Key Terms: Plate Tectonics (AQA A Level Geography): Revision Note

Structure of the Earth - key terms

Asthenosphere – A semi-molten, plastic-like layer of the upper mantle on which tectonic plates float; it flows slowly under pressure.

Continental crust – The thicker (up to 70 km), less dense layer of the Earth’s crust, composed mainly of granite.

Core – The central part of the Earth, made up of a solid inner core and a semi-molten outer core, composed mostly of iron and nickel.

Crust – The Earth’s outermost solid layer, split into oceanic and continental types.

Inner core – The solid centre of the Earth, mainly composed of iron, and the hottest part of the planet.

Lithosphere – The rigid outer part of the Earth, made up of the crust and the uppermost part of the mantle.

Mantle – The thickest layer of the Earth, located between the crust and the core, consisting of upper and lower sections with varying densities and temperatures.

Oceanic crust – The thinner (5–10 km), denser type of crust, composed mainly of basalt.

Outer core – The liquid layer of the Earth's core, responsible for generating the Earth’s magnetic field.

Radioactive decay – The breakdown of unstable elements (e.g. uranium and potassium) within the Earth's core, releasing heat that drives mantle convection.

Plate tectonics types - key terms

Convection currents – Circular movements of molten rock in the mantle caused by heat from the core.

Continental drift theory – The early idea proposed by Alfred Wegener that continents were once joined together (Pangaea) and have since drifted apart.

Gravitational sliding (ridge push) – A tectonic process where newly formed oceanic crust at mid-ocean ridges slides down a slope under gravity, helping to move tectonic plates.

Palaeomagnetism – The study of the Earth's magnetic field recorded in rocks; provides evidence for sea floor spreading at mid-ocean ridges.

Plate tectonic theory – The scientific explanation for the movement of Earth's lithospheric plates and associated phenomena like earthquakes and volcanoes.

Sea floor spreading – The process by which new oceanic crust is formed at mid-ocean ridges and moves away, confirmed by symmetrical magnetic patterns on either side.

Slab pull – A tectonic process where the subducting (sinking) oceanic plate is pulled into the mantle by gravity, driving plate motion.

Tectonic plates – Large sections of the Earth’s lithosphere that move over the asthenosphere, forming boundaries where geological activity occurs.

Processes and landforms of plate margins - key terms

Collision boundary – Where two continental plates meet and are forced upwards, creating fold mountains; no subduction or volcanic activity occurs.

Constructive (divergent) plate margin – A boundary where plates move apart, forming new crust, mid-ocean ridges, rift valleys, and mild volcanic activity.

Conservative (transform) plate margin – A boundary where plates slide past each other, often causing earthquakes (e.g. the San Andreas Fault) but no volcanoes.

Destructive (convergent) plate margin – A boundary where plates move towards each other; one plate subducts, forming trenches, volcanoes, and earthquakes.

Fold mountains – High mountain ranges formed by the compression and uplift of the crust at convergent boundaries (e.g. the Andes, Himalayas).

Island arc – A chain of volcanic islands formed above a subduction zone at an oceanic–oceanic destructive boundary (e.g. Japan).

Mid-ocean ridge – An underwater mountain range formed at a constructive boundary where new oceanic crust is created by rising magma.

Rift valley – A deep valley formed when land stretches and drops between faults at a constructive boundary on land (e.g. East African Rift Valley).

Subduction zone – The area where an oceanic plate descends into the mantle beneath another plate, creating deep-sea trenches and volcanic arcs.

Trench – A deep depression in the ocean floor formed at a subduction zone; examples include the Peru-Chile Trench and Mariana Trench.

Magma plumes - key terms

Hot spot – A location in the mantle where radioactive decay produces heat, forming a stationary magma plume that can cause volcanic activity far from plate margins.

Island chain – A line of volcanic islands formed as a tectonic plate moves over a stationary magma plume; older islands move away and become extinct (e.g. Hawaii).

Magma plume – An upwelling of superheated rock rising through the mantle from near the core, potentially breaking through the crust to form volcanoes.