Geologists hammer golden spikes into cliffs to mark the edges of time
To define where one slice of Earth's history ends and the next begins, geologists choose a precise point in a real rock face and call it a golden spike. The boundary that closed the age of dinosaurs is fixed at Oued Djerfane in Tunisia, marked by iridium from the Chicxulub asteroid, a layer found worldwide.
The geologic time scale divides Earth's roughly 4.54 billion years into standard intervals. It is maintained by the International Commission on Stratigraphy, part of the International Union of Geological Sciences, and combines two approaches: ordering rock layers by their relative position, and dating them with techniques such as radiometric measurement. Regional schemes once competed because equivalent rocks look different from place to place, so the commission has worked to pin each boundary to a single reference section, formally a Global Boundary Stratotype Section and Point. Everything above the spike belongs to the younger interval.
Numbers can move while the spikes stay put. In early 2022 the start of the Cambrian was revised from 541 to 538.8 million years ago, yet the physical boundary in the rock did not change; only its measured age did. Ages are written in Ma, millions of years, so the base of the Jurassic, at 201.4 million years plus or minus 0.2, carries an uncertainty of 200,000 years. The oldest divisions, from the Hadean through most of the Proterozoic, are still defined by round-number ages rather than rock features.
The ordering rests on common-sense principles. Undisturbed layers get younger upward. Sediments settle roughly flat and spread sideways until they thin out. A rock that cuts across another must be younger, fragments inside a rock must be older than it, and gaps called unconformities record erosion or pauses in deposition. Fossils follow one another in a dependable sequence, letting geologists match strata across oceans.
The hierarchy runs from four eons through ten eras and 22 periods to 37 formal epochs and 96 formal ages. We live in the Quaternary Period, Holocene Epoch and Meghalayan Age. Names mix logic and history: Paleozoic, Mesozoic and Cenozoic mean old, middle and new life, while Cretaceous refers to a type of rock, Permian to a place and Ordovician to an ancient tribe. The commission now wants new divisions named after geographic features near their reference sections.
Source: Geologic time scale
