Basics of Geology
Rock types
Rock types are divided into three main groups based on their origin: igneous rocks, sedimentary rocks, and metamorphic rocks. These groups together form the rock cycle, in which rocks can transform into other types through geological processes (Stanley, 1999).
Igneous rocks
Igneous rocks make up a large part of the Earth’s crust and play an important role in shaping the Earth’s surface (Gradstein et al., 2020). Igneous rocks form when magma cools and crystallizes.
Igneous rocks are classified into intrusive (plutonic) rocks and extrusive (volcanic) rocks. Examples of intrusive rocks include granite and gabbro, while extrusive rocks include basalt, andesite, and rhyolite.
Sedimentary rocks
Sedimentary rocks are formed when material transported by erosion and weathering accumulates and solidifies. Sedimentary rocks preserve information about Earth’s history and the evolution of life, as they often contain fossils (Stanley, 1999).
Sedimentary rocks can be divided into three main groups, which make up 95% of all sedimentary rocks: mudstones, sandstones and conglomerates, and carbonate rocks (Tieteen termipankki, 2025).
Sedimentary rocks can also be classified based on their origin into three categories:
Clastic sedimentary rocks are formed from mechanically weathered fragments of pre-existing rocks. This group includes mudstones, sandstones, claystones, breccias, and conglomerates.
Chemical sedimentary rocks are formed when dissolved minerals crystallize. Examples of chemical sedimentary rocks include some types of limestone and banded iron formations.
Organic and biogenic sedimentary rocks are formed from organic matter. Organic sedimentary rocks originate from plants, bacteria, and the remains of animals that have been buried and compacted over time (Stanley, 1999). Coal is an example of a rock formed from plant remains. Similarly, oil shale forms from the remains of algae, plankton, and microscopic organisms. Biogenic sedimentary rocks form when organisms produce minerals in their shells, bones, and structural parts, which eventually harden into rock (Gradstein et al., 2020).
Metamorphic rocks
Metamorphic rocks are types of rocks that have undergone transformation due to high pressure and temperature deep within the Earth’s crust. Metamorphic processes can alter igneous rocks, sedimentary rocks, or pre-existing metamorphic rocks into new rocks without melting them. The most notable examples include marble, gneiss, schist, and quartzite. (Stanley, 1999; Gradstein et al., 2020)
Sources
Gradstein, F. M., Ogg, J. G., Schmitz, M., & Ogg, G. (2020). Geologic Time Scale 2020. Elsevier.
Stanley, S. M. (1999). Earth System History. W.H. Freeman & Company.