Metamorphic Rocks

Metamorphism usually happens where plates are coming together; rocks are heated and are under high pressure. Click on image for full size (18K GIF) L.Gardiner/Windows Original

Have you have heard that caterpillars metamorphose into butterflies? Well, rocks can metamorphose too! They don't grow wings like a butterfly. But they do change! Rocks metamorphose when they are in a place that is very hot and pressure is high. You can find such a place where Earth's tectonic plates are coming together. There, the colliding plates squish rocks, and hot pools of magma heat them deep underground.

Some rocks only change a little, while others change a lot. When a rock is metamorphosed, its mineral crystals change. Usually, the same chemical ingredients are used to form new crystals during metamorphism. Sometimes new types of minerals grow that weren't in the rock before.

Often, flat minerals like mica become lined up perpendicular (at a right angle) to the direction of pressure. When minerals within a metamorphic rock are organized this way, it is called foliation. Some metamorphic rocks are foliated and others are non-foliated.

Any type of rock, can be metamorphosed. The rocks are changed either in small areas of contact metamorphism or large areas of regional metamorphism.

Figure out foliation!
Try this experiment to figure out how minerals become oriented when a rock is under pressure.

1. Add mica flakes to a piece of play dough. The dough will represent a rock undergoing metamorphism.
2. Knead the dough until the mica flakes are well-incorporated roll your dough into a ball.
3. Break the ball of dough in half. The flakes will be oriented in all different directions just as they would be in an unmetamorphised rock.
4. Take one half of the ball and flatten it with the palm of your hand. The pressure that you are applying to the dough is like the pressure placed on our rocky Earth by the forces of plate tectonics.
5. Break the flattened dough in half and look at what has happened to the mica flakes!