How does the geological time scale work?
A hierarchy of named intervals dividing Earth's history, defined by changes in the rock and fossil record rather than by round numbers — which is why the boundaries fall at irregular dates.
The hierarchy, from longest to shortest: eons, eras, periods, epochs and ages.
The four eons. The Hadean, Archean and Proterozoic together span roughly the first four billion years — around 88% of Earth's history — and are collectively informal referred to as the Precambrian. The Phanerozoic, meaning "visible life", covers the last 539 million years or so and contains everything most people know.
The Phanerozoic eras:
Palaeozoic — Cambrian through Permian, ending in the largest known extinction event.
Mesozoic — Triassic, Jurassic, Cretaceous. The dinosaurs.
Cenozoic — from 66 million years ago to now.
How boundaries are defined. Originally by fossil assemblages — the appearance or disappearance of particular species in the rock record. Modern practice defines boundaries by a Global Boundary Stratotype Section and Point (GSSP) — a physical marker, informally a "golden spike", driven into a specific rock layer at a specific location, chosen as the reference definition for that boundary worldwide.
Ages in years are then determined by radiometric dating and are revised as measurements improve, while the boundary itself is the rock layer, not the number.
Why boundaries fall where they do. They mark events — mass extinctions, major climatic shifts, significant evolutionary transitions. The Cretaceous–Palaeogene boundary is defined around the extinction event, and is marked globally by an iridium-rich clay layer.
The five major mass extinctions punctuate the scale, and are among the clearest boundary markers.
The Anthropocene — a proposed epoch marking human influence on the geological record. After lengthy deliberation, the relevant body rejected its formalisation as an epoch, a decision that remains contested; the term is widely used informally regardless.
The Holocene is the current formal epoch, beginning around 11,700 years ago with the end of the last glacial period.
The scale is maintained by the International Commission on Stratigraphy and is periodically revised.