What is the primary difference between the lithosphere and the asthenosphere?

The lithosphere is the rigid, brittle outer layer consisting of the crust and upper mantle, while the asthenosphere is the semi-molten, plastic layer beneath it. The lithosphere is broken into plates that move over the flowing asthenosphere.

How does continental crust differ from oceanic crust in terms of subduction?

Oceanic crust is thinner and denser (basaltic), causing it to subduct into the mantle when it meets other plates. Continental crust is thicker and less dense (granitic), making it too buoyant to subduct; instead, it crumples to form mountains.

What distinguishes a constructive margin from a destructive margin?

At a constructive margin, plates move apart and new crust is created by rising magma. At a destructive margin, plates move together and crust is either destroyed through subduction or uplifted into mountain ranges.

Why is it a misconception that tectonic plates float on a 'sea of liquid magma'?

The mantle is actually solid rock, not liquid. However, the asthenosphere behaves 'plastically' over long periods, meaning it can flow very slowly under pressure, similar to how thick honey or putty moves.

What is the error in assuming mantle convection is the only force driving plate tectonics?

While convection provides the initial heat engine, 'Slab Pull' is often the dominant force. The weight of a subducting plate sinking into the mantle pulls the rest of the plate behind it more effectively than convection currents alone.

Why do conservative (transform) margins lack volcanic activity?

Volcanoes require magma to reach the surface, which usually happens through subduction melting or crustal thinning. At conservative margins, plates slide horizontally without creating gaps or subducting, so no magma is generated or released.

Define 'Palaeomagnetism' and its role in plate tectonic theory.

Palaeomagnetism is the study of the Earth's ancient magnetic field preserved in rocks. Symmetrical patterns of magnetic reversals on either side of mid-ocean ridges provide evidence that new crust is being created and moving outward.

What is a 'Subduction Zone'?

A subduction zone is a specific area at a convergent boundary where a denser oceanic plate sinks beneath a less dense plate into the asthenosphere. This process recycles crust and leads to deep-sea trenches and volcanic activity.

What is 'Ridge Push'?

Ridge push, or gravitational sliding, is a process at constructive margins where the higher elevation of the mid-ocean ridge causes the newly formed, warm crust to slide downward and outward under the force of gravity.

Explain the formation of an 'Island Arc'.

An island arc is a chain of volcanic islands formed at an oceanic-oceanic convergent boundary. As one oceanic plate subducts under another, it melts, and the resulting magma rises through the overriding plate to form a curved line of volcanoes.

Key Terms: Plate Tectonics | AQA A-Level Geography

1. The Layered Earth: Lithosphere and Asthenosphere

The lithosphere is the cool, rigid outermost layer of the Earth, comprising the crust and the uppermost portion of the mantle. It is broken into several large and small segments known as tectonic plates.
Beneath the lithosphere lies the asthenosphere, a semi-molten or 'plastic' layer of the upper mantle. Because it can flow slowly under high pressure and temperature, it acts as a lubricant that allows the rigid plates above to move.
The distinction between these two layers is mechanical: the lithosphere behaves as a brittle solid that breaks under stress, while the asthenosphere behaves as a ductile solid that deforms over geological time.

Diagram showing the rigid lithosphere subducting into the plastic asthenosphere at a convergent boundary.

2. The Engine of Motion: Convection and Gravity

Mantle convection is the primary driver of plate movement, where heat from the core (generated by radioactive decay) causes molten rock to rise, cool, and sink in circular patterns.
Slab pull is considered the most significant force in plate motion; it occurs when the older, colder, and denser edge of an oceanic plate sinks into the mantle at a subduction zone, pulling the rest of the plate along with it.
Ridge push (or gravitational sliding) occurs at mid-ocean ridges, where the elevated position of the newly formed crust creates a gravitational force that pushes the plate away from the ridge crest.

3. Constructive (Divergent) Plate Margins

At constructive margins, plates move away from each other, allowing magma to rise from the mantle to fill the gap and cool into new crust.
When this occurs under the ocean, it forms a mid-ocean ridge, a continuous underwater mountain range characterized by volcanic activity and shallow earthquakes.
If a constructive margin develops on a continent, it creates a rift valley, where the land stretches and sinks between parallel faults, potentially eventually forming a new ocean basin.

4. Destructive (Convergent) Plate Margins

Destructive margins occur where plates move toward each other. If an oceanic plate meets a continental plate, the denser oceanic plate is forced downward into the mantle in a process called subduction.
This interaction creates a trench (a deep-sea depression) and a volcanic arc on the overriding plate as the subducting slab melts and magma rises to the surface.
When two continental plates collide, neither subducts due to their low density; instead, the crust is crumpled and forced upward to form massive fold mountains.

5. Conservative (Transform) Margins

At conservative margins, plates slide past one another horizontally. Because crust is neither created nor destroyed, these boundaries are characterized by intense friction and the buildup of tectonic stress.
The sudden release of this stress results in powerful earthquakes along fault lines. Unlike other margins, conservative boundaries typically lack volcanic activity because there is no subduction or spreading to allow magma to reach the surface.

6. Key Distinctions: Crustal Types

Continental crust is thick (up to 70 km), relatively old, and composed primarily of granite, which has a lower density than oceanic materials.
Oceanic crust is thin (5-10 km), geologically young, and composed mainly of basalt, which is significantly denser than granite.

Feature	Continental Crust	Oceanic Crust
Composition	Granite	Basalt
Density	Lower (approx. 2.7 $g/cm^3$ )	Higher (approx. 3.0 $g/cm^3$ )
Thickness	30-70 km	5-10 km
Age	Can be billions of years old	Usually less than 200 million years

7. Exam Strategy & Tips

Identify the Margin by the Feature: If a question mentions a 'trench' or 'fold mountains,' immediately associate it with a convergent (destructive) boundary. If it mentions a 'ridge' or 'rift,' think divergent (constructive).
Density is Destiny: Always remember that density determines which plate subducts. Oceanic crust always sinks beneath continental crust because it is denser.
Check for Volcanoes: If a boundary has earthquakes but NO volcanoes, it is almost certainly a conservative (transform) margin or a continental-continental collision zone.
Evidence of Movement: Use palaeomagnetism (magnetic stripes on the seafloor) as the definitive evidence for seafloor spreading and plate movement.

Key Terms: Plate Tectonics

Summary

1. The Layered Earth: Lithosphere and Asthenosphere

2. The Engine of Motion: Convection and Gravity

3. Constructive (Divergent) Plate Margins

4. Destructive (Convergent) Plate Margins

5. Conservative (Transform) Margins

6. Key Distinctions: Crustal Types

7. Exam Strategy & Tips

Key Terms: Plate Tectonics

Summary

1. The Layered Earth: Lithosphere and Asthenosphere

2. The Engine of Motion: Convection and Gravity

3. Constructive (Divergent) Plate Margins

4. Destructive (Convergent) Plate Margins

5. Conservative (Transform) Margins

6. Key Distinctions: Crustal Types

7. Exam Strategy & Tips