1.2 Earth's structure and composition

Earth's structure and composition form the foundation of geophysics. Understanding what the planet is made of and how it's organized at depth is essential for interpreting seismic data, explaining plate tectonics, and modeling processes like volcanism and magnetic field generation.

Seismic waves are the primary tool for probing Earth's interior. Because P-waves and S-waves respond differently to solid and liquid materials, they act as remote sensors that map out boundaries between layers thousands of kilometers below the surface.

Earth's Interior Layers

Major Layers and Their Depths

The Earth is divided into four main layers, each with distinct physical and chemical properties.

• Crust: The outermost layer, ranging from about 5 km thick beneath the oceans to roughly 70 km thick under continental mountain belts. It's composed of solid rocks and minerals.
• Mantle: Extends from the base of the crust down to about 2,900 km depth. The mantle is made of hot, dense silicate rock. In the upper mantle (roughly 100–200 km depth), a partially molten zone called the asthenosphere allows tectonic plates to move.
• Outer core: A liquid layer from approximately 2,900 to 5,100 km depth, composed primarily of iron-nickel alloy.
• Inner core: The innermost layer, from about 5,100 km to the Earth's center at 6,371 km. It's solid iron-nickel alloy under immense pressure and temperature.

Temperature and Pressure Changes with Depth

Both temperature and pressure increase with depth. This gradient drives the different physical states of each layer.

• In the outer core, temperatures are extremely high (roughly 4,000–5,000°C), but the material remains liquid because the pressure, while enormous, isn't sufficient to force it into a solid state at those temperatures.
• In the inner core, the pressure is so great (estimated at over 360 GPa) that it forces the iron-nickel alloy into a solid state despite temperatures comparable to the surface of the Sun (~5,000–6,000°C).

The key takeaway: it's the balance between temperature and pressure that determines whether a given layer is solid, liquid, or able to flow plastically.

Composition and Properties of Earth's Layers

Density and Composition Variations

Density increases systematically from the surface to the center, reflecting changes in both composition and pressure.

Physical State and Behavior of Materials

• The crust and uppermost mantle (together called the lithosphere) behave as rigid, brittle solids. This is the layer that breaks during earthquakes.
• The asthenosphere (upper mantle, roughly 100–200 km depth) is solid but can deform plastically over geological timescales. This ductile flow is what allows tectonic plates to move.
• The lower mantle is also solid but convects very slowly, driving large-scale mantle circulation.
• The outer core is liquid. Convection currents in this electrically conducting fluid generate Earth's magnetic field through a process called the geodynamo.
• The inner core is solid. It may rotate slightly faster than the rest of the Earth, though this is still debated.

These physical states directly control processes like plate tectonics, volcanism, and seismic wave propagation.

Seismic Waves and Earth's Structure

Types of Seismic Waves and Their Propagation

Seismic waves, generated by earthquakes or artificial sources (like explosions used in exploration), travel through the Earth and carry information about what they've passed through.

• P-waves (primary waves) are compressional: they push and pull material in the direction of travel. They propagate through both solids and liquids.
• S-waves (secondary waves) are shear waves: they move material perpendicular to the direction of travel. They can only propagate through solids because liquids don't support shear stress.

The velocity of both wave types depends on the density and elastic properties of the material. When waves cross a boundary between layers of different properties, they refract (bend) or reflect, just like light passing between air and water.

Evidence for Earth's Internal Structure

Several key observations from seismology constrain the depth and nature of each layer:

1. The Mohorovičić discontinuity (Moho): A sharp increase in seismic velocity marks the boundary between the crust and mantle. Andrija Mohorovičić identified it in 1909 from the arrival times of seismic waves at different distances from an earthquake.
2. S-wave shadow zone: S-waves are not detected at angular distances greater than about 104° from an earthquake source. This tells us the outer core is liquid, since S-waves can't travel through it.
3. P-wave shadow zone: P-waves are absent between roughly 104° and 140° from the source. This results from refraction of P-waves as they enter the liquid outer core, bending them away from that zone. Faint P-wave arrivals within the shadow zone were later explained by reflections off the solid inner core.
4. Seismic tomography: By analyzing travel times from thousands of earthquakes recorded at stations worldwide, geophysicists build 3D velocity models of the interior. These images reveal heterogeneity within layers, such as hot upwellings (mantle plumes) and cold downwellings (subducting slabs).

Rocks and Minerals of the Crust and Mantle

Rock Types and Formation Processes

The crust and upper mantle are composed of three fundamental rock types, classified by how they form:

• Igneous rocks form from the cooling and solidification of magma or lava. Basalt (fine-grained, from rapid cooling at the surface) dominates oceanic crust, while granite (coarse-grained, from slow cooling at depth) is characteristic of continental crust.
• Sedimentary rocks form from the accumulation and lithification of sediments produced by weathering and erosion. Sandstone (from sand-sized grains) and limestone (often from biological carbonate material) are common examples.
• Metamorphic rocks form when pre-existing rocks are transformed by elevated temperature and pressure without fully melting. Gneiss (from granite or sedimentary protoliths) and marble (from limestone) are typical examples.

Common Rock-Forming Minerals

• Silicate minerals are by far the most abundant in the crust and upper mantle. Key examples include quartz, feldspar, mica, pyroxene, and olivine. Olivine and pyroxene dominate the upper mantle, while quartz and feldspar are more common in crustal rocks.
• Carbonate minerals like calcite and dolomite are the main constituents of sedimentary limestones and dolostones. They often form through biological activity (shells, coral) or chemical precipitation from seawater.
• Oxide minerals such as magnetite and hematite are important in some igneous and metamorphic rocks. Magnetite is particularly relevant in geophysics because it carries a record of Earth's magnetic field at the time the rock formed.

Factors Influencing Rock and Mineral Distribution

The types of rocks and minerals you find in a given region depend on its geological context:

• Tectonic setting is a major control. Mid-ocean ridges (divergent boundaries) produce basaltic oceanic crust. Subduction zones (convergent boundaries) generate a wider range of igneous rocks, from basalt to andesite to granite, through processes like partial melting and magmatic differentiation. Transform boundaries primarily deform existing rocks rather than creating new ones.
• Magmatic processes such as partial melting, fractional crystallization, and magma mixing determine the specific composition of igneous rocks. For example, partial melting of mantle peridotite at a mid-ocean ridge produces basaltic magma because lower-melting-point minerals melt first.
• Regional geological history matters too. A region that has experienced multiple tectonic events, burial and uplift, or repeated metamorphic episodes will have a more complex rock record than a stable continental interior (craton) that has remained relatively undisturbed for billions of years.