Plate margins: the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-continent, ocean-ocean and continent-continent collision); the Benioff zone and subduction; the characteristic rocks, structures, earthquakes and volcanoes produced at each margin type.

What this dot point is asking

OCR wants you to describe the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins, distinguish the ocean-continent, ocean-ocean and continent-continent destructive sub-types, explain subduction and the Benioff zone, and give the characteristic rocks, structures, earthquakes and volcanoes of each margin type.

The answer

Constructive (divergent) margins

Plates move apart, and new crust forms to fill the gap.

• Process. The mantle rises and undergoes decompression melting, producing low-viscosity basaltic magma that fills the gap and forms new oceanic crust.
• Features. Mid-ocean ridges with a central rift valley; on land, continental rifts (for example the East African Rift).
• Rocks. Basalt and gabbro.
• Earthquakes. Shallow and generally moderate.
• Volcanoes. Frequent but gentle (effusive), basaltic eruptions; shield volcanoes and fissures.

Destructive (convergent) margins

Plates move together; what happens depends on the crust involved.

• Ocean-continent. Dense oceanic crust subducts beneath buoyant continental crust. Water from the slab triggers melting; the magma is andesitic to rhyolitic (it interacts with continental crust), forming explosive stratovolcanoes in a volcanic arc (for example the Andes). Deep ocean trench; shallow-to-deep earthquakes; intense folding and high-pressure metamorphism.
• Ocean-ocean. One oceanic plate subducts beneath another, forming an island arc of andesitic volcanoes (for example Japan) and a trench, with shallow-to-deep earthquakes.
• Continent-continent (collision). Neither plate subducts (both are buoyant), so the crust thickens and crumples, forming high fold mountains (for example the Himalayas) with large-scale folding and thrusting and shallow but powerful earthquakes; little or no volcanism once collision is complete.

Conservative (transform) margins

Plates slide past each other horizontally.

• Process. No crust created or destroyed; friction locks the plates, stress builds, then the plates suddenly slip.
• Features. Linear transform faults (for example the San Andreas Fault).
• Earthquakes. Shallow but often powerful.
• Volcanoes. None, because there is no melting mechanism.

Examples in context

Example 1. The Andes (ocean-continent). Subduction of the Nazca Plate beneath South America produces a trench, a chain of explosive andesitic stratovolcanoes, and deep Benioff-zone earthquakes: the textbook ocean-continent margin.

Example 2. The Himalayas (continent-continent). The collision of India with Asia thickened the crust into the world's highest mountains, with intense folding and thrusting and powerful shallow earthquakes but no subduction-related volcanic arc.

Try this

Q1. State the type of magma and the volcano style at a constructive margin. [2 marks]

• Cue. Basaltic, low-viscosity magma; gentle (effusive) eruptions forming shield volcanoes or fissures.

Q2. Explain what the Benioff zone is and why it dips. [2 marks]

• Cue. It is the inclined plane of earthquake foci marking the descending subducted slab; it dips because the slab sinks into the mantle at an angle, giving shallow earthquakes near the trench and deeper ones down-dip.

Q3. Explain why fold mountains form at a continent-continent margin rather than a volcanic arc. [2 marks]

• Cue. Both plates are buoyant continental crust, so neither subducts; the crust thickens and crumples into fold mountains, and without subduction there is no melting to feed a volcanic arc.