Structure of the Earth and Convection Currents

Mechanical Layers: Lithosphere vs Asthenosphere

While the crust and mantle are defined by what they are made of, scientists also divide the outer Earth by how it physically behaves.

• The Lithosphere is the rigid, brittle outer shell of the planet. It includes the entire crust and the very top, solid portion of the mantle. This brittle layer is broken into giant puzzle pieces called Tectonic Plates.
• Just below the lithosphere lies the Asthenosphere ($35$–$400\text{ km}$ deep). This is a semi-molten layer of the upper mantle. Because it experiences $1$–$5\%$ partial melting, it behaves as a rheid (a solid material that flows plastically). This allows the rigid lithospheric plates above to "float" and slide over it.

The Internal Heat Engine and Convection

To understand why tectonic plates move, we must look at the Earth's internal heat engine. The primary source of this heat is Radioactive Decay.

Unstable isotopes, such as Uranium-238 and Potassium-40, break down in the core and mantle, releasing immense amounts of geothermal energy. This extreme heat creates a Convection Current in the asthenosphere.

1. Heat from the core warms the semi-molten magma in the lower asthenosphere.
2. This magma expands, becomes less dense, and slowly rises toward the lithosphere.
3. As the rising magma hits the rigid base of the lithosphere, it is forced to spread sideways, creating a frictional drag (called Basal Drag) that pulls the tectonic plate along.
4. As the magma moves away from the heat source, it cools, becomes denser, and sinks back down to be reheated, completing the cycle.

Modern Theories: Slab Pull and Ridge Push

While convection currents were long thought to be the sole driver of plate movement, modern geologists (and the updated OCR specification) emphasize gravity-driven forces.

• Slab Pull: This is considered the dominant force driving tectonic movement. At destructive boundaries, the cold, dense oceanic plate sinks into the mantle due to gravity. The immense weight of this sinking slab pulls the rest of the plate down behind it.
• Ridge Push: At constructive boundaries (like mid-ocean ridges), hot magma rises and cools to form new lithosphere. Because this new crust is hot and less dense, it sits higher than the surrounding seafloor. As it cools and condenses, gravity causes it to slide downhill, pushing the older plate away.

Plate Movement Rates

The Earth's lithosphere is cracked into several major tectonic plates, including the Pacific Plate, Eurasian Plate, North American Plate, and the Nazca Plate. These plates move incredibly slowly, typically between $1\text{ cm}$ and $10\text{ cm}$ per year—roughly the speed that human fingernails grow.

Worked Example

If the Nazca Plate is subducting beneath the South American Plate at an average rate of $8\text{ cm}$ per year, how far will it have travelled in $2.5\text{ million}$ years? Give your answer in kilometres.

Step 1: Identify the values.

• $\text{Speed} = 8\text{ cm/year}$
• $\text{Time} = 2,500,000\text{ years}$

Step 2: Calculate total distance in centimetres.

• $\text{Distance} = 8 \times 2,500,000 = 20,000,000\text{ cm}$

Step 3: Convert centimetres to kilometres.

• $20,000,000\text{ cm} = 200,000\text{ m}$
• $200,000\text{ m} = 200\text{ km}$