Metamorphic rocks

What are metamorphic rocks?

The word "metamorphosis" means "change".

Metamorphic rocks are existing rocks that have been changed. The rocks may be changed by heat. When this happens, all the minerals in the rock melt and then form new crystals.

Sometimes rocks are changed by huge pressure. The pressure squeezes the rocks so much that the new rock has lots of thin layers.

Sometimes rocks are changed by heat and pressure together. Then the new rock has different minerals and lots of layers.

What causes the heat and pressure?

Layers of sedimentary rocks buried beneath the Earth's surface. Magma rises up from the mantle and forms an intrusive igneous rock mass, this huge mass of extremely hot liquid rock pushes solid rocks aside. This causes all the surrounding rocks to be affected by the heat and the pressure. These rocks are changed into metamorphic rocks.

Types of metamorphic rock

Let's see which metamorphic rocks form when sandstone, limestone, shale and granite are heated until they melt and are compressed by pressure.

Quartzite

When sandstone is heated, all the sand grains melt and make new quartz crystals. This forms the metamorphic rock called quartzite. It looks similar to sandstone but the crystals shine and the rock is much harder than sandstone.

This rock is so hard that it is sometimes used to make sculptures.

Marble

When limestone is heated, it changes into a metamorphic rock called marble. All the minerals melt and form new crystals. Marble is extremely hard and very shiny. It can be white, red, blue, yellow, brown or green depending on which substances were in the original limestone. Marble is a very beautiful rock and it lasts for a very long time. People use marble to carve statues and gravestones and decorate public buildings and places of worship.

Slate

When shale is put under intense pressure it forms thin layers - this is the metamorphic rock called slate. Remember that layers of sediments build up on the sea bed. The bottom layers are buried by the top layers. If there are many layers, the bottom layer is put under huge pressure from the layers above. Burial changes the rock into a metamorphic rock with many layers like slate. People use slate for making roof tiles and floor tiles because it is hard and long-lasting.

Gneiss

When granite is heated, all the minerals melt and make new crystals. Pressure causes the new rock to have stripes or bands of crystals. The metamorphic rock is called gneiss. Gneiss is very hard. People crush this metamorphic rock to make roads.