Stratigraphy

Stratigraphy is a branch of geology (the science of the Earth) that focuses on the layers of rock—especially sedimentary rocks formed from the breakdown and settling of matter—how they are positioned, what they are made of, and when they formed. The term comes from "stratum," meaning layer.

The primary purpose of stratigraphy is to construct a chronological sequence: each rock layer corresponds to a period of time, and by comparing layers from different locations, a picture emerges of how the landscape changed over millions of years. It is also used to search for petroleum, natural gas, groundwater, and minerals.

Basic principles

The principle of superposition states that when layers are undisturbed, the lower layer is older than the one above it. The principle of original horizontality holds that sedimentary layers were originally deposited horizontally; if they are now tilted, they have been disturbed afterward. The principle of cross-cutting shows that anything cutting through a layer—such as a fault or an intrusion—is younger than that layer.

Subdivisions

Lithostratigraphy divides layers by their appearance and composition; the basic unit is called a formation. Biostratigraphy uses fossils to compare layers at distant locations; widespread species that existed for a limited time are called index fossils. Chronostratigraphy assigns layers to defined time intervals. There is also magnetostratigraphy, based on changes in the Earth's magnetic field preserved in rocks.

Time and chronology

Stratigraphy provides relative time (one thing is older than another), whereas radiometric dating—based on the decay of radioactive atoms—gives ages in years. The two are combined to construct the Geologic Time Scale, divided into eons, eras, periods, and epochs. Locations worldwide where the boundary between two periods is formally defined are called Global Stratotype Section and Points and lie on the Earth.

Application in related fields

Archaeology uses a similar method: excavations are divided into layers, and each artifact found is dated according to the layer it came from. Paleoclimatology likewise relies on layers of ice and sediment to detect changes in past climate.