Geological formation

A geological formation, or simply formation, is a large body of rock with a similar set of physical features (lithology). These features set it apart from the rock around it. It has a certain place in the layers of rock found in an area (the stratigraphic column). Formations are the basic unit of lithostratigraphy, which is the study of strata, or rock layers.

A formation must be big enough to be mapped on the surface or traced underground. Its thickness does not define it, and it can vary a lot. Formations are usually flat and layered, but not always. A formation may be made of one type of rock, of alternating layers of two or more types, or of a mix of different rocks. It just needs to be different from the rock around it.

The idea of a geological formation goes back to the start of modern science about the Earth. Abraham Gottlob Werner used the term in his theory of how the Earth formed. He developed this theory between 1774 and his death in 1817. Over time, the idea became more exact. Today it is written down in guides such as the North American Stratigraphic Code and similar guides in other regions.

Geological maps show where different formations appear at the surface. These maps are useful in fields such as structural geology. They help geologists understand how the Earth's crust has moved in an area. They also help predict where valuable minerals may be found underground.

Defining formations

Formations are drawn so that the rock inside each one is as similar as possible. They are defined by their rock type, which is called lithology. This includes the rock's chemical and mineral makeup, its texture, its color, and its depositional structures. It can also include fossils that act as rock pieces, or plant-like materials such as coal or kerogen. The taxonomy of fossils cannot be used to define a formation. How different neighboring rocks must be depends on how complicated the geology of an area is. A formation must be shown on geological maps for its region, and its thickness can range from less than a meter to several thousand meters.

Formations are usually named after a lasting natural or human-made feature of the place where they were first described. The name combines that place name with either "Formation" or a describing word. For example, the Morrison Formation is named for the town of Morrison, Colorado, and the Kaibab Limestone is named after the Kaibab Plateau in Arizona. A new name cannot copy an existing formation name, and the first name used for a formation takes priority. A formal formation also has a stratotype, which is usually a type section that shows the full thickness of the rock. Other reference sections may be added if the formation is not fully visible in one place or changes a lot from side to side. Geologists may also use biostratigraphic units based on paleontological evidence, chronostratigraphic units based on the age of rocks, and chemostratigraphic units based on chemistry. These are useful, but they are not defined in the same way as formations.

Usefulness of formations

Scientists who study rock layers, called strata, use the idea of formations. The study of these layers is called stratigraphy. A formation is the basic unit of stratigraphy. Formations may be grouped into groups of strata or divided into members. Members do not need to be shown on maps at the same scale as formations. However, they must look clearly different from the rock around them when they are present.

Defining and recognizing formations helps geologists match up rock layers that are far apart. This can be done between outcrops and other places where rock strata can be seen. Formations were first used as important markers of geologic time. Scientists used their relative ages and the law of superposition to do this. The parts of the geological time scale were described and put in order by scientists in the 18th and 19th centuries. Formations can be defined for sedimentary rock layers, weaker metamorphic rocks, and volcanic rocks. Some rocks, such as intrusive igneous rocks and very strongly changed metamorphic rocks, are usually not called formations. Instead, they are described as lithodemes.

Other uses of the term

"Formation" is also used informally to describe the odd shapes that rocks take on. These shapes come from natural forces that wear rocks away or build them up. Once the force stops acting on a formation, it no longer changes. Some well-known cave formations include stalactites and stalagmites.