Geologic time scale

The geologic time scale divides Earth’s history into nested intervals, from eons to ages, using geological and biological evidence to mark boundaries.

Eon: The largest formal unit of geologic time, containing multiple eras. Eons form the scale’s broadest divisions.
Global Boundary Stratotype Section and Point: An internationally designated reference point defining the lower boundary of a geological stage. Many formal boundaries are anchored to a specific rock section and marker.
Precambrian: The informal span of Earth history before the Cambrian Period, encompassing most of the planet’s existence. It contains the scale’s vast early history before abundant complex fossils appear.
Nicolaus Steno: A seventeenth-century Danish scientist whose principles of superposition and original horizontality helped establish stratigraphic reasoning. His principles supplied basic rules for reading the sequence of layered rocks.
Anthropocene: A proposed geological epoch defined by the substantial influence of human activity on Earth’s systems. Its status as a formal unit remains disputed, unlike the established Holocene.
Era: A formal unit of geologic time smaller than an eon and larger than a period. Eras subdivide eons into major chapters of Earth history.
Radiometric dating: A method for determining the age of materials from predictable radioactive decay. It supplies numerical ages for boundaries first identified in rock sequences.
Cambrian explosion: A geologically rapid diversification of animal life evident in the fossil record around the beginning of the Cambrian Period. Its fossil record helps distinguish the Cambrian’s distinctive biological transition.
Georges Cuvier: A French naturalist who used comparative anatomy and fossils to establish extinction as a fact. Fossil succession and extinction became evidence for divisions in Earth history.
Golden spike: The informal name for a physical marker used to identify a Global Boundary Stratotype Section and Point. Debates over proposed boundaries often turn on choosing a clear, correlatable marker.

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