Major groups of sedimentary, metamorphic, and igneous rocks

Major Sedimentary Rock Groups

Clastic Sedimentary Rocks, like sandstone and shale, form from consolidated fragments of existing rocks. These fragments are often weathered debris.
Chemical Sedimentary Rocks, including evaporites such as halite and gypsum and precipitates like limestone and flint, form when minerals precipitate out of solution.
Organic Sedimentary Rocks are composed of the remains of living organisms. Examples are coal, created from compressed plant remains, and coquina, formed from cemented shells.

Major Metamorphic Rock Groups

Foliated Metamorphic Rocks exhibit banding or layering, often appearing striped or lined. These include gneiss and schist. The alignment of minerals in this process results in the layered effect.
Non-Foliated Metamorphic Rocks lack bands or other directional features. Examples include marble and quartzite. They are the result of pressure and heat without deformation.
Metasomatism is the process where a rock changes composition via fluid transport of chemicals. This produces rocks like skarn.

Major Igneous Rock Groups

Intrusive Igneous Rocks are formed from magma that cools slowly beneath the Earth’s surface, allowing large crystals to form. Granite is an example.
Extrusive Igneous Rocks cool from lava at the surface quickly, preventing large crystal growth. An example of this group is basalt.
Types of igneous rock can also be classified based on silica content: Felsic rocks (rich in silica, e.g. granite), Intermediate rocks (moderate silica level, e.g. andesite), Mafic rocks (lower silica content, e.g. basalt) and Ultramafic rocks (very low silica level, e.g. peridotite).

The principles of geology involve analysing these rock types and their features to understand Earth’s history. This knowledge is used in fields from geological hazard prediction and mitigation, to the extraction of valuable fuel and mineral resources.