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The internationally agreed calendar of Earth's 4.6-billion-year history, divided into eons, eras, periods, epochs and ages by rock and fossil evidence.
The Geological Time Scale is the system of chronological dating that divides Earth's roughly 4.6-billion-year history into named intervals, based on the sequence of rock strata and the fossils they contain. It is maintained by the International Commission on Stratigraphy (ICS), a body of the International Union of Geological Sciences (IUGS), which ratifies every boundary. The units run in a nested hierarchy — eon, era, period, epoch, age — from the largest to the smallest, and each boundary is anchored either to a physical reference point in a rock section called a Global Boundary Stratotype Section and Point (GSSP, informally a 'golden spike') or, for the oldest intervals, to a fixed numerical age.
Type: ConceptFive-level hierarchy — Eon > Era > Period > Epoch > Age, each nested inside the one above it
Four eons — Hadean, Archean, Proterozoic and Phanerozoic; the first three together are informally called the Precambrian and cover about 88% of Earth's history
The Phanerozoic Eon, beginning about 539 million years ago with the Cambrian explosion of visible life, has three eras — Palaeozoic, Mesozoic and Cenozoic
Boundaries are defined by GSSPs, physical 'golden spikes' driven into a specific rock section chosen as the global reference for that boundary
Mass extinctions mark several major boundaries — the Permian-Triassic and the Cretaceous-Palaeogene extinction that ended the non-avian dinosaurs about 66 million years ago
We currently live in the Phanerozoic Eon, Cenozoic Era, Quaternary Period, Holocene Epoch and Meghalayan Age
The scale is revised as evidence accumulates — the Holocene was formally subdivided into three ages only in July 2018
Frequency: Moderate to high — geological time appears in UPSC Prelims physical geography, and the Meghalayan Age and the 2024 Anthropocene rejection are recurring current-affairs hooks
About 4.6 billion years
Hadean, Archean, Proterozoic, Phanerozoic
About 539 million years ago
Palaeozoic, Mesozoic, Cenozoic
About 66 million years ago — end of the non-avian dinosaurs
About 11,700 years ago
11,700 to 8,200 years ago
8,200 to 4,200 years ago
4,200 years ago to the present; GSSP at Mawmluh Cave, Meghalaya
Rejected as a formal epoch by the ICS in March 2024; upheld by the IUGS
A boundary in geological time is not simply a number of years. Wherever possible it is defined by a Global Boundary Stratotype Section and Point — one specific, physically accessible rock section somewhere in the world, at one specific layer, agreed by the ICS as the global reference for that boundary. Every other rock sequence on Earth is then correlated against it. For the base of the Meghalayan Age the reference is a stalagmite in Mawmluh Cave near Cherrapunji, whose oxygen-isotope record captures an abrupt drying about 4,200 years ago — the 4.2 kilo-year BP event, a two-century megadrought associated with the decline of several early civilisations, including the mature phase of the Indus Valley Civilisation. A second reference core from an ice sheet supplements it. This is also why the Anthropocene proposal failed: the Anthropocene Working Group nominated Crawford Lake in Canada with a 1952 start marked by plutonium fallout from nuclear testing, but the Subcommission on Quaternary Stratigraphy judged the evidence base too narrow to carry a formal epoch.
A GSSP is like a master metre-bar kept in a vault. Nobody argues about how long a metre is by re-measuring a table; they compare against the reference. Geologists do the same with time — they point to one agreed layer of rock and correlate everything else to it.
The Geological Time Scale is the common language in which all Earth history is written — it is how coal seams, oil-bearing basins, fossil beds and mountain-building episodes are dated and correlated across continents. For India, the scale carries direct national interest: the Deccan Traps erupted around the Cretaceous-Palaeogene boundary, the Himalaya began rising after the Indian plate collided with Asia in the Cenozoic, and the Gondwana rock system holds most of the country's coal. The Meghalayan Age gave India its first formally recognised unit of global geological time, and the reference site at Mawmluh Cave has since featured on India's tentative list for UNESCO recognition. The Anthropocene debate matters for a different reason: even though the ICS refused to ratify it in 2024, the term continues in wide scientific and policy use as shorthand for the era of dominant human influence on Earth systems — a distinction between formal stratigraphy and useful concept that makes a good Mains point.
Hierarchy: Eon > Era > Period > Epoch > Age; maintained by the ICS under the IUGS
Earth is about 4.6 billion years old; four eons — Hadean, Archean, Proterozoic, Phanerozoic
Precambrian = the first three eons, about 88% of Earth's history
We are in: Phanerozoic > Cenozoic > Quaternary > Holocene > Meghalayan
Holocene began about 11,700 years ago; split in July 2018 into Greenlandian, Northgrippian and Meghalayan
Meghalayan Age (4,200 years ago onward) has its GSSP in Mawmluh Cave, Cherrapunji, Meghalaya — the 4.2 ka BP megadrought
Anthropocene was REJECTED as a formal epoch in March 2024 — it remains an informal term
From largest to smallest: eon, era, period, epoch and age. Eons are divided into eras, eras into periods, periods into epochs and epochs into ages.
The Meghalayan Age, within the Holocene Epoch, Quaternary Period, Cenozoic Era and Phanerozoic Eon. The Meghalayan began about 4,200 years ago.
Because the Global Boundary Stratotype Section and Point for its base is a stalagmite in Mawmluh Cave near Cherrapunji in Meghalaya, whose isotope record captures the abrupt global drying of about 4,200 years ago.
No. In March 2024 the Subcommission on Quaternary Stratigraphy voted against the proposal and the IUGS upheld that decision, so the Anthropocene is not formally recognised. We remain in the Holocene, although the term is still widely used informally.
A Global Boundary Stratotype Section and Point is a single agreed rock section and layer, physically accessible somewhere in the world, that serves as the global reference for a boundary in geological time.