Major Groups of Rocks

From Ohio History Central

Geologists have named three major groups of rocks on the planet Earth. Within each group a particular source provides parent material for the formation of rocks. And a natural process forms rocks from that parent material. The three groups are defined as follows:

Igneous

Source of Material

Magma, the extremely hot, molten rock that occurs in the Earth’s interior. Liquid magma also may contain suspended grains of minerals and dissolved gasses.

Forming Process

Cooling and consolidation.

Sedimentary

Source of Material

Material in solution in either fresh or sea water, or particles of matter suspended in water.

Forming Process

Precipitation or deposition.

Metamorphic

Source of Material

Either igneous or sedimentary rock.

Forming Process

Change resulting from high temperatures, high pressures, or both.

Ohio has all three kinds of rocks. Only sedimentary rocks, however, make up the bedrock at the surface of the state. This means that all of the surface bedrock of Ohio was laid down under water, either fresh or marine. Therefore, at sometime in the geologic past, all of what is now Ohio was covered with water for long periods of time.

Both metamorphic and igneous rocks lie on top of the surface of Ohio. But these rocks are not attached to the solid bedrock of the state. The great glaciers of the last ice age carried these pieces of metamorphic and igneous into what is now Ohio, leaving them behind as boulders, stones and pebbles.

Igneous, metamorphic and sedimentary rocks lie deep beneath the surface of Ohio, covered by thick layers of younger sedimentary rocks. These deeply buried rocks form the foundation on which ancient bodies of water deposited the layers of sedimentary rock that we see today on the surface. And the upper sedimentary layers provided the foundation on which Ohio’s soils of today developed.

Therefore, the foundation on which the State of Ohio was formed and grew, the ground on which Ohio’s residents today live, work and play, is sort of like a layer cake. The oldest, bottom part is made up of igneous and metamorphic rocks; the not-so-old middle layers consist of sedimentary rock; and the relatively recent upper region contains soil mixed with material dropped by the great glaciers.

Resources

  • Coogan, Allan H. Ohio’s Surface Rocks and Sediments; Chapter 3 in Fossils of Ohio, edited by Rodney M. Feldmann; Ohio Geological Survey, Bulletin 70, 1996.

See Also