusion of hot liquid magma forces its way up through these deep layers of rock. The magma bakes the surrounding rocks causing them to change, or metamorphose.

So all around the outer boundary of the intrusion of magma you will have the formation of metamorphic rocks. Now, keep in mind that because there is a somewhat direct contact with the heat source, contact metamorphism takes place over a relatively small area.

Regional Metamorphism

It is a different story for our second type of metamorphism: regional metamorphism. This is a type of metamorphism where rock minerals and texture are changed by heat and pressure over a wide area or region. This name is also fairly easy to recall if you remember that regional metamorphism takes place over a wide region.

In fact, this type of metamorphism is often associated with the building of mountain ranges. To explain this type of metamorphism it helps to recall that the Earth’s surface and upper layers are made up of plates. These plates can slide toward each other, and as you can imagine, at the boundary where these plates meet, there is an intense amount of pressure as well as some heat. This is much like the pressure and heat that would build up if you pressed your hands together for a long time.This pressure and heat is so intense that it causes the rocks found along the boundary to change or metamorphose into metamorphic rocks. So we see that contact metamorphism and regional metamorphism not only differ by the amount of area over which they occur, they also differ by the amount of pressure involved. Contact metamorphism does happen underground and therefore there is some pressure involved, but it is not nearly as much as we see with regional metamorphism.

Lesson Summary

There are two types of metamorphism: contact metamorphism and regional metamorphism. Contact metamorphism is a type of metamorphism where rock minerals and texture are changed, mainly by heat, due to contact with magma. Regional metamorphism is a type of metamorphism where rock minerals and texture are changed by heat and pressure over a wide area or region.

Contact and regional metamorphism differ in both the area and pressure involved. With contact metamorphism we see rocks change, mainly due to heat in a relatively small region. With regional metamorphism we see rocks change, due to heat and pressure, over a wide region.

Learning Outcomes

After concluding this lesson, you should be able to:

  • Define metamorphism
  • Describe contact metamorphism and contact with magma
  • Identify that regional metamorphism takes place over wide areas due to pressure from earth’s plates
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