The three types of plate boundary (constructive, destructive and conservative), the processes and features at each, and the driving forces of plate motion (mantle convection, ridge push and slab pull).

What this dot point is asking

WJEC wants you to describe the three types of plate boundary, the processes and landforms produced at each, and the forces that drive plate motion. This is the working core of plate tectonics and feeds directly into the geohazards theme: nearly all earthquakes and volcanoes occur at plate boundaries.

The answer

The three types of boundary

Plates interact in three ways. At a constructive (divergent) boundary plates move apart and new crust is created. At a destructive (convergent) boundary plates move together and crust is destroyed by subduction or crumpled in collision. At a conservative (transform) boundary plates slide past one another and crust is neither created nor destroyed.

Constructive (divergent) boundaries

Plates move apart, mantle rises and melts by decompression, and new basaltic crust is created, building a mid-ocean ridge (such as the Mid-Atlantic Ridge). Features include the ridge itself, a central rift valley, shallow earthquakes and gentle (effusive) basaltic volcanism. On land, divergence produces a continental rift (such as the East African Rift).

Destructive (convergent) boundaries

Plates move together; the outcome depends on what collides. Oceanic-continental: the denser oceanic plate subducts beneath the continent, forming an ocean trench, an explosive volcanic arc, deep earthquakes and fold mountains (the Andes). Oceanic-oceanic: one oceanic plate subducts beneath another, forming a trench and a volcanic island arc (Japan). Continental-continental: neither plate subducts readily, so the crust crumples into great fold-mountain belts (the Himalayas) with shallow earthquakes but little volcanism.

Conservative (transform) boundaries

Plates slide horizontally past one another along a transform fault. No crust is created or destroyed and there is no volcanism, but stress builds and releases in powerful, shallow earthquakes (the San Andreas Fault, California).

The driving forces

Three forces move the plates. Mantle convection drags the base of the plates. Ridge push arises because new, hot, high-standing crust at a ridge slides down and pushes the plate away. Slab pull, now regarded as the dominant force, is the pull of the dense, cold subducting slab sinking into the mantle and dragging the plate behind it.

Examples in context

The Mid-Atlantic Ridge. A constructive margin running the length of the Atlantic, building new basaltic crust and surfacing in Iceland, with a rift valley and shallow earthquakes. The Andes. An oceanic-continental destructive margin where the Nazca plate subducts beneath South America, producing a trench, explosive volcanoes and fold mountains. The San Andreas Fault. A conservative margin where the Pacific and North American plates slide past one another, generating major Californian earthquakes but no volcanoes.

Try this

Q1. State what is created or destroyed at each of the three boundary types. [3 marks]

• Cue. Constructive: new crust created; destructive: crust destroyed (subduction); conservative: crust neither created nor destroyed.

Q2. Name the inclined zone of deep earthquakes at a subduction zone. [1 mark]

• Cue. The Benioff zone.

Q3. Identify the driving force now regarded as most important, and explain it. [2 marks]

• Cue. Slab pull; the dense, cold subducting slab sinks into the mantle and drags the rest of the plate behind it.