A brief history of Earth
Four and a half billion years in one page: a molten start, a slow build-up of oxygen and life, continents that gather and scatter, and a climate set over millions of years by volcanoes and weathering.
- Name the four eons and the three eras of the Phanerozoic, and see how short the age of animals is
- Place the key events, from the oldest minerals to the Great Oxidation Event, in order
- Explain the supercontinent cycle and the causes of the biggest mass extinctions
- Trace India's geological story from its ancient cratons to the Himalaya
- Connect past climates to the long-term carbon cycle
The geological time scale
Geologists divide Earth's history into named units, a bit like historians use dynasties. The largest are eons, split into eras, which are split into periods (such as the Jurassic) and then epochs. The boundaries were originally drawn where the rock record changes sharply, often at a mass extinction, and have since been dated in years.
Ages are written in shorthand: Ga means billions of years ago and Ma means millions of years ago. How do we know these dates? Mostly from radioactive decay in minerals, combined with the layer-ordering rules in Stratigraphy.
- Earth forms, about 4.54 Ga.
- Oldest zircon crystals, about 4.4 Ga.
- Stromatolites, the oldest widely accepted fossils, about 3.5 Ga.
- Oxygen builds up in the air, about 2.4 Ga.
- Snowball Earth, about 720–635 Ma.
- Animals boom in the Cambrian explosion, about 539 Ma.
The early Earth: Hadean and Archaean
The Earth formed about 4.54 billion years ago from dust and rock orbiting the young Sun. Soon afterwards, at roughly 4.5 Ga, a Mars-sized body is thought to have struck it; debris from that impact collected to form the Moon. The first eon, the Hadean (named after Hades), was a time of heavy bombardment and partly molten surfaces, and almost no rock survives from it.
What does survive are tiny crystals of the mineral zircon, which is so tough that it outlasts the rocks it grew in. Zircons from the Jack Hills of Western Australia are about 4.4 billion years old, and their chemistry suggests liquid water and some crust existed even then.
During the Archaean (4.0–2.5 Ga), the first lasting continental crust formed, as small blocks that would later weld together into the stable cores of today's continents, called cratons. Life appeared early. The oldest widely accepted evidence is stromatolites about 3.5 billion years old: layered mounds built by mats of microbes trapping sediment. Disputed traces go back to about 3.7–3.8 Ga.
The Proterozoic: oxygen, iron and ice
For its first two billion years, the atmosphere had almost no oxygen. Then microbes that photosynthesise, cyanobacteria, which had been releasing oxygen for a long time, finally won out over the processes that removed it. About 2.4 billion years ago oxygen began to build up in the air: the Great Oxidation Event.
Oxygen also changed the chemistry of the oceans. Iron dissolved in seawater reacted with oxygen made by microbes, at first only locally in shallow water, and settled out as rust-coloured layers, building banded iron formations. They formed mainly before and around the rise of oxygen in the air. Most of the biggest formed between about 2.5 and 1.8 Ga, but others go back as far as 3.8 Ga. They are now the world's main source of iron ore. India's, in the Singhbhum, Bastar and Dharwar cratons, are mostly older than 2.6 billion years (see Sedimentary & surface deposits).
Continents also began a long rhythm. Plate tectonics periodically herds most of the land into one supercontinent, which later breaks apart, and the pieces drift and eventually reassemble. The best-known supercontinents are Columbia (also called Nuna, about 1.8 Ga), Rodinia (about 1 Ga) and Pangaea (about 335–175 Ma). Pangaea began to rift about 200 Ma, and new oceans had split it apart by about 180–175 Ma.
- Supercontinent. Most of the land is in one mass; heat may build up in the mantle beneath it.
- Rifting. The crust stretches and thins, a rift valley drops down and melt rises beneath it.
- Ocean opens. Seafloor spreads from a new mid-ocean ridge as the pieces drift apart.
- Ocean closes. Old seafloor sinks back into the mantle at a trench, feeding volcanoes, and the continents converge until they collide. A new supercontinent forms every few hundred million years.
Near the end of the Proterozoic, the climate swung to an extreme. Between about 720 and 635 Ma, ice sheets reached the tropics, possibly covering almost the whole planet: Snowball Earth. The planet escaped because volcanoes kept releasing CO2 while ice-covered rock could barely weather to remove it, so CO2 built up until the greenhouse effect melted the ice. (You can try this yourself in the thermostat model at the end of this module.)
The Phanerozoic: the age of visible life
The Phanerozoic ("visible life") began about 539 million years ago with the Cambrian explosion, when most of the major animal groups appear in the fossil record in a geologically short time, many with shells and skeletons that fossilise well. A few shelled animals, such as the tube-shaped Cloudina, had already appeared shortly before. The Phanerozoic is divided into three eras: the Palaeozoic ("old life"), Mesozoic ("middle life", the age of dinosaurs) and Cenozoic ("new life", the age of mammals).
The history of life since then has been punctuated by mass extinctions, short intervals when a large fraction of all species died out. Five stand out. At least three of them line up with eruptions of large igneous provinces, outpourings of flood basalt big enough to change the climate through their CO2 and sulphur gases.
| Extinction | Age | Leading explanation |
|---|---|---|
| 1 · End-Ordovician | ~445 Ma | Rapid glaciation and falling sea level |
| 2 · Late Devonian | ~372 Ma | Several pulses; oxygen-starved oceans; causes still debated |
| 3 · End-Permian | ~252 Ma | The worst of all: roughly 80–90% of marine species lost. Linked to the Siberian Traps, flood basalts whose gases caused rapid warming and ocean acidification |
| 4 · End-Triassic | ~201 Ma | Flood basalts as Pangaea began to split (the Central Atlantic Magmatic Province) |
| 5 · End-Cretaceous | 66 Ma | About three-quarters of species, including all non-bird dinosaurs. An asteroid hit what is now Chicxulub, Mexico, while the Deccan Traps were erupting in India; how much each contributed is still debated |
India through time
India's rocks span almost the whole time scale, which makes it a good place to see Earth history on the ground. The orange marks on the timeline above show where each of these fits.
| Rocks | Age | What they record |
|---|---|---|
| Singhbhum and Dharwar cratons | Archaean, >3 Ga | Some of India's oldest crust, in Jharkhand–Odisha and Karnataka. Ancient continental nuclei, with gold-bearing greenstone belts (Kolar, Hutti) and banded iron formations |
| Cuddapah and Vindhyan basins | Proterozoic, roughly 1.9–0.6 Ga | Thick piles of sandstone, shale and limestone laid down in great basins on top of the cratons. Mostly undeformed, so they preserve sedimentary layers very well |
| Gondwana basins (e.g. Damodar valley) | Mainly Permian, ~300–250 Ma | When India was part of the southern continent Gondwana, swampy forests in rift valleys were buried and became coal. Most of India's coal comes from these basins |
| Rajmahal Traps | ~118 Ma | Flood basalts in Jharkhand, erupted as India broke away from Antarctica and Australia |
| Deccan Traps | ~66 Ma | Vast flood basalts erupted as India passed over a mantle plume, now the Réunion hotspot |
| The Himalaya | ~50 Ma to today | India's collision with Asia, still pushing north at about 4–5 cm a year (see Plate tectonics) |
Over the last 150 million years India made one of the fastest journeys of any continent: it broke away from Gondwana, crossed the Indian Ocean over two mantle plumes (leaving the Rajmahal and then the Deccan flood basalts behind) and crashed into Asia. To follow that journey step by step, use the time slider in Plate tectonics.
Two tools work together. The rules of Stratigraphy (superposition, cross-cutting and so on) put rocks in order. Radiometric dating of minerals such as zircon, which measures how much of a radioactive element has decayed, attaches numbers in years. Fossils link layers of the same age from place to place.
The Cenozoic: a cooling world
About 50 million years ago the Earth was much warmer than today, with no permanent ice at the poles. Since then it has cooled, in steps. Ice sheets grew on Antarctica around 34 Ma, and in the Northern Hemisphere around 2.7 Ma. The Quaternary period, from 2.58 Ma to today, is an age of ice: glaciers have advanced and retreated dozens of times, paced by small regular changes in Earth's orbit. The last glacial peak was only about 20,000 years ago, when sea level was about 120 m lower.
Humans have now changed the planet enough to leave a mark in the rock record: plastics, concrete, radioactive fallout and a spike in CO2. Many scientists call this the Anthropocene. In 2024, though, geology's governing bodies voted against adding it to the time scale as a formal epoch, so officially we still live in the Holocene, which began 11,700 years ago. The word is still widely used informally.
What sets the planet's temperature over millions of years? Mostly the amount of CO2 in the air, and that is controlled by a slow balance. Volcanoes and mid-ocean ridges steadily release CO2. Rain, which is a weak acid because it dissolves CO2, slowly dissolves silicate rocks such as basalt and granite: chemical weathering. Rivers carry the products to the sea, where they end up locked in carbonate rock such as limestone, taking the carbon with them. The crucial twist is that weathering runs faster when it is warm and wet. That makes it a negative (balancing) feedback: anything that pushes the climate one way triggers a response that pushes it back.
The planet starts in balance, with volcanoes adding CO2 exactly as fast as weathering removes it. At time zero, change how much CO2 the volcanoes release, or bury the land under ice, and watch CO2 and temperature find a new balance.
Relative to today's natural rate, roughly a few hundred million tonnes of CO₂ a year.
A deliberately simple model. Temperature rises about 3 °C for each doubling of CO2; weathering speeds up about 9% for each degree of warming; the air and ocean are treated as one well-mixed pool; an ice-covered planet reflects so much sunlight that it is about 40 °C colder, and the ice melts once CO2 has climbed a few hundred times higher. Human emissions are left out. The timescales are only rough: real Snowball episodes lasted millions to tens of millions of years.
Past climates show the long-term carbon cycle at work. When CO2 rises, the planet warms, rain increases and rock weathers faster, which draws CO2 back down. Snowball Earth ended when that sink was switched off under the ice. Over the Cenozoic, CO2 fell and the planet cooled. One influential hypothesis is that the rise of the Himalaya and Tibet helped cause this, by exposing vast amounts of fresh rock to monsoon rain and speeding up weathering. It is still debated, but it is a natural version of the same idea behind enhanced rock weathering. The catch is speed: natural weathering adjusts CO2 over hundreds of thousands of years, while we are changing it in decades.
Earth's history also shapes where ores are found. Some deposit types belong to particular periods. Banded iron formations, the source of most of the world's iron ore, formed mainly before and around the rise of oxygen in the atmosphere, so explorers look for them in rocks of that age.