Formation of the Earth: How Our Planet Was Born

The formation of the Earth took place about 4.54 billion years ago, when dust, rock and metal orbiting the young Sun gathered into a single planet through countless collisions. Over tens of millions of years the growing Earth melted, separated into an iron core and a rocky mantle, and slowly cooled. Understanding this process, and measuring when it happened, was one of the great scientific achievements of the 20th century.

Background: a planet from a disc

The Earth was one product of the formation of the Solar System. About 4.568 billion years ago, a collapsing cloud of gas and dust formed the Sun, surrounded by a spinning disc of leftover material. In the warm inner part of that disc, only rock-forming minerals and metals could exist as solids.

These solids clumped into planetesimals, bodies a few kilometres to a few hundred kilometres across. Gravity then drew planetesimals together into larger planetary embryos, some the size of the Moon or Mars. Earth was built from this material in the zone between the orbits of Venus and Mars.

What happened: accretion

The process of growing a planet by collisions is called accretion. It happened in stages, each more violent than the last.

In the first stage, planetesimals collided at relatively low speeds and stuck together. Larger bodies grew faster than small ones because their stronger gravity pulled in more material, a runaway effect that produced dozens of planetary embryos within perhaps a million years or so.

In the final stage, these embryos crossed each other’s orbits and crashed together in giant impacts. Computer simulations suggest this phase lasted tens of millions of years, perhaps up to about 100 million years. Earth may have been formed from around ten or more such embryos, together with a mass of smaller debris.

Building blocks and the final impact

Scientists compare the chemistry of Earth’s rocks with different types of meteorites to work out its raw materials. Earth’s isotopes match most closely those of enstatite chondrites, a rare class of meteorite formed in the inner Solar System, although no single known meteorite type matches the planet exactly. Some material from farther out, richer in water and carbon, was probably added as well.

Volatile substances such as water, carbon and nitrogen make up only a tiny fraction of Earth’s mass, but they mattered enormously for its later history. How much arrived during the main phase of accretion, and how much came later with asteroids and comets, is still debated.

The last major collision is widely thought to have been with a Mars-sized body, often called Theia. That impact is the leading explanation for the formation of the Moon, and it probably happened between about 4.51 and 4.45 billion years ago.

A molten planet

The early Earth was extremely hot. Three sources of heat combined:

• Impacts. Each collision turned enormous amounts of motion into heat.
• Radioactivity. Short-lived isotopes such as aluminium-26, plus longer-lived uranium, thorium and potassium, released heat as they decayed.
• Core formation. As dense iron sank towards the centre, it released gravitational energy as heat.

As a result, much of the planet melted, probably more than once, forming what scientists call a magma ocean that may have extended hundreds of kilometres deep. After the Moon-forming impact, parts of the Earth’s surface may briefly have been vaporised altogether.

Forming the core

In a molten or partly molten planet, heavy iron and nickel separated from lighter silicate rock and sank to the centre, forming the core. Lighter materials rose to form the mantle and, eventually, the crust. This separation into layers is called differentiation.

Scientists can time core formation using the hafnium-tungsten clock. Hafnium-182 decays to tungsten-182 with a half-life of about 9 million years. Because tungsten tends to follow iron into the core while hafnium stays in the rock, the amount of tungsten-182 in the mantle records when the core separated. These measurements suggest that the core was largely formed within roughly the first 30 to 100 million years of Solar System history.

Today the core makes up about a third of Earth’s mass. Its liquid outer part, churning as the planet loses heat, generates Earth’s magnetic field, which shields the surface from much of the solar wind.

Cooling and the first crust

Once the giant impacts ended, the surface cooled relatively quickly. A thin solid crust formed over the magma ocean, and water vapour and other gases released from the interior built up a thick early atmosphere.

The oldest direct evidence from this period comes from tiny crystals of the mineral zircon. The Jack Hills zircons of Western Australia, dated to about 4.4 billion years, carry chemical hints that some crust had already interacted with liquid water. This suggests the surface may have cooled enough for oceans within roughly 100 to 200 million years of the planet’s formation, although the extent of those oceans is debated.

The first 500 million years or so of Earth’s history are known as the Hadean Eon, a name derived from Hades, the Greek underworld, and introduced by the geologist Preston Cloud in 1972. Almost no rocks survive from it, because later geological activity recycled the early crust.

How the age of the Earth was measured

For most of history, the age of the Earth was a matter of religious tradition or philosophical guesswork. In 1650 Archbishop James Ussher of Armagh used biblical chronology to date the Creation to 4004 BC. In the 18th century the French naturalist the Comte de Buffon estimated, from cooling experiments with iron spheres, that the Earth was about 75,000 years old.

In the 19th century, geologists who studied rock layers argued that Earth must be far older. The physicist William Thomson, later Lord Kelvin, calculated how long a hot Earth would take to cool and in 1862 gave a range of 20 to 400 million years, later narrowing it to around 20 to 40 million years. His figures were too low partly because he did not know about radioactivity, discovered in 1896, which generates heat inside the Earth.

Radiometric dating

Radioactivity also provided the solution. Radioactive elements decay at fixed rates, so the ratio of a parent element to its decay products acts as a clock. In 1907 the American chemist Bertram Boltwood used uranium-lead ratios to date minerals at hundreds of millions of years or more. In 1913 the British geologist Arthur Holmes published “The Age of the Earth”, giving the oldest rocks then known an age of about 1.6 billion years, and spent decades refining the timescale.

The decisive step came from the American geochemist Clair Patterson. Earth’s own oldest rocks have been destroyed, so Patterson turned to meteorites, which formed at the same time as the planets. Working at the California Institute of Technology, he measured lead isotopes in fragments of the Canyon Diablo meteorite, which made Meteor Crater in Arizona. In 1956 he published an age of 4.55 billion years, with an uncertainty of about 70 million years.

Patterson’s work also had an unexpected legacy. While fighting lead contamination in his laboratory, he discovered how much lead from petrol had spread through the environment, and he campaigned for decades for leaded fuel to be banned.

The modern accepted figure, 4.54 billion years with an uncertainty of about 1 percent, rests on many meteorite measurements and on lead isotope data from Earth and the Moon.

Myths and facts

• Myth: the Earth formed at the same instant as the Sun. In fact, the first solids condensed about 4.568 billion years ago, and Earth took tens of millions of years to finish assembling. Its “age” refers to this period of formation rather than a single moment.
• Myth: the oldest rocks prove the Earth’s age. The oldest known rocks are only about 4.03 billion years old. The planet’s age comes from meteorites and lead isotope models, not from Earth rocks.
• Myth: the early Earth was a uniform ball of fire for hundreds of millions of years. Evidence from zircons suggests that solid crust, and possibly liquid water, existed by about 4.4 billion years ago.

By the numbers

Timeline

• c. 4,568 million years ago: The first solids condense in the disc around the young Sun.
• c. 4,568–4,550 million years ago: Planetesimals and planetary embryos grow in the inner Solar System.
• c. 4,540 million years ago: Earth is largely assembled; the commonly cited age of the planet.
• c. 4,510–4,450 million years ago: A giant impact forms the Moon and remelts much of the Earth.
• c. 4,400 million years ago: The oldest known zircon crystals form, hinting at early crust and water.
• c. 4,030 million years ago: The Acasta Gneiss, the oldest known intact rock, forms in what is now Canada.
• AD 1862: Lord Kelvin publishes his estimate of the Earth’s age from cooling.
• AD 1913: Arthur Holmes publishes “The Age of the Earth”.
• AD 1956: Clair Patterson publishes the meteorite-based age of about 4.55 billion years.

Why it matters

The way the Earth formed determined almost everything about it: its size, its iron core and magnetic field, its rocky surface and the water and carbon it inherited. Those conditions made possible the origin of Earth’s oceans and, later, the appearance of life. Measuring the planet’s true age also transformed science, giving geology and evolution the vast span of deep time they require.