Stratum

A stratum is a distinct layer of rock or sediment, uniform in composition, color, or grain size. Strata typically form when rock-forming material such as sand, clay, or limestone accumulates in water or on land and then compacts and solidifies over a long period.

The study of strata and their arrangement is called stratigraphy. It is one of the foundations of geology, because the order of strata allows scientists to sequence events in time.

Formation

Most strata are sedimentary. Weathered material from rocks is carried by water, wind, or ice and settles in low-lying places such as sea beds, basins, or river deltas. As new sediment accumulates on top, the underlying material compacts and hardens into solid rock. Strata can also form from volcanic activity, such as ash layers deposited from eruptions.

Principles of stratigraphy

A fundamental principle is original superposition: if strata have not been disturbed, the lowest layer is oldest and the uppermost is youngest. Another principle is original horizontality—strata are deposited roughly horizontal. When a stratum is later found tilted or folded, this indicates past earth movement or tectonic activity.

Fossils and dating

Many strata contain fossils of ancient life. Some fossils—called index fossils—existed for only a short time but spread over a wide area, making them useful for correlating strata from different locations. Other methods such as radiometric dating are used when volcanic rocks are present in the sequence.

Significance

Strata are used to locate mineral resources such as oil, gas, groundwater, and ore deposits. They also help assess earthquake hazard and reconstruct past climates on Earth.