Birth of the Earth

The story

A planet built by collisions

The Earth was not born in a day, and it was not born calm. In the nebula around the young Sun, at a distance where it was hot enough to leave only rocks and metals, billions of small bodies kept colliding and joining together. The bigger ones pulled in the others and grew. Scientists estimate that the Earth formed over a few tens of millions of years, and that its age is about 4.54 billion years, with an uncertainty of about one per cent.

How do you measure an age like that? You cannot use any Earth rock from the very first hour, because the Earth has changed all the time and the oldest rocks have been destroyed and remade. Instead we use meteorites, which are leftovers that have stayed almost the same since the Solar System formed. In 1953 an American geochemist, Clair Patterson, measured in the Canyon Diablo meteorite, which fell in Arizona, the amounts of different kinds of lead. These form from the decay of uranium, which happens at a steady, known rate, like a clock. The result was about 4.55 billion years, still very close to the accepted figure today.

A sea of molten rock

The young planets were extremely hot, for three reasons. The first was the constant collisions, which turn the energy of movement into heat. The second was radioactive elements, which release energy from the inside when they decay. The third was gravity itself, which squeezed the planet as it grew. The outer part of the Earth melted into an ocean of magma, that is, molten rock, perhaps hundreds of kilometres deep.

In this ocean of rock something fundamental happened: the material sorted itself by weight. Iron and nickel, which are heavy, sank towards the centre and formed the core. The lighter rocks stayed on top and formed the mantle, and at the surface, as it cooled, a thin crust appeared. That is why the Earth is still made of layers, like an onion.

Inside the Earth

Nobody has ever gone deeper than 12 kilometres: the deepest hole ever dug, in Russia, on the Kola Peninsula, reaches about 12,262 metres, a tiny amount compared with the Earth's radius of about 6,370 kilometres. Yet we know what the inside is like, thanks to earthquakes. Seismic waves travel through the planet and change speed and direction when the material changes. By studying them, seismologists have found that under the crust (between 5 and 70 kilometres thick) lies the mantle, of hot rock that flows slowly for almost 2,900 kilometres; then a liquid outer core of iron and nickel; and at the centre a solid inner core, about three quarters the size of the Moon.

The inner core was discovered in 1936 by the Danish seismologist Inge Lehmann, who noticed that certain earthquake waves behaved strangely and realised that they must be bent by a solid sphere inside the liquid core. The movement of the liquid metal in the outer core produces the Earth's magnetic field, the invisible shield that deflects much of the Sun's charged particles and makes a compass useful for finding your way.

The Hadean eon

The first period in the history of the Earth, up to about 4 billion years ago, is called the Hadean, after Hades, the Greek god of the underworld, because of the idea of a fiery world. In fact scholars think that quite early on the surface had cooled enough to have a solid crust and even liquid water. We do not know how hot or how calm that world was: very little remains of the Earth of those days, because the planet keeps recycling its rocks.