Geological Time Scale: Geography Optional UPSC

• The geological time scale (GTS) is the standard calendar of Earth history: it divides about 4.567 billion years into eons, eras, periods, epochs and ages, each tied to a rock record and dated in millions of years ago (Ma).
  • It is built from two kinds of evidence: relative order (which rock layer and which fossil came first) and absolute ages from radioactive decay.
• The time scale starts where the story of the origin of the Earth ends, and it is the frame for dating every landform, orogeny, glaciation and extinction.
• The International Commission on Stratigraphy (ICS) keeps the official version, the International Chronostratigraphic Chart; the dates below follow its v2026/06 edition.

Age of the Earth

Why Dating the Earth Is Difficult

• Geological processes are so slow that no one sees them complete in a lifetime.
  • James Hutton (1788) wrote that Earth history shows “no vestige of a beginning, no prospect of an end”.
  • His idea that slow, present-day processes shaped the past (the root of uniformitarianism) demanded vast, “deep” time.
• Before radioactivity was understood, every estimate rested on an assumed constant rate of some process, and each gave a different answer.

Early Methods of Estimating the Age

Religious Estimates

• Archbishop James Ussher (1650), working from Biblical genealogies, placed Creation on 23 October 4004 BC, an Earth only about 6,000 years old.
• Hindu cosmology measures time in kalpas, manvantaras and yugas; the elapsed part of the present cycle works out to roughly 1.97 billion years.
  • This is closer to the scientific order of magnitude, but it is scriptural deduction, not measurement.

Oceanic Salinity

• Principle: the first oceans were fresh; rivers have since carried dissolved salts from weathered land, so age of oceans = total salt in the oceans ÷ salt added per year.
• John Joly (1899) used the sodium budget and got about 80–90 million years for the oceans.
  • Adding an assumed interval before the oceans formed still gave an Earth of only about 120 million years, far too young; ocean-floor rocks alone are dated to about 180–200 million years.
• A variant allowed for faster salt supply during mountain-building episodes, raising the ocean’s age to about 1,500 million years and the Earth’s to about 1,875 million years.
• Weaknesses:
  • Salt is recycled: it is buried in evaporites and sediments, so the ocean is not a simple accumulating tank.
  • Rates of weathering vary in space and time, and hydrothermal vents at ridges also add dissolved salts.

Sedimentation

• Principle: divide the total thickness of the world’s sedimentary rocks (often taken as about 100 miles or 160 km) by the yearly rate of deposition, then multiply for the time before the first sediments.

• Weaknesses:
  • Deposition rates differ with climate, relief, lithology and structure.
  • Old sediments are compacted under younger load and eroded again, so the true total thickness cannot be measured.
  • Gaps (unconformities) record time with no rock at all.

Erosion

• Principle: land is lowered about 1 foot in 10,000 years; eroding the 100-mile sediment pile would take about 5.28 billion years, and doubling it gives an Earth of over 10 billion years.
• The result is too old; erosion rates vary with lithology, structure, relief, vegetation, gradient and climate, so no single rate holds.

Tidal Friction of the Moon

• George Howard Darwin (1879) argued that the Moon split from a fast-spinning Earth and has receded ever since, while tidal friction slowed Earth’s rotation.
• Back-calculating the recession gave ages of about 4 billion years.
• Correction: laser ranging now measures the recession at about 3.8 cm a year; projected back uniformly this gives about 10 billion years, which shows the rate has not been constant (it depends on ocean-basin shape and tidal resonance). The fission origin of the Moon is also rejected in favour of a giant impact.

Lord Kelvin’s Cooling Earth

• William Thomson, Lord Kelvin (1862) assumed the Earth began fully molten (about 7,000 °F) and has cooled by conduction, and used the geothermal gradient and rock conductivity to find when the crust solidified.
  • He first gave 20–400 million years, and later narrowed it to 20–40 million years (1897).
• Why it failed:
  • It ignored radiogenic heat: decay of uranium, thorium and potassium keeps the interior of the Earth hot, and radioactivity was unknown in 1862.
  • It ignored mantle convection, which carries heat up far more efficiently than conduction.

Other Early Estimates

• Pleochroic haloes: Joly also used the dark rings that radioactive inclusions burn into mica; their size and darkness hint at age.
• Henry Norris Russell (1921) used uranium–lead ratios in the crust and bracketed the Earth’s age at a few billion years.
• Biological evolution was once thought to need about 1 billion years.

Radiometric Dating: The Modern Answer

• Pierre Curie and Albert Laborde (1903) showed that radium releases heat; Ernest Rutherford (1904–05) proposed that radioactive decay could date rocks.
• Bertram Boltwood (1907) found that uranium decays to lead and obtained the first uranium–lead ages, up to about 2.2 billion years.
• Arthur Holmes (1913), in The Age of the Earth, built the first geological time scale with radiometric dates.
• Principle: a radioactive parent isotope decays to a stable daughter at a fixed rate, its half-life; the parent–daughter ratio in a mineral gives the time since it crystallised.

• Radiocarbon (C-14) dating measures the radioactivity of carbon in once-living matter; it dates only the latest part of the Quaternary, such as Holocene terraces, peat and archaeological sites.
• Relative dating still orders the rocks that radiometric ages then calibrate:
  • Nicolas Steno (1669): law of superposition, younger layers lie on older ones in undisturbed sedimentary rocks.
  • William Smith (1815): faunal succession, each fossil assemblage occupies a fixed place in the sequence; he drew the first geological map of a country.
  • Palaeomagnetic reversals recorded in lavas and ocean-floor stripes add another dated sequence.

The Accepted Age

• Clair Cameron Patterson (1956) measured lead isotopes in the Canyon Diablo meteorite and fixed the age of the Earth and meteorites at 4.55 ± 0.07 billion years; today’s value is about 4.54 billion years.
• The oldest solids of the Solar System (calcium–aluminium-rich inclusions in meteorites) date to 4.567 billion years, which the ICS takes as the base of the Hadean.
• Oldest terrestrial material:
  • Jack Hills zircon, Western Australia: 4.404 billion years, the oldest mineral.
  • Acasta Gneiss, Canada: about 4.0 billion years, among the oldest rocks.
  • Isua belt, Greenland: about 3.8 billion years.
• Indian example: the Singhbhum craton (Jharkhand–Odisha) preserves crust over 3.3 billion years old; zircon and sandstone evidence shows it rose above sea level about 3.3–3.2 billion years ago, among the earliest land on Earth.

The Faint Young Sun Paradox

• Carl Sagan and George Mullen (1972) noted that the young Sun was 25–30% fainter, which should have frozen the early Earth, yet sedimentary rocks show liquid water over 3.8 billion years ago.
• Explanations:
  • James Kasting (1993): a much stronger greenhouse, mainly a high concentration of carbon dioxide, kept the oceans warm.
  • Minik Rosing and colleagues (2010): minerals in 3.8-billion-year-old banded iron formations of Isua, Greenland, cap carbon dioxide at only a few times today’s level; instead the Earth absorbed more sunlight because of small continents (a dark ocean surface) and thinner clouds (few biological cloud-forming particles).
• Current view: the paradox is not fully settled; most models combine greenhouse gases (carbon dioxide and methane) with a lower albedo.

The Geological Time Scale

Units of Geological Time

• Time units (geochronologic) have matching rock units (chronostratigraphic): rocks formed during a period make up a system.

• Era: a long interval marked by a distinctive stage in the history of life, such as the Palaeozoic of “ancient life”.
• Period: the basic unit, during which one standard rock system forms; it contains two or more epochs.
• Epoch: a shorter interval with distinctive deposits, climate or life.
• Each boundary in the Phanerozoic is fixed by a Global Boundary Stratotype Section and Point (GSSP), the “golden spike”: one reference point in one rock section worldwide.
  • Numerical ages change as dating improves; only the GSSP defines a Phanerozoic boundary.
• Older schemes:
  • Rocks were once grouped as Primary, Secondary, Tertiary and Quaternary, and the Precambrian as the Archaeozoic (Azoic) and Proterozoic eras; later tables added the Palaeozoic, Mesozoic and Cenozoic, giving five eras.
  • Today the Archean and Proterozoic are eons, “Primary” and “Secondary” are obsolete, and “Tertiary” is informal, replaced by the Paleogene and Neogene.

The Geological Clock

• A geological clock shows all of Earth history as a spiral dial; in the version of Frank Press and Raymond Siever (1974) each turn is 1 billion years, each “hour” 100 million years and each “minute” 10 million years.
• Squeezed into one day, the story runs:
  • The Cambrian begins at about 9:10 pm.
  • Non-bird dinosaurs die out at about 11:39 pm.
  • Homo sapiens appears in the last 6 seconds, and the Holocene fills the last quarter of a second.

The Time Scale at a Glance

Names and Their Meanings

Significance of the Time Scale

• It shows how the diversity of life grew, collapsed in extinctions and recovered.
• It lets fossils and rocks be read as records of past environments: reefs, deserts, coal swamps, ice sheets.
• It reveals how catastrophes (impacts, flood basalts, glaciations) reset conditions on Earth.
• For geomorphology, it dates orogenies, erosion surfaces, terraces and glaciations, and so measures the “time” in structure–process–time.

Earth History Through the Eras

Precambrian (4,567–538.8 Ma)

• The Precambrian covers the Hadean, Archean and Proterozoic eons, about 88% of Earth history.
• Hadean: the Earth cooled from a molten state; a solid crust formed, degassing built a dense early atmosphere, and condensing water vapour made the first oceans.
• Archean: the first continental nuclei (cratons) grew; microbial mats built stromatolites from about 3.5 billion years ago.
• Proterozoic:
  • Photosynthesis oxygenated the air in the Great Oxidation Event (about 2.4–2.1 billion years ago).
  • Supercontinents assembled and broke up; the breakup of Rodinia (about 750–600 Ma) set the stage for the Palaeozoic.
  • Rocks suffered repeated metamorphism and several glaciations, including near-global “Snowball Earth” ice in the Cryogenian.
  • Soft-bodied multicellular animals (the Ediacaran biota) lived in the seas, while the land was bare.
• Indian example: the Dharwar, Singhbhum, Bastar, Aravalli and Bundelkhand cratons are Archean; the Aravalli range is a Proterozoic fold belt, and the Cuddapah and Vindhyan basins hold Proterozoic sandstones and limestones.

Palaeozoic Era (538.8–251.9 Ma): Ancient Life

• The Cambrian explosion filled warm, shallow seas with shelled animals such as trilobites; the seas repeatedly transgressed and regressed over the continents.
• Much of the building limestone and the coal of Europe and eastern North America formed in this era.
• Continents converged to form Pangaea by the late Palaeozoic, and collisions raised the Caledonian and Hercynian (Variscan) mountains, including the Appalachians.

• Late Palaeozoic “Age of Amphibians”: amphibians, which evolved from fish in the Late Devonian, dominated the Carboniferous swamps.
• End-Permian extinction (251.9 Ma), the “Great Dying”: the largest in Earth history, wiping out an estimated over 80% of marine species and about 70% of land vertebrate species.
  • Cause: the Siberian Traps flood basalts released carbon dioxide and sulphur dioxide, causing extreme warming, ocean acidification and oxygen-starved seas.
• Indian example: the Talchir tillite records the Late Carboniferous–Early Permian glaciation of Gondwana, and the Lower Gondwana coal of the Damodar valley (Jharia, Raniganj) formed in Permian swamps; they are key evidence for continental drift.

Mesozoic Era (251.9–66 Ma): Middle Life, “Age of Reptiles”

• The era opened with the continents joined as Pangaea; it rifted apart in the Jurassic, opening the Atlantic and splitting Gondwana.
• Reptiles flourished in the drier climate, helped by waterproof skin and shelled eggs.

• Mammals were small, warm-blooded and furred, traits that later helped them survive changing environments.
• Plants: gymnosperms (seed-bearing, non-flowering, such as pines and cycads) dominated until flowering plants (angiosperms) spread in the Cretaceous.
• End-Cretaceous (K–Pg) extinction, 66 Ma:
  • A 10-km asteroid struck Chicxulub (Yucatán, Mexico); dust and soot blocked sunlight, photosynthesis collapsed, and the food chain failed from plants to plant-eaters to predators.
  • Non-bird dinosaurs, pterosaurs, marine reptiles and ammonites vanished; birds, mammals, crocodiles and many others survived.
  • The Deccan Traps erupted in pulses around the same time and added climatic stress.
• Indian example: India broke from Gondwana and drifted north; the Rajmahal Traps (Early Cretaceous) and Deccan Traps (about 66 Ma) mark its passage over mantle hotspots, and the Lameta beds of the Narmada valley (Jabalpur, Dhar, Balasinor) hold dinosaur eggs and nests.

Cenozoic Era (66 Ma–Present): Recent Life, “Age of Mammals”

• Mammals radiated into land, air and sea (bats, whales, dolphins); grasses spread and fed grazing herds; flowering plants became dominant.
• Mountain building: India’s plate-tectonic collision with Asia about 50 million years ago raised the Himalaya, while the Alps and Rockies also rose.
  • Their uplift and weathering drew down carbon dioxide and helped cool the global climate towards the ice ages.

• The classical account of Himalayan mountain building recognises three phases of uplift: the Greater Himalaya (Eocene–Oligocene), the Lesser Himalaya (Miocene) and the Siwaliks (Pliocene–Pleistocene).
• Indian example: the Siwalik molasse holds a rich fossil record of apes, elephants and horses, the Karewas of Kashmir are Plio-Pleistocene lake beds, and the Indo-Gangetic plain is Quaternary alluvium; Assam oil and Mumbai High oil lie in Tertiary rocks.

Quaternary Period (2.58 Ma–Present)

• Pleistocene (2.58 Ma–11,700 years ago): ice sheets repeatedly covered much of the northern hemisphere and sea level fell by over 100 m at glacial peaks.
  • The classical scheme recognised four glacials: Günz, Mindel, Riss and Würm in the Alps and Nebraskan, Kansan, Illinoian and Wisconsinan in North America, separated by interglacials such as the Aftonian, Yarmouthian and Sangamonian.
  • Ocean-core oxygen isotopes now reveal dozens of glacial cycles, paced by orbital changes.
  • Retreating Pleistocene ice sheets left the Great Lakes of North America, the glacial lakes of Scandinavia, Switzerland and northern Italy, and the fjords of Norway.
  • The genus Homo is about 2.8 million years old, and Homo sapiens about 300,000 years old.
• Holocene (11,700 years ago–present): the present warm interglacial, the time of farming, animal domestication and cities.
  • Since 2018 it has three ages: Greenlandian (from 11,700 years), Northgrippian (from 8,200 years) and Meghalayan (from 4,200 years).
  • Indian example: the base of the Meghalayan Age is a golden spike in a stalagmite of Mawmluh Cave, Meghalaya, recording the 4.2-kiloyear mega-drought linked to the decline of several early civilisations, including phases of the Harappan.

The Big Five Mass Extinctions

The Anthropocene Question

• Paul Crutzen and Eugene Stoermer (2000) proposed that humans have made a new epoch, the Anthropocene, marked by carbon dioxide rise, plastics, radionuclides and changed landscapes.
  • An expert working group proposed a start in the early 1950s, with Crawford Lake, Canada, as the reference section.
• In March 2024 the Subcommission on Quaternary Stratigraphy rejected the proposal and the IUGS upheld the vote.
• The Anthropocene therefore remains an informal term, still widely used for the scale of human impact on landforms; the Holocene continues as the official epoch.

A Living Time Scale

• The chart is revised almost every year: recent editions moved the base of the Ordovician to 486.85 Ma (from 485.4) and of the Cretaceous to about 143.1 Ma (from about 145), and the 2026 edition refined Triassic and Permian stage ages.
• Several Cambrian stages still lack GSSPs, and the Precambrian boundaries, long fixed by round numbers, are being redefined by rock-based golden spikes.
• Plate reconstructions push the record further: sea-floor spreading keeps the oldest ocean floor to about 180–200 million years, so everything older is read from the continents alone.