Exploring the Internal Structure of the Earth: How Scientists Study Our Planet

The interior of the Earth is arranged in concentric shells — a thin outer crust, a thick mantle beneath it, and a dense metallic core at the centre. Understanding the internal structure of the Earth has occupied scientists for centuries, because no one can travel to the centre to look directly. What we know comes instead from seismic waves, gravity measurements, laboratory experiments, and the behaviour of volcanoes and the magnetic field.

How the Earth and its shells came to exist

The layered structure of the Earth formed through planetary differentiation, a process in which a hot, partly molten young planet separated into shells of different density. Many millions of years ago our planet was, in all likelihood, a fiery-liquid and intensely heated body — almost a dwarf "sun", a small reddish star that poured heat and light into surrounding space. As gravity pulled the heaviest material inward, iron and nickel sank to form the core while lighter silicate rock rose to form the mantle and crust.

Ideas about the origin of the Earth

The Earth originally had a somewhat different shape, and it was during this early period that its permanent companion, the Moon, is thought to have separated from it. In any case the Moon separated from the Earth before the Earth's crust formed — no less than roughly four billion years ago. The Moon itself is differentiated, with a thin crust, a silicate mantle and a small iron-rich core, mirroring on a smaller scale the structure of the Earth. About the results of the mutual gravitational attraction between the Earth and the Moon, read more: Ocean tides.

The Moon — the Earth's eternal companion

The Moon serves as a natural comparison for studying the Earth's interior.

Transformations of the Earth and the formation of its shells

Millions of years passed before the major transformations of the Earth produced the world we recognise today. Over that time the planet became covered with a sufficiently thick layer of crust. It took a long time for large accumulations of water to gather, for the continents, seas and oceans to take their final form, and for an atmosphere — an air envelope reaching outward in every direction to a distance of up to 1,000 kilometres — to form around the Earth.

How is the internal structure of the Earth studied?

The interior of the Earth is studied mostly by indirect methods, because direct access reaches only a few kilometres down. Scientists combine deep mining and drilling, geological study of rock strata, the analysis of earthquake waves, gravity measurements and laboratory experiments on rocks under extreme pressure and temperature. Each method covers a different depth range, and together they build a picture of the whole 6,371-kilometre radius.

The difficulty of direct exploration: mines, tunnels and boreholes

Direct exploration of the Earth's interior reaches only the outermost few kilometres. Mines, tunnels and boreholes allow conclusions about the internal structure of the Earth only to a depth of about 4–4.5 kilometres.

Geological study of rock strata

Geology extends our knowledge a little further by reading the layered rocks exposed at and near the surface. By studying the various strata, geology makes it possible to judge the structure of the Earth only to a depth of about 10–20 kilometres. This includes tracking the rock cycle, in which rock is continuously created, destroyed and recycled, but it still tells us nothing direct about the deep mantle or the core.

The seismic method: studying earthquakes

The seismic method is the single most powerful tool for revealing the deep interior of the Earth. Conclusions about the structure at the remaining 6,330–6,350 kilometres of depth can be drawn only from thorough, all-round study of earthquakes together with measurements of gravity at different points on the Earth's surface. During an earthquake, seismic waves travel through the Earth like waves in a solid, elastic body, and the way these earthquake waves speed up, slow down, bend or stop at different depths reveals the boundaries between layers.

Major discoveries in geophysics came from interpreting these seismic waveforms:

• Andrija Mohorovičić, a scientist from Croatia, discovered in 1909 a sharp jump in seismic wave velocity that marks the boundary between the crust and the mantle — now called the Mohorovičić discontinuity, or simply the "Moho".
• R.D. Oldham used the absence of certain waves to argue that the Earth has a distinct core.
• Inge Lehmann, analysing earthquake waves in 1936, showed that the core is not uniform: she identified a solid inner core inside the liquid outer core.

Modern receiver-array stations continue this work with great precision. Researchers including John Vidale and Keith Koper of the USC Dornsife College of Letters, Arts and Sciences, together with colleagues such as Guanning Pang, Ruoyan Wang and Wei Wang, have analysed repeating earthquakes from the South Sandwich Islands recorded at receiver arrays near Fairbanks, Alaska, and Yellowknife, Canada. Their waveform-analysis methods, reported in Nature Geoscience and supported by bodies such as the National Science Foundation and the Chinese Academy of Sciences, revealed both that the inner core's rotation has been slowing relative to the surface and that its surface is changing shape over time.

Gravimetry and the measurement of gravity

Gravimetry is the science of measuring the force of gravity, and it reveals how mass is distributed inside the Earth. It is of great importance for determining masses within the Earth and is widely used in geodetic surveying. The foundations of gravity science trace back to Isaac Newton, whose law of gravitation made it possible to weigh the planet; from such measurements we know the Earth's average density is about 5.5 times that of water — far denser than surface rock, which proves the interior must be much heavier.

Gravity studies have been carried out not only on land but also at sea. For example, gravity was measured on the Black, Japanese and Sea of Okhotsk waters from submarines, using special pendulum instruments, so that the readings would be free of the disturbances caused by waves at the surface.

What are the internal shells of the Earth?

The Earth is divided into three main chemical shells — the crust, the mantle and the core — and into mechanical layers defined by how the rock behaves. The chemical (compositional) classification asks what each layer is made of, while the physical (mechanical) classification asks how it deforms. The two schemes overlap but do not match exactly, which is why both are used.

The Earth's crust: structure, composition and thickness

The crust is the thin, outermost solid shell of the Earth and the only layer we can sample directly. Down to a depth of nearly 10 kilometres the crust is, surprisingly, about half oxygen by weight; silicon, aluminium and iron also occupy a large place in its composition (more on this: Ferrous and non-ferrous metals and their ores). Other elements gradually enter the mixture in smaller amounts. The crust ranges from only a few kilometres thick under the oceans to several tens of kilometres under the continents.

Continental and oceanic crust

The crust comes in two distinct types that differ in thickness, density and composition:

• Continental crust is thick (roughly 30–70 kilometres), relatively light, and rich in granitic, silica-aluminium material. It builds the continents and reaches its greatest thickness beneath young mountain ranges such as the Himalayas.
• Oceanic crust is thin (about 5–10 kilometres), denser, and basaltic, rich in silica and magnesium. It floors the Pacific Ocean and other basins and is created at each mid-ocean ridge, then destroyed where it sinks into ocean trenches.

The Earth's mantle and its properties

The mantle is the thick shell between the crust and the core, and it makes up the bulk of the Earth's volume. Between the central core and the solid outer shell lies an intermediate layer — a stiff, viscous mass called magma or sima (named for the silicon and magnesium it contains alongside other elements). This intermediate layer is about a thousand kilometres thick in the older description, but modern measurements show the full mantle extends to about 2,900 kilometres. The lower mantle, or mesosphere, is hot, dense and under enormous pressure, and the dominant rock there is peridotite, a magnesium-iron silicate.

The asthenosphere: its movement and properties

The asthenosphere is the soft, partly molten upper-mantle layer on which the rigid plates ride. Although solid, it deforms slowly like extremely thick tar — it has a very high viscosity but still flows over geological time. Above it lies the lithosphere, the rigid shell of crust plus uppermost mantle that is broken into tectonic plates. Slow convection in the mantle and movement within the asthenosphere drag these plates along, which is the engine of plate tectonics.

The Earth's core: outer and inner

The core is the dense metallic centre of the Earth, made mostly of iron and nickel, and it is divided into a liquid outer core and a solid inner core. The central part of the Earth's spheroid is an intensely heated "metallic" core with a radius of more than three thousand kilometres. Based on earthquake studies — waves travel inside the Earth as in a solid, elastic body — it was concluded that there is no fiery-liquid or gaseous core at the very centre, even though the temperature there exceeds three thousand degrees, and that the central core is in fact even harder than steel.

The composition and density of the core

The core's composition and density are known from seismic data, laboratory work and meteorite studies. The material making up the central core of our planet is 9–10 times denser than water; the temperature of this part of the Earth, by some estimates, exceeds three thousand degrees, and the pressure reaches up to three million atmospheres. The idea that the core is iron-rich is supported by the composition of chondrite meteorites, thought to resemble the early material of the planets, and by high-pressure experiments. Francis Birch showed how to interpret seismic velocities in terms of iron at high pressure, and Don L. Anderson advanced the modern picture of the core and mantle.

The crystalline structure of the inner core

The inner core is solid crystalline iron, kept solid by immense pressure despite temperatures rivalling the Sun's surface. Recent receiver-array studies suggest that the surface of the Earth's inner core may undergo viscous deformation — slowly changing its topography — and that the inner core's rotation, once thought to run slightly faster than the rest of the planet, has been slowing relative to the surface. Because the spinning core is coupled to the rest of the Earth, such changes can produce tiny variations in the length of the day.

The chemical and physical layers of the Earth

The Earth's layers can be classified two ways, and the difference matters. The chemical scheme divides the planet by composition into crust, mantle and core. The physical (mechanical) scheme divides it by behaviour into the rigid lithosphere, the soft asthenosphere, the stiff mesosphere (lower mantle), the liquid outer core and the solid inner core. The crucial mismatch is at the top: the lithosphere includes both the crust and the uppermost, rigid part of the mantle, so the mechanical and chemical boundaries do not coincide there.

Temperature and pressure deep inside the Earth

Temperature and pressure both rise steadily with depth inside the Earth. The presence of high temperature at the centre is shown by the simple fact that, going down into the depths of the Earth, the temperature rises by almost exactly one degree for every 33 metres. In earlier times this gradient was extended naively, leading to the conclusion that the centre reached about 200,000 degrees and that the central core was fiery-liquid — a figure now known to be far too high.

The rise of temperature with depth is real, but we now know that this steep near-surface gradient holds only for a thin layer of the planet's upper shell and is explained mainly by the presence in that layer of radioactive elements (thorium, radium, uranium and others). The slow decay of these radioactive isotopes is one of the main sources of the Earth's internal heat, alongside heat left over from the planet's formation, and it is this heat that escapes outward as the Earth's surface heat flow.

The geothermal gradient and radioactive elements

The geothermal gradient — the rate at which temperature increases downward — is steep near the surface but flattens deeper down. The episode early in the Earth's history when heavy iron sank rapidly toward the centre, releasing a great burst of heat, is sometimes called the "iron catastrophe"; it is closely tied to planetary differentiation. Today, balancing radioactive heating against this primordial heat, scientists estimate the inner core temperature at roughly 5,000–6,000 degrees Celsius rather than the impossible figures once proposed.

The Earth's magnetic field and its origin

The Earth's magnetic field is generated by motion in the liquid outer core, a process known as the geodynamo. As the molten, electrically conducting iron of the outer core churns and forms turbulent eddies — driven by heat escaping from the inner core and by the planet's rotation — it acts like a self-sustaining dynamo, generating the magnetic field that shields the surface from solar radiation. The field can exist only where the iron is hot and fluid; above the Curie temperature, solid iron loses its permanent magnetism, so the field must come from moving liquid metal rather than from a permanently magnetised core.

The link between core structure and changes in day length

Changes deep in the core can subtly alter how long a day lasts. Because the liquid outer core, the solid inner core and the rocky mantle exchange angular momentum, a slowing or speeding of the inner core's rotation shifts mass and momentum within the planet. The same studies that detected the slowing and reshaping of the inner core also connect these changes to measurable, though extremely small, variations in the length of the Earth's day.

Volcanism and the movement of continents

Volcanic eruptions are the clearest visible proof that the deep interior of the Earth holds very high temperatures. Sometimes the craters of volcanoes hurl out, with enormous force, intensely heated and molten masses of varied material — gases, vapours, ash and solidified pieces of lava — as well as large blocks of the rocks through which the erupting products pass.

The formation of continents and mountain building

The continents and great mountain ranges are built and reshaped by the slow movement of tectonic plates. Driven by mantle convection, the plates of the lithosphere collide, pull apart and slide past one another: where they collide, crust crumples upward to raise ranges such as the Himalayas; where they pull apart at a mid-ocean ridge, new oceanic crust is born; and where one plate dives beneath another at ocean trenches, old crust is consumed. This is the modern theory of plate tectonics, and it ties together volcanism, earthquakes, mountain building and the slow drift of the continents into one framework.

Conclusions about the internal structure of the Earth

The newest research into the internal structure of the Earth leads to a consistent picture of nested, differentiated shells:

• The central part of the Earth is an intensely heated metallic core with a radius of more than three thousand kilometres, split into a liquid outer core and a solid inner core.
• The temperature of this central region, by some estimates, exceeds three thousand degrees, and the pressure reaches up to three million atmospheres.
• The material of the central core is 9–10 times denser than water.
• The Earth's solid outer shell — crust and rigid upper mantle — is a little over a thousand kilometres thick in the older reckoning and is almost three times denser than water; the full crust-and-mantle system above the core reaches about 2,900 kilometres.

Authoritative agencies such as the U.S. Geological Survey (USGS), part of the Department of the Interior, continue to monitor earthquakes and refine this model, while university teams publish new findings in journals such as Nature Geoscience. Taken together, the seismic, gravimetric and laboratory evidence shows the Earth as a layered, dynamic body — a metallic core driving a magnetic field, a convecting mantle moving the plates, and a thin crust on which all surface life and geology unfold.