The history of Earth

The Earth formed about 4.54 billion years ago by accretion, the process in which grains of dust around the young Sun stuck together into ever larger bodies that collided and merged.

The Moon is thought to have formed from the debris thrown into orbit when a young planet, called Theia, struck the Earth about 4.5 to 4.4 billion years ago, a scenario whose date and details are still debated.

Plate tectonics, the regime in which rigid plates of the Earth's outer shell slide, dive and collide, replaced a stagnant lid, a single unbroken shell, about 3 billion years ago, with estimates from 3.8 to 2.2 billion years ago.

Oxygen accumulated in the Earth's atmosphere and stayed there for the first time about 2.4 billion years ago, after hundreds of millions of years in which iron, volcanic gases and rock absorbed it as fast as microbes produced it.

Atmospheric oxygen stayed at a thousandth of today's level at most from 1.6 to 0.8 billion years ago, according to chromium isotopes measured in sediments of that age.

Snowball Earth is the name of two glaciations of the Cryogenian period, between 717 and 635 million years ago, during which ice sheets reached the sea at every latitude, the tropics included.

The Late Palaeozoic Ice Age is the span of about a hundred million years, late in the Palaeozoic era of 539 to 252 million years ago, during which ice sheets spread repeatedly over much of Gondwana, formed by today's southern continents.

Pangaea was a supercontinent that united nearly all the land in a single mass from pole to pole, surrounded by a single ocean, Panthalassa, with an interior a thousand kilometres from any coast, from 335 to 175 million years ago.

Pangaea, the supercontinent that joined all the continents, broke up between 201 and 135 million years ago by rifting, the stretching of the crust until it parts, and a flood of basalt preceded each episode.

The Cretaceous thermal maximum, between 94 and 83 million years ago, is the warmest interval of the last half billion years, when the Earth had no permanent ice and sea level was the highest of that half billion years.

Atmospheric carbon dioxide fell, about 34 million years ago, below the level at which ice can survive an Antarctic summer, and an ice sheet spread across the continent in about eight hundred thousand years.

Ice sheets have grown and melted over North America and northern Europe dozens of times during the last 2.6 million years, to the rhythm of the Milankovitch cycles, slow variations in the Earth's orbit and axis.

The Industrial Revolution, from 1750, returned to the atmosphere within decades carbon buried over hundreds of millions of years, and corals and tree rings recorded the industrial warming year by year.

James Hutton examined two sets of rock layers meeting at right angles at Siccar Point, on the Scottish coast, in 1788, and inferred from them a sequence of events too long for any short chronology, which is the idea of deep time.

The Great Acceleration is the sharp and simultaneous rise, after 1950, of some two dozen measures of human activity and of the Earth's response, from population and energy to carbon dioxide and plastics.

Clair Patterson obtained 4.55 billion years for the age of the Earth in 1956, by measuring the lead in meteorites and in deep-ocean sediment.

Sea-floor spreading, in which rock forms at the mid-ocean ridges and is carried away on both sides, was demonstrated between 1963 and 1968 from magnetic stripes that mirror one another on either side of those ridges.

The EPICA Dome C ice core, three kilometres of Antarctic ice read layer by layer since 2004, contains bubbles of ancient air sealed as the snow compacted, and gives past temperature and carbon dioxide side by side.