Rocks are any naturally occurring solid masses or aggregates of minerals or mineraloid matter (no consistent crystal structures). They are categorized by the minerals included, their chemical composition and the way in which they  form. Rocks are usually grouped into three main groups:

  • igneous rocks, 
  • metamorphic rocks and 
  • sedimentary rocks. 

Rocks form the Earth's outer solid layer, the crust.

Igneous rocks are formed when magma cools in the Earth's crust, or lava cools on the ground surface or the seabed. 


Pillow basalts ( a type of Igneous rock) on the Pacific seafloor (Hawaii): Courtesy of NOAA/wikipedia

Metamorphic rocks are formed when existing rocks are subjected to such large pressures and temperatures that they are transformed — something that occurs, for example, when continental plates collide.  Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite.

Sedimentary rocks are formed by diagenesis (changing from one type of sedimentary rock to another type of sedimentary rock) or lithification (compacting and releasing fluid) of sediments, which in turn are formed by the weathering, transport and deposition of existing rocks.


Lower Antelope Canyon was carved out of the surrounding sandstone by both mechanical weathering and chemical weathering. Wind, sand, and water from flash flooding are the primary weathering agents. image: Moondigger/wikipedia

Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation:

  •  clastic sedimentary rocks, 
  • biochemical (biogenic) sedimentary rocks, 
  • chemical sedimentary rocks, and 
  • a fourth category for "other" sedimentary rocks formed by impacts, volcanism, and other minor processes.

Sedimentary rocks include: Conglomerates and breccias , Sandstones, Mudrocks, Biochemical sedimentary rocks (limestone, coal, chert such as microorganism exoshells/diatoms)

The scientific study of rocks is called petrology (from the greek word petros), which is an essential component of geology

Source adapted from: Wikipedia contributors. (2019, July 21). Rock (geology). In Wikipedia, The Free Encyclopedia. Retrieved 03:19, July 29, 2019, from