2.1 Internal structure of the Earth

Earth's internal structure consists of distinct layers that differ in composition, density, and physical state. Understanding these layers is essential for explaining plate tectonics, volcanism, and Earth's magnetic field. Scientists have mapped this structure primarily through the behavior of seismic waves, since no drill has ever reached beyond the crust.

Earth's Layers

Composition and Density

The crust is Earth's outermost layer, composed of solid rocks and minerals. It's by far the thinnest layer, but it varies quite a bit depending on location:

• Oceanic crust is 5–10 km thick and made mostly of basalt (denser, ~3.0 g/cm³)
• Continental crust is 30–70 km thick and made mostly of granite (less dense, ~2.7 g/cm³)

The mantle sits between the crust and the core. At roughly 2,900 km thick, it makes up about 84% of Earth's total volume. It's composed of dense, iron- and magnesium-rich rock, primarily peridotite. Density increases with depth: the upper mantle ranges from 3.4–4.4 g/cm³, while the lower mantle reaches 4.4–5.6 g/cm³.

A key point here: the mantle is mostly solid, not liquid. It behaves as a very slow-flowing solid over long timescales, which is what allows convection and plate movement.

Core Characteristics

The outer core is a liquid layer beneath the mantle, composed primarily of iron and nickel.

• Approximately 2,300 km thick (from ~2,890 km to ~5,150 km depth)
• Temperatures range from about 4,000–6,000°C
• Density ranges from 9.9–12.2 g/cm³
• Because it's liquid metal in motion, it generates Earth's magnetic field through convection currents (the geodynamo)

The inner core is a solid sphere at Earth's center, also made of iron and nickel.

• Radius of approximately 1,220 km (from ~5,150 km to ~6,370 km depth)
• Temperatures reach 5,000–7,000°C, yet it remains solid because the immense pressure at that depth raises the melting point of iron above the actual temperature
• Highest density of any layer: 12.8–13.1 g/cm³

This density progression from crust to core is why Earth's average density (5.5 g/cm³) is so much higher than the density of surface rocks. Most of Earth's mass is concentrated in the iron-rich core.

Mantle Divisions

Lithosphere and Asthenosphere

These two divisions cut across the crust-mantle boundary because they're defined by mechanical behavior (rigid vs. flowing) rather than chemical composition.

• The lithosphere includes the crust plus the rigid uppermost mantle. It ranges from ~50 km thick under oceans to ~200 km thick under continents. This is the layer that's broken into tectonic plates.
• The asthenosphere lies beneath the lithosphere, extending from roughly 80–200 km down to about 660 km depth. It's solid but partially molten and ductile enough to flow very slowly. This flow is what allows tectonic plates above to move.

Think of it this way: the lithosphere is like a set of rigid rafts floating on the slowly deformable asthenosphere beneath.

Mohorovičić Discontinuity (Moho)

The Moho is the boundary between the crust and the mantle. Croatian seismologist Andrija Mohorovičić discovered it in 1909 when he noticed that seismic waves from a nearby earthquake arrived faster than expected at distant stations. The explanation: waves that traveled deeper hit a layer where they suddenly sped up.

• P-wave velocity jumps from about 6.7–7.2 km/s (crust) to 7.6–8.6 km/s (mantle) at this boundary
• The Moho sits at ~5–10 km depth beneath oceanic crust and ~20–90 km beneath continental crust
• This velocity jump reflects the change from crustal rocks (granite/basalt) to denser mantle rock (peridotite)

Seismic Evidence

Seismic Waves and Earth's Interior

Seismic waves are the primary tool for mapping Earth's interior. When an earthquake occurs, it sends out waves that travel through the planet, and their speed, direction, and arrival times reveal what they passed through.

The two main body wave types behave differently:

• P-waves (primary/compressional) travel through both solids and liquids. They're faster and arrive first at seismograph stations.
• S-waves (secondary/shear) travel only through solids. They cannot propagate through liquid.

How seismic waves reveal layer boundaries:

1. Wave velocity changes with the density and rigidity of the material. A sudden velocity change at a specific depth indicates a boundary between layers (the Moho is the classic example).
2. S-waves disappear entirely when they encounter the outer core. Seismograph stations on the far side of an earthquake receive P-waves but no direct S-waves, confirming the outer core is liquid.
3. P-waves bend (refract) as they enter the liquid outer core, creating a P-wave shadow zone between 103° and 143° from the earthquake's epicenter. Stations in this zone receive no direct P-waves. This shadow zone's geometry helped scientists determine the size and depth of the outer core.