SRAC Presents: Mineralogy for Kids

Sedimentary Rocks: In this session - Ted Keir will explain rocks formed from sedimentation

In most places on the surface of the earth, the igneous rocks which make up the majority of the crust are covered by a thin layer of loose sediment or debris. Sedimentary rock is made as layers of this debris get compacted and cemented together with other igneous rocks. Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks:

Clastic: your basic sedimentary rock. Clastic sedimentary rocks are accumulations of clasts: little pieces of broken up rock which have piled up and been "lithified" by pressure and cementation.

Chemical: many of these form when standing water evaporates, leaving dissolved minerals behind. These are very common in arid lands, where seasonal lakes occur. Thick deposits of salt and gypsum can form due to repeated flooding and evaporation over long periods of time.

Organic: any accumulation of sedimentary debris caused by organic processes. Many animals use calcium for shells, bones, and teeth. These bits of calcium can pile up on the seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock.