Ask-a-Geologist Q&A #16: What Types of Rocks Were Changed by High Temperature and High Pressure?

What rocks were changed from high temperature and high pressure?

This Ask-a-Geologist question was sent to us by a reader who is interested in the rock metamorphism that caused a rock to change from one type to another while it is underground. Cory wrote to us and asked, “What is the term for a rock that has changed from high heat and high pressure?”

Well Cory, what you’re talking about is a metamorphic rock. Metamorphic rocks used to be one thing, like maybe an igneous rock or sedimentary rock, and over time they are changed from temperatures and pressures and also from mineral-rich waters in the areas into another type of rock.

Sedimentary Sandstone Rock before metamorphism.

So a good example of this is a sandstone. It used to be a sedimentary rock, and over time, from high heat and pressure, it will change into a quartzite, which is a metamorphic version of that sandstone.

Quartzite metamorphic rock that used to be sandstone

Now, this term comes from the Greek words, meta, which means to change, and morph, which means form. So metamorphic means a rock that has changed its form. Geologists will talk about several different types of metamorphism, but there are a couple that you might be interested in.

Types of Rock Metamorphism

There are many types of metamorphism. Four that you may be interested in learning about are Contact metamorphism, Regional metamorphism, Cataclastic metamorphism, and Hydrothermal metamorphism.

Contact Rock Metamorphism

The first one is contact metamorphism. Now, this occurs around cooling magma chambers. Magma chambers are associated with volcanoes. They have super hot liquid rock, and as these areas start to cool, they alter the rocks that are along the edges of the magma chambers. So if you picture it, there’s a big magma chamber underground. The rocks that are already there surrounding it are subject to this high heat from this liquid rock in the magma chamber, and also from a lot of the mineral-rich waters that are in the area, and they change along these boundaries. It’s usually over small areas, but it does change along these boundaries of the cooling magma.

Regional Rock Metamorphism

Another type is regional metamorphism. Now, this occurs over huge areas at a time. It can occur over thousands of square miles at a time. It usually occurs along plate boundaries where one is being pushed under another. It can also occur in areas where there’s just a huge buildup of rock layers and sediment on top of other rocks. And then as these rocks travel deeper into the earth’s crust, the heat goes up because the deeper you go, the hotter it is, and then the pressure builds up from the rocks on top of the original rocks. This change in regional metamorphism usually occurs from high heat and high pressure, not as much from the water solutions. It can also associate a lot with mountain buildings, and it causes just a huge change over a very, very large area all at once.

Cataclastic Rock Metamorphism

Another type that you might be interested in is called cataclastic metamorphism. It occurs along fault zones. In these areas where the faults occur, there’s a lot of pressure on the rocks, and because of this pressure, it can cause something called brittle deformation. Really that’s a fancy word for the rocks tend to break and they break apart in these areas, and the pressure causes the change of the rocks in the area. This type of metamorphism isn’t really subject to a lot of temperature changes, but just from pressure changes.

Hydrothermal Rock Metamorphism

And then the fourth type that you might be interested in is called hydrothermal metamorphism. And this really occurs in areas where there’s not a whole lot of pressure, not a whole lot of high temperatures, but it occurs from mineral-rich water solutions flowing through the area. And these water solutions actually cause a chemical change in the existing rock, and they change into a new metamorphic form.

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