Metamorphic rocks are formed when rocks are subjected to high temperature and pressure. These conditions can occur deep within the Earth, where the temperature and pressure are much higher than at the surface. Metamorphic rocks can also be formed when rocks are heated by lava or magma from a volcano.

The changes that occur to rocks during metamorphism can be physical, chemical, or both. Physical changes include changes in the texture, grain size, and mineral composition of the rock. Chemical changes include changes in the chemical composition of the rock, such as the addition or removal of certain elements.

The type of metamorphic rock that is formed depends on the composition of the original rock and the conditions of metamorphism. Some of the most common types of metamorphic rocks include:

* Slate: A fine-grained, foliated rock that is formed from the metamorphism of mudstone or shale.

* Phyllite: A fine-grained, foliated rock that is formed from the metamorphism of slate.

* Schist: A medium-grained, foliated rock that is formed from the metamorphism of phyllite or other fine-grained metamorphic rocks.

* Gneiss: A coarse-grained, foliated rock that is formed from the metamorphism of schist or other coarse-grained metamorphic rocks.

* Quartzite: A hard, non-foliated rock that is formed from the metamorphism of sandstone.

* Marble: A hard, non-foliated rock that is formed from the metamorphism of limestone or dolomite.

Metamorphic rocks are important because they provide information about the conditions that existed deep within the Earth. They can also be used to date geological events, such as the formation of mountain ranges.