Interior of the Earth

We live on the thin outer skin of the Earth, yet beneath our feet lies a vast hidden world stretching thousands of kilometres down to the planet's centre. No one has ever been there, and no one ever will, because the deepest mine and the deepest drill barely scratch the surface. So how do we know what the inside of the Earth is made of? The answer is a detective story written in earthquake waves, and it reveals an Earth built of three great layers.

The ways of learning about the interior fall into two groups.

Direct sources are the materials we can actually touch. Mines and deep boreholes let us study the rocks of the upper crust, and the lava that pours out of volcanoes brings up material from below. But these reach only a few kilometres down, a tiny fraction of the journey to the centre.

Indirect sources carry us the rest of the way. The most powerful of these is the study of earthquake waves. These waves travel right through the planet and change their behaviour according to the material they pass through. Other clues come from the Earth's gravity, its magnetism, and meteorites that fall from space. Meteorites are thought to be made of similar stuff to the Earth's deep interior. By putting these clues together, scientists have mapped the layers without ever seeing them.

The crust is the thin, solid, outermost layer of the Earth, the ground we build our lives upon. It is extremely thin compared with the whole planet, like the skin on an apple.

What is the crust made of? By mass, the most abundant element in the crust is oxygen, at about 46 per cent. Silicon comes second. This surprises many students, because oxygen feels like a gas of the air. In the crust, however, oxygen is locked into solid minerals, combined with silicon and metals such as aluminium, iron, calcium, sodium, potassium and magnesium. The familiar term "silica" itself is a compound of silicon and oxygen.

The crust is not the same everywhere. The crust beneath the continents is thicker and made of lighter rocks rich in silica and aluminium, a combination known as sial. The crust beneath the oceans is thinner but made of heavier rocks rich in silica and magnesium, known as sima. This difference in density between continental and oceanic crust matters a great deal for how the surface behaves.

Below the crust lies the mantle, the thickest layer by volume, reaching to a depth of about 2,900 kilometres. It is made of dense, solid rock, but it is not completely rigid.

The upper part of the mantle includes a zone called the asthenosphere. Here the rock is so hot that it behaves like a very slow, soft solid that can flow over long periods. This is the main source of the molten magma that reaches the surface during volcanic eruptions. The solid crust together with the rigid uppermost mantle is called the lithosphere. This layer is broken into great plates. Their slow movement shapes the continents and oceans.

The deepest part of the Earth is the core, beginning at about 2,900 kilometres down and reaching the centre at roughly 6,400 kilometres. It is made mainly of the heavy metals iron and nickel, a mixture sometimes called nife (from the chemical symbols Ni and Fe).

The core has two parts. The outer core is in a liquid state. The inner core, despite the enormous heat, is solid. The pressure there is so immense that it keeps the metal from melting. The swirling of the liquid outer core is believed to generate the Earth's magnetic field. This is the invisible shield that makes a compass needle point north.

The proof for these layers comes from seismic waves, the energy released by an earthquake. Two kinds matter here. They differ not only in speed but in how the rock particles actually move as the wave passes.

Primary waves, or P-waves, are the faster of the two and arrive first. They are longitudinal waves: the particles of the material vibrate to and fro in the same direction as the wave travels, like a push travelling along a line of railway coaches. Because solids, liquids and gases can all be compressed and stretched in this way, P-waves can pass through all three.

Secondary waves, or S-waves, are slower. They are transverse waves: the particles vibrate at right angles to the direction in which the wave travels, like the shake travelling along a rope. Liquids cannot be sheared sideways, so S-waves, crucially, cannot travel through liquids.

This single difference is the key clue. When scientists tracked waves from earthquakes around the world, they found a shadow zone on the far side of the Earth where no S-waves arrived. The only explanation was that the waves had met a liquid layer that stopped them. That liquid layer is the outer core. The way P-waves bend at each boundary mapped out the other layers too. In this way the inside of the Earth was discovered without anyone ever going there.

An earthquake is the sudden release of energy stored in the rocks of the lithosphere. The release happens at a break in the rocks called a fault.

• Development of a fault: when stressed rock finally fractures and a new fault forms, the stored energy escapes as seismic waves.
• Movement along a fault: rock masses on either side of an existing fault can slip past each other suddenly, releasing energy in the same way. This is the most common cause.
• Volcanic eruptions: the violent movement of magma and the explosive force of an eruption also shake the crust and generate earthquakes.

Note what does not belong on this list. The slow folding of rocks bends them gradually over millions of years. Because the strain is released little by little, folding by itself does not directly cause earthquakes.

Two different scales describe an earthquake, and they measure different things.

• Intensity: the visible effect of the shaking on people, buildings and the ground. Intensity is measured on the Mercalli scale, which is based on observed damage.
• Magnitude: the energy released by the earthquake itself. Magnitude is measured on the Richter scale, which is calculated from the measured amplitude of the seismic waves on a seismograph.

The Richter scale is logarithmic. Each whole number step means a tenfold increase in wave amplitude, not a hundredfold. In terms of energy released, each step is roughly a 31.6-fold increase. A magnitude 7 earthquake therefore shakes the ground ten times harder than a magnitude 6 and releases about thirty times more energy.

A volcano is an opening in the crust through which material from the interior escapes to the surface. Molten rock below the surface is called magma. Once it erupts and flows out, it is called lava. An eruption throws out far more than lava alone.

• Lava flows: molten rock that spreads over the surface and cools into igneous rock.
• Pyroclastic debris: solid fragments of shattered rock and hardened magma, ranging from large volcanic bombs to small cinders.
• Ash and dust: the finest particles, which can rise high into the atmosphere and travel great distances.
• Gases: large volumes of water vapour and carbon dioxide, together with nitrogen compounds and sulphur compounds such as sulphur dioxide and hydrogen sulphide, and minor amounts of chlorine and hydrogen.

Volcanoes are not scattered randomly. They cluster where the lithospheric plates meet, because plate boundaries give magma a path to the surface. The Pacific Ring of Fire is the most famous belt. The Caribbean Sea has an active volcanic island arc where one plate sinks beneath another. The Black Sea and Caspian Sea regions also lie in a tectonically disturbed zone of young fold mountains. The Baltic Sea, by contrast, sits on an ancient, stable continental shield far from any plate boundary, so volcanic eruptions do not occur there.

The early Earth was lifeless. Its first atmosphere and oceans were stocked with simple compounds released partly by volcanic outgassing. Life is thought to have arisen from chemical reactions among these compounds in the early oceans, often pictured as a primordial soup. The elements primarily responsible were carbon, hydrogen and nitrogen. These are the building blocks of organic molecules such as amino acids, from which the first living cells were assembled. Life appeared roughly 3.8 billion years ago, and the fossil record in sedimentary rocks traces its slow evolution ever since.

The most recent part of that story is human evolution. Modern humans shared the planet with several archaic human species. The best known are the Neanderthals. Another is the Denisovans, an extinct early human species identified from DNA extracted from fossil fragments found in Denisova Cave in Siberia. Denisovans were related to, but distinct from, both Neanderthals and modern humans, and they interbred with our ancestors. Traces of Denisovan DNA survive in some present-day populations of Asia and Oceania.

The solid Earth is made of rocks, and every rock belongs to one of three families.

Igneous rocks form when hot molten magma cools and hardens. Because they form first, they are called the primary rocks. Granite and basalt are examples.

Sedimentary rocks form when small particles worn from older rocks are carried by water or wind and settle in layers. Over a long time, those layers are pressed together and harden. They often appear as visible bands and may contain fossils. Sandstone and limestone are examples.

Metamorphic rocks form when existing igneous or sedimentary rocks are buried deep in the Earth. Great heat and pressure change them, but they do not melt completely. Clay turns into slate, and limestone turns into marble.

These three types are not fixed forever. Over vast stretches of time, one type can change into another. Igneous rock can be worn down into sediment. Sediment can be cooked into metamorphic rock. Any rock can melt and cool again as igneous rock. This endless transformation is called the rock cycle.