Metamorphic+Rocks

Metamorphic Rocks

Rock that is changed by high temperature and pressure without being melted is called metamorphic rock. Most metamorphic rocks are very hard. Many metamorphic rocks have mineral crystals that are lined up or arranged in stripes or bands. High temperature softens mineral crystals. Then high pressure locks them together. Sometimes the crystals flow into stripes.

To understand how pressure can change a rock, suppose a handful of small clay pellets are loosely gathered into a ball. Someone places several heavy books on the ball. What will happen? The clay will become a dense, flat sheet.

Now suppose the clay ball is a rock deep inside Earth. The weight of everything above the rock squashes it. The rock is also very hot, because temperature rises as you go deeper inside Earth. The high temperature makes the rock soft. So the rock, like the clay, changes shape. It becomes denser. It also may form layers and folds.

Metamorphic rocks form in other places, too. They form deep under the ocean floor, where there is a lot of pressure. They also form near volcanoes, where the temperature is high. The picture on these pages shows how different metamorphic rocks form.

Slate Gneiss

Marble

Click here for a video on how metamorphic rocks are formed:

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