Summary
Highlights
Rocks are classified by their formation. Igneous rocks form from cooling magma, sedimentary rocks from compacted sediments, and metamorphic rocks undergo change (metamorphosis) due to intense heat or pressure.
Contact metamorphism occurs when existing rocks come into contact with intense heat, typically from magma or lava intrusions. The heat 'burns' and changes the surrounding rocks.
Regional metamorphism is primarily caused by immense pressure, often found at convergent plate boundaries where two plates collide. This pressure metamorphoses the rocks trapped between them.
Shale, a sedimentary rock, can progressively metamorphose into slate, phylite, schist, and then gneiss with increasing heat and pressure. Gneiss often exhibits distinct bands of minerals due to intense pressure. Further heat and pressure beyond gneiss can lead to melting and the formation of igneous rocks.
Information about metamorphic rocks, including types of metamorphism, composition, crystal size, banding, rock names, and map symbols, can be found in reference tables on page 7.