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Inside the Earth: layers, seismic evidence and rocks

The Earth: its place, motions and interior · Part 3 / 3 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

How the interior is known from direct and indirect sources, P, S and surface waves, the shadow zone, the crust, mantle and core with the Moho, Gutenberg and Lehmann boundaries, the elements and minerals of the crust, igneous, sedimentary and metamorphic rocks, and the rock cycle.

Parts 1 and 2 looked at the Earth from outside; this part goes underneath. The Earth's mean radius is about 6,371 km, yet no hole drilled by people has gone much deeper than 12 km, so nearly all knowledge of the interior is reasoned out from indirect clues, above all the way earthquake waves travel. Those clues show a planet built in shells: a thin crust outside, a thick rocky mantle below it and a dense metallic core at the centre. The second half of the part turns to the crust itself: its elements and minerals, and the three families of rock that keep turning into one another.

Evidence about the interior comes from direct and indirect sources. Direct sources are real material from below: deep mines, such as South Africa's gold mines at roughly 4 km; drilling, such as the Kola Superdeep Borehole in north-western Russia, which reached about 12.2 km in 1989; and volcanic eruptions, since magma rises from depths no drill can reach. Indirect sources are inferences from measurements at the surface. Temperature, pressure and density all increase with depth; the Earth's average density, about 5.5 g/cm³, is roughly twice that of surface rock, so heavier material must lie inside. Meteorites are thought to resemble the deep material, and gravity and magnetic surveys show how mass and magnetic minerals are spread. The richest indirect source is the record of earthquake waves.

An earthquake is a sudden release of energy when strained rocks break and slip along a fault. The point underground where the energy is released is the focus, or hypocentre; the point on the surface directly above it is the epicentre. The energy spreads as seismic waves, recorded by a seismograph. P waves, or primary waves, are the fastest and arrive first; they push and pull material along their line of travel, as sound does, and cross solids, liquids and gases. S waves, or secondary waves, come next; they shake material at right angles to their path, like a rope flicked at one end, and move only through solids. Surface waves run along the Earth's outer skin; they are the slowest, arrive last and do the most damage to buildings.

Seismograph records from around the world reveal a telling pattern. Measuring distance from the epicentre as an angle at the Earth's centre, both P and S waves arrive directly up to about 105°. Beyond about 105°, direct S waves never arrive. Between about 105° and 145°, direct P waves are missing too; this belt is the shadow zone. Beyond 145°, P waves appear again. Two conclusions follow. S waves cannot cross liquid, so their absence shows that the outer core is liquid. P waves bend sharply as they enter and leave the core, which steers them away from the 105°–145° belt. Because S waves are missing everywhere beyond 105°, their shadow is much larger than that of P waves. Every earthquake has its own shadow zone, measured from its own epicentre.

Picture the crust as the skin on a giant fruit. No layer lies above it, none below it is thinner, and all of it together fills under one per cent of the Earth's volume. Beneath the continents the crust averages roughly 30 to 40 km in depth and thickens to around 70 km below lofty mountain belts like the Himalaya; it is old, relatively light and made mostly of granite-like rock rich in silica and aluminium, hence the old name SIAL. Oceanic crust, under the deep sea floor, is only about 5–10 km thick, younger and heavier, made mostly of basalt rich in silica and magnesium, hence SIMA. Being lighter and thicker, continental crust stands high, while oceanic crust lies low and forms the ocean basins. The crust ends below at the Mohorovičić discontinuity, the Moho for short, a boundary at which earthquake waves abruptly travel faster; the Croatian seismologist Andrija Mohorovičić identified it in 1909 while studying an earthquake near Zagreb.

Below the Moho lies the mantle, reaching down to about 2,900 km and making up more than four-fifths of the Earth's volume. It is dense rock rich in silicates of iron and magnesium. S waves pass through it, so it behaves as a solid, yet it is hot enough to creep very slowly over millions of years, like an extremely stiff toffee. In the upper mantle lies the asthenosphere from roughly 100 km to a few hundred kilometres down; the rock there is near its melting point and partly molten in places, and much of the magma that feeds volcanoes forms there. Above it, the crust and the rigid uppermost mantle together form the lithosphere, commonly put at about 100 km thick. The lithosphere is broken into plates that move over the soft asthenosphere, the basis of plate tectonics in the next chapter.

The core begins at the Gutenberg discontinuity, about 2,900 km down, where P waves slow sharply and S waves stop. The outer core, down to about 5,150 km, is liquid metal; slow currents in it act like a dynamo and generate the Earth's magnetic field, which shields the planet from charged particles from the Sun and makes a compass needle point north. The inner core, from about 5,150 km to the centre at about 6,371 km, is solid although its temperature is estimated at 5,000–6,000 °C, comparable to the surface of the Sun, because enormous pressure keeps it from melting. The boundary between the outer and inner core is commonly called the Lehmann discontinuity, after the Danish seismologist Inge Lehmann, who proposed a solid inner core in 1936. The core is mainly iron with some nickel, hence the old name NIFE, and it is the densest layer.

The crust is built from elements combined into minerals, and minerals combined into rocks. Count up the crust's weight and just eight elements together account for close to 98 per cent of the total. Top of that list by weight is oxygen, near 46 percent; silicon follows at close to 28, aluminium near 8 and iron near 5, and the remaining four, calcium, sodium, potassium and magnesium, supply somewhere between 2 and 4 percent apiece. For the planet as a whole the order changes: because the core is mostly metal, iron is the most abundant element of the whole Earth by mass. Each mineral has a fixed chemistry and an orderly, repeating arrangement of atoms. With oxygen and silicon dominant, most minerals are silicates; feldspars are the commonest group, and quartz is another very common mineral. A rock is a natural mixture of minerals: granite, for instance, is mainly feldspar and quartz with a little mica.

igneous rocks are born when molten rock cools and sets solid; their name borrows the Latin 'ignis', fire. Geologists label them primary because sedimentary and metamorphic rocks are ultimately made, directly or indirectly, from material that first solidified this way. Where the cooling happens sets their texture. magma cooling slowly deep underground grows large crystals, giving coarse-grained intrusive, or plutonic, rocks such as granite. Lava cooling quickly at the surface keeps its crystals tiny, giving fine-grained extrusive, or volcanic, rocks such as basalt. Igneous rocks are generally hard, are not layered and contain no fossils. India's best-known example is the Deccan Traps, a vast pile of basalt lava flows covering much of Maharashtra and parts of neighbouring states, laid down about 66 million years ago, around the end of the age of dinosaurs. Weathering of that basalt across ages produced the dark Deccan soil, and a large part of the plateau's cotton crop now grows in it.

sedimentary rocks are made from fragments of older rocks and the remains of living things. Rivers, wind, glaciers and waves carry weathered particles and drop them as sediment, mostly under water. Layer settles on layer; the weight above compacts the layers below and natural cements bind the grains, a hardening called lithification. So these rocks are usually stratified, and they are the ones that hold fossils. By origin they are mechanically formed from rock fragments (sandstone, conglomerate, shale), organically formed from plant and animal remains (coal, and limestone or chalk from shells and coral), or chemically formed when minerals come out of solution (rock salt, gypsum, some limestones). Coal, petroleum and natural gas occur in them, and the red sandstone of Delhi's Red Fort is a familiar example.

metamorphic rocks are existing rocks, whether igneous, sedimentary or already metamorphic, changed by great heat, great pressure or both, without melting. Their minerals recrystallise and often line up in bands or sheets, a texture called foliation. Heat from nearby magma causes contact, or thermal, metamorphism; the squeezing and heating of wide areas during mountain building causes regional metamorphism. Four pairs are asked again and again: limestone becomes marble, sandstone becomes quartzite, shale becomes slate and granite becomes gneiss. The white marble of the Taj Mahal, quarried at Makrana in Rajasthan, began as limestone laid down under water. Metamorphic rocks are usually harder and more compact than their parent rocks.

The three families are not fixed: over geological time any rock can become any other, a recycling called the rock cycle. Igneous rock exposed at the surface weathers into sediment and then sedimentary rock. Buried, heated or squeezed, igneous or sedimentary rock becomes metamorphic. Carried deeper still, any rock can melt into magma and cool into new igneous rock, and uplift brings rocks back to the surface to be worn down again. The Sun's energy, working through rain, rivers, wind and ice, drives weathering and erosion at the surface; the Earth's internal heat drives melting, metamorphism and the movement of plates. So the two halves of this part connect: the layered interior known from seismic waves supplies the heat and the magma, the crust is where rocks are made and remade, and the same forces shape the landforms, earthquakes and volcanoes of the next chapter.

Exam traps

• Solids, liquids and gases all carry P waves, but S waves stop the moment they meet anything that is not solid. The absence of S waves beyond about 105° from the epicentre is the evidence for a liquid outer core.
• Direct P waves are missing between about 105° and 145° from the epicentre, but S waves are missing everywhere beyond 105°, so the area left without S waves covers far more of the globe than the band without direct P waves.
• Energy is released at the focus (hypocentre), below the ground; the epicentre is the spot on the ground lying straight above it.
• Boundaries in order: Moho (crust and mantle), Gutenberg (mantle and core, about 2,900 km), Lehmann (outer and inner core, about 5,150 km). Questions shuffle these.
• Oxygen is the most abundant element in the crust, followed by silicon; for the Earth as a whole, iron is the most abundant by mass.
• Do not equate the lithosphere with the crust alone, since the rigid top layer of the mantle belongs to it too; beneath it sits the asthenosphere, a weak zone of the upper mantle from which much magma rises.
• The inner core is solid although it is the hottest part, because of enormous pressure; it is the outer core that is liquid.
• Fossils are found in sedimentary rocks. Granite is intrusive igneous and basalt extrusive igneous; marble comes from limestone, quartzite from sandstone, slate from shale and gneiss from granite.

Recap

• The interior is known mostly indirectly. Direct sources: mines, drilling (Kola, about 12 km), volcanic material. Indirect: temperature, pressure and density, meteorites, gravity, magnetism and, above all, seismic waves.
• Focus underground, epicentre on the surface above it; P waves fastest and pass through all states; S waves only through solids; surface waves slowest and most destructive.
• Shadow zone: no direct P waves between about 105° and 145°; no S waves beyond 105°; hence a liquid outer core.
• Crust: continental (thick, granitic, light, SIAL) and oceanic (thin, basaltic, dense, SIMA); the Moho (1909) at its base.
• Mantle to about 2,900 km; asthenosphere weak, partly molten, a source of magma; lithosphere = crust + rigid upper mantle, broken into plates.
• Core: liquid outer core (source of the magnetic field) and solid inner core; Gutenberg at about 2,900 km, Lehmann at about 5,150 km; iron and nickel (NIFE).
• Crust by weight: O > Si > Al > Fe > Ca > Na > K > Mg; whole Earth: iron first. Feldspar and quartz are common minerals; rocks are mixtures of minerals.
• Igneous (granite, basalt, Deccan Traps), sedimentary (layers, fossils, coal, Red Fort sandstone), metamorphic (marble, quartzite, slate, gneiss), all linked by the rock cycle.

Practice questions

Sources

• NASA Science: Earth facts and the Earth's interior and magnetic field explainers (National Aeronautics and Space Administration, US government) — https://science.nasa.gov/earth/facts/
• United States Geological Survey: 'This Dynamic Earth' and educational material on the Earth's interior, minerals and the rock cycle (US government publication) — https://pubs.usgs.gov/publication/7000097
• USGS Earthquake Hazards Program: seismic waves, focus and epicentre, seismographs, and the structure of the Earth from seismology — https://www.usgs.gov/programs/earthquake-hazards
• Geological Survey of India, Ministry of Mines: geology of India, the Deccan Traps and rock types — https://www.gsi.gov.in