Inside the Earth

Stand anywhere on Earth and there are about 6,370 kilometres of planet beneath you. Humans have explored almost none of it. The deepest mine goes down four kilometres; the deepest borehole ever drilled, in Russia's Kola Peninsula, reached 12 kilometres before the rock became too hot and soft to continue. That is a fifth of one percent of the way to the centre. Yet we know, in surprising detail, what lies below.

Listening to earthquakes

The Earth cannot be dug into, but it can be listened to. Every large earthquake sends vibrations through the whole planet, and seismometers on the far side record their arrival. Some waves travel faster through dense rock, some cannot pass through liquid at all, and all of them bend when they cross from one material into another. By timing waves from thousands of earthquakes arriving at thousands of stations, geologists have built an image of the interior much as a doctor builds an ultrasound picture of a body. The result is a planet in layers.

The crust

The outermost layer, the one we live on, is the crust. Under the oceans it is only five to ten kilometres thick, made of dense dark basalt that erupted from undersea volcanoes. Under the continents it is thicker, up to seventy kilometres beneath mountain ranges, and made of lighter granite-like rock. Compared with the planet as a whole, the crust is proportionally thinner than the shell of an egg. It is also cracked into about a dozen large plates that slide slowly over the layer beneath.

The mantle

Below the crust, and making up more than 80 percent of Earth's volume, is the mantle, nearly 2,900 kilometres of hot rock. It is not molten, as many people imagine; it is solid, but so hot, between about 1,000 and 3,700 degrees Celsius, that over millions of years it flows like extremely thick toffee. Hot rock rises from the depths, cools near the top, and sinks again, in vast slow currents that take hundreds of millions of years to complete a cycle. Those currents drag the crustal plates around on the surface, building mountains and opening oceans. Where the mantle rock rises and the pressure drops, some of it melts, and that melt feeds volcanoes.

The mantle is mostly silicate minerals rich in magnesium and iron. Diamonds form here, at depths of 150 kilometres and more, and are carried up in rare violent eruptions.

The core

At about 2,900 kilometres down, the rock gives way abruptly to metal. The core is mostly iron with some nickel, and it comes in two parts. The outer core, 2,200 kilometres thick, is liquid, at temperatures of 4,000 to 5,000 degrees. Below it, the inner core is a solid ball of iron about 2,440 kilometres across, roughly the size of the Moon. It is solid despite being hotter still, around 5,400 degrees, as hot as the surface of the Sun, because the pressure at the centre, more than three million times that at the surface, forces the atoms into a rigid crystal. The inner core is slowly growing as the planet cools and more liquid iron freezes onto it, at a rate of roughly a millimetre a year.

The heat down there is partly leftover from Earth's formation, when colliding debris and sinking iron released enormous energy, and partly from the decay of radioactive elements such as uranium, thorium and potassium scattered through the mantle and crust.

The invisible shield

The outer core is the reason compasses point north. Liquid iron is an electrical conductor, and as it churns, driven by heat and twisted by the planet's spin, it generates electric currents, which in turn produce a magnetic field. This self-sustaining machine is called the geodynamo. The field reaches far out into space and deflects most of the solar wind, the stream of charged particles from the Sun that would otherwise erode the atmosphere over billions of years, as it has done on Mars.

The field is not steady. Its strength rises and falls, the magnetic poles wander by tens of kilometres a year, and every few hundred thousand years, on an irregular schedule, the whole field flips, north becoming south. Volcanic rocks lock in the direction of the field as they cool, and they record hundreds of reversals. The last happened 780,000 years ago. The field has weakened about ten percent over the past two centuries, which may or may not be the start of another.

Why it matters

Everything on the surface depends on the interior. The mantle's slow churning builds continents and recycles carbon between rock, air and sea, keeping the climate habitable over geological time. The core's magnetic field protects the atmosphere. Earth is still geologically alive, unlike the Moon or Mars, because it is large enough to have stayed hot inside for four and a half billion years. We live on the cool thin skin of a planet that is still cooking.