Structure and plate motion

Density increases with depth toward the core. Continental crust is the least dense ($\approx 2,700–2,800\,\text{kg/m}^3$), oceanic crust is denser ($\approx 3,000\,\text{kg/m}^3$), the mantle ranges from $3,300\,\text{kg/m}^3$ to $5,600\,\text{kg/m}^3$, the outer core hits $9,900–12,200\,\text{kg/m}^3$, and the inner core reaches an extreme $12,800–13,100\,\text{kg/m}^3$.

Worked Example: Unit Conversion

In exams, you may need to convert density from $\text{g/cm}^3$ to $\text{kg/m}^3$. To do this, multiply the value by $1,000$. For example, if the density of oceanic crust is given as $3.0\,\text{g/cm}^3$:

Interpreting Cross-Sections

Peeling an apple gives you a great sense of scale for the Earth; the skin is just as thin compared to the fruit as the crust is to the rest of the planet. In exams, you may be asked to identify layers on a cross-sectional diagram.

Boundaries between layers with different properties are called discontinuities. For instance, the Moho discontinuity separates the crust and mantle, while the Gutenberg discontinuity separates the mantle and outer core.

On a typical 10 cm scaled diagram of the Earth's interior:

• Crust: Drawn as a single thin line ($\approx 0.1\,\text{mm}$).
• Mantle: The largest segment ($\approx 4.5\,\text{cm}$).
• Outer Core: The second largest ($\approx 3.5\,\text{cm}$).
• Inner Core: The central circle ($\approx 1.9\,\text{cm}$).

The Lithosphere and Asthenosphere

Why do tectonic plates move instead of staying firmly in place? To understand this, geologists classify the upper layers of the Earth based on their mechanical properties rather than just their chemistry.

The lithosphere is the rigid outer shell of the Earth, consisting of the crust and the uppermost solid part of the mantle. It is about 100 km deep on average (varying from 5 km under oceans to 200 km under mountains), with temperatures ranging from surface levels up to approximately 1,000°C at its base. Because it is rigid and brittle, it breaks under stress, which is where earthquakes occur.

Directly below the lithosphere sits the asthenosphere, which extends down to about 400–660 km. Reaching temperatures of at least 1,300°C, this layer is semi-molten. It behaves like a soft solid (often compared to porridge or toothpaste) that flows slowly. Because the lithosphere is less dense than the asthenosphere, the rigid tectonic plates effectively float on top of this moving layer.

Heat Sources and Convection Currents

Every time you boil a pan of thick soup, you can watch hotter liquid rise from the bottom, move sideways at the surface, cool, and sink back down—this is exactly what happens inside the Earth's mantle.

The core's immense internal heat comes from two sources. The primary source is radioactive decay, which is the breakdown of unstable isotopes (like Uranium-238 and Potassium-40) releasing thermal energy. The secondary source is primordial heat left over from the Earth's formation.

This heat drives convection currents in the mantle through a step-by-step mechanism:

1. Radioactive decay in the core heats the lower mantle rock.
2. The heated rock expands, becomes less dense, and slowly rises toward the crust.
3. Upon reaching the rigid lithosphere, the rock is forced sideways in a horizontal flow.
4. As it moves away from the heat source, the rock cools, becomes denser, and sinks back toward the core in a continuous circular motion.

Worked Example: 3-Mark "Explain" Question

Explain how the core’s internal heat source generates convection currents. (3 marks)

• Point 1: Heat from the core is generated by radioactive decay.
• Point 2: This causes mantle material to heat up, become less dense, and rise toward the crust.
• Point 3: The rock moves horizontally, cools, becomes denser, and sinks back down to complete the circular convection cell.

Mechanisms of Plate Motion

Understanding the forces inside the Earth explains why continents drift apart at 2–5 cm per year, roughly the speed your fingernails grow. Tectonic plate movement is driven by a combination of forces acting together.

As convection currents flow horizontally beneath the lithosphere, they exert frictional drag on the underside of the tectonic plates, helping to pull them along.

At constructive (divergent) boundaries, new rock is formed at elevated mid-ocean ridges. As magma rises and cools into dense rock, gravity causes the plate to slide down the slope away from the ridge. This gravitational sliding is known as ridge push.

At destructive (convergent) boundaries, cold, dense oceanic plates sink into the mantle. The sheer weight and density of this sinking slab pulls the rest of the trailing plate down with it due to gravity. This mechanism is called slab pull, and it is widely considered the strongest primary driver of tectonic plate movement.

Worked Example: 3-Mark "Explain" Question

Explain the process of slab pull. (3 marks)

• Point 1: At subduction zones, the denser oceanic plate sinks into the mantle.
• Point 2: This occurs because the plate is cooler and denser than the surrounding mantle.
• Point 3: The weight of this sinking "slab" pulls the rest of the plate along with it due to gravity.