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

What this dot point is asking

Eduqas wants you to describe the processes and characteristic features of constructive (divergent), destructive (convergent) and conservative (transform) margins; to distinguish the ocean-ocean, ocean-continent and continent-continent destructive sub-types; to explain subduction, the Benioff zone and the difference between decompression and flux melting; and to give the diagnostic rocks, structures, earthquakes and volcanoes that let you identify a margin from its features.

The answer

Constructive (divergent) margins

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

• Process. As the plates separate, the underlying mantle rises and the drop in pressure triggers decompression melting, producing low-viscosity basaltic magma that wells up to fill the gap and crystallises as new oceanic crust.
• Features. Mid-ocean ridges with a central rift valley and high heat flow; on continents, rift valleys (for example the East African Rift).
• Rocks. Basalt (erupted) and gabbro (cooled at depth), with pillow lavas where basalt erupts underwater.
• 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 kind of crust involved.

• Ocean-continent. Dense oceanic crust subducts beneath buoyant continental crust. Water from the slab triggers flux melting; the magma interacts with continental crust and becomes andesitic to rhyolitic, building explosive stratovolcanoes in a continental volcanic arc (for example the Andes). Diagnostic features: a deep trench, shallow-to-deep earthquakes, intense folding, and high-pressure low-temperature (blueschist) metamorphism.
• Ocean-ocean. One oceanic plate subducts beneath another, and the chain of andesitic volcanoes rising through the overriding oceanic plate forms an island arc (for example Japan), with a trench and shallow-to-deep earthquakes.
• Continent-continent (collision). Both plates are buoyant continental crust, so neither subducts. The crust thickens and crumples into high fold mountains (for example the Himalayas), with large-scale folding and thrusting and shallow but powerful earthquakes, and little or no volcanism once collision is complete.

Conservative (transform) margins

Plates slide past each other horizontally.

• Process. No crust is created and none is destroyed. Friction locks the plates, elastic strain builds up, then they suddenly slip.
• Features. Linear transform faults (for example the San Andreas Fault), with offset streams and ridges across the fault and sheared, crushed rock (fault breccia) along it.
• 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 oceanic Nazca Plate beneath South America produces a deep 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 powerful shallow earthquakes but no volcanic arc, because neither buoyant continent could subduct.

Try this

Q1. State the type of magma and the volcano style at a constructive margin, and name the melting process that produces them. [3 marks]

• Cue. Basaltic, low-viscosity magma giving gentle (effusive) eruptions that build shield volcanoes and fissures; produced by decompression melting of rising mantle.

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

• Cue. It is the inclined plane of earthquake foci that traces the descending subducted slab; it dips because the dense oceanic slab sinks into the mantle at an angle, giving shallow earthquakes near the trench and progressively 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 a subducting slab there is no flux melting to feed a volcanic arc.