The deepest hole ever dug
From 1970 onwards, Soviet scientists spent nearly twenty years drilling a hole into the Kola Peninsula in the far north of Russia, trying to go as deep as they could. They reached about 12.2 kilometres, the deepest hole ever made, before the rock became so hot, around 180°C, that it behaved almost like plastic and the drill could go no further. That is still only about a third of the way through the continental CRUST, and a tiny scratch compared with the 6,371 kilometres to the centre of the Earth.
So nobody has ever seen the deeper layers. Everything we know about them comes from indirect clues, above all the way earthquake waves travel through the planet.
A scientist who found a hidden core
When a big EARTHQUAKE strikes, SEISMIC waves spread through the whole Earth, and they speed up, slow down and bend as they pass through different materials. By timing their arrival at stations around the world, scientists can work out what is inside. Some waves cannot pass through liquid at all, and the shadow they leave on the far side of the planet showed that the OUTER CORE is molten.
In 1936 a Danish scientist, Inge Lehmann, noticed faint waves arriving in places where, if the core were entirely liquid, there should be none. She worked out that there must be a solid INNER core inside the liquid one. It is a ball of IRON and NICKEL about as hot as the surface of the Sun, but kept solid by the enormous PRESSURE of everything above it.
A slow-moving mantle
Between the crust and the core lies the MANTLE, the thickest of the LAYERS. It is solid rock, not a sea of lava, but it is so hot that over millions of years it flows very slowly, like extremely stiff toffee. Hot rock rises, cools and sinks again in huge loops called CONVECTION currents, and these help drag the TECTONIC PLATES that make up the Earth's surface.
Where the plates meet, at a BOUNDARY, they grind past each other, pull apart or crash together, and most earthquakes and volcanoes happen along these edges. A FAULT is a crack where the rock on either side has moved. OCEANIC crust, under the seas, is thinner and heavier than the crust of the continents, and where the two meet the ocean floor sinks back down into the mantle. Deep below, some of that rock melts to form MAGMA.
The idea everyone laughed at
In 1912 a German scientist, Alfred Wegener, suggested that the continents had once been joined in a single landmass and had slowly drifted apart. He pointed out that the coasts of South America and Africa fit together like jigsaw pieces, and that matching fossils and rocks are found on both. Most geologists dismissed the idea, because he could not explain what moved the continents. He died on an expedition in Greenland in 1930, decades before discoveries about the ocean floor in the 1950s and 1960s proved him largely right. Our mountains page shows what happens when plates collide. The layers get denser towards the centre, and the DENSITY, the HEAT, the MOLTEN rock and the BEDROCK all tell the same story of a planet still on the move.