Explore rock layers, learn dating methods, and discover how stratigraphy reveals the story of Earth's geological past.
Stratigraphy is the branch of geology that studies rock layers (strata) and their properties, with the goal of understanding Earth's history. By analyzing the sequences of rock layers, the fossils they contain, and the events that led to their formation, stratigraphy provides a window into the Earth's past spanning billions of years.
This principle states that in a sequence of undisturbed rock layers, the oldest layers are at the bottom and the youngest are at the top. Each new layer of sediment is deposited on top of previously existing layers.
This fundamental principle allows geologists to determine the relative ages of rocks without relying on absolute dating methods.
Sedimentary layers are typically deposited in nearly horizontal layers. If we observe tilted or folded layers today, they must have been deformed after deposition by tectonic forces.
This principle helps geologists identify rocks that have been deformed by mountain-building events or other tectonic processes.
A rock layer extends laterally until it thins out or encounters a different depositional environment. Even if a layer is not continuously visible, we can infer its presence across regions based on its properties and fossil content.
This principle allows geologists to correlate rock layers across distances and reconstruct ancient environments.
Geological structures (like faults or igneous intrusions) that cut through existing rocks are younger than the rocks they cut through. This allows us to determine the relative ages of features.
For example, a granite intrusion that cuts through sandstone layers must be younger than the sandstone.
Fossils appear in a predictable sequence through time. Different time periods are characterized by distinct fossil assemblages. This allows geologists to date rock layers based on the fossils they contain.
Index fossils—fossils that are distinctive and wide-ranging—are particularly useful for correlating and dating rock sequences.
Relative dating determines whether one rock or fossil is older or younger than another without specifying an actual age. Methods include:
Absolute (radiometric) dating provides actual ages in years based on the radioactive decay of elements in rocks. Common methods include:
These methods rely on known decay rates and the assumption that the system has remained closed since the rock formed.
The geologic time scale is a chronological representation of Earth's history, divided into eons, eras, periods, and epochs. The major divisions include:
Earth's earliest time. Simple life forms emerged. Little oxygen in atmosphere.
More complex organisms developed. Oxygen accumulated in atmosphere. Multicellular life emerged.
Characterized by abundant visible fossils. Divided into three major eras:
Unconformities are surfaces of erosion or non-deposition that represent missing time in the rock record. They reveal that not all time is represented in every location:
Younger layers rest on tilted older layers. Indicates deformation between deposits.
Younger layers rest on eroded older layers with roughly parallel surfaces. Indicates erosion.
Sedimentary layers rest on eroded igneous or metamorphic rock. Indicates long gap in time.
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