Destructive margins: subduction, trenches and island arcs

Overview

Deep sea trench, accretionary prism and why destructive margins produce the world's largest earthquakes.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What happens at a destructive plate margin?

   Model answer

   Two plates move towards each other and one is forced down beneath the other into the mantle, a process called subduction. The descending plate is heated and gradually destroyed, which is why the margin is called destructive. Subduction produces a deep sea trench at the surface, an inclined zone of earthquakes, and a line of volcanoes on the overriding plate parallel to the trench.

2. 2

   When two plates converge, what decides which one is subducted?

   Model answer

   Density. Oceanic lithosphere is denser than continental lithosphere, so where the two meet the oceanic plate always descends. Where two oceanic plates meet, the older one subducts, because oceanic lithosphere cools and thickens with age and so becomes progressively denser. Where two continental plates meet neither is dense enough to sink, so subduction cannot be sustained and a collision margin results instead.

3. 3

   Describe what happens to a slab as it descends into the mantle.

   Model answer

   The slab bends downwards at the trench, fracturing its upper surface, and descends at an angle typically around 45 degrees. It remains cold and brittle for a long time because rock conducts heat slowly, which is why it generates earthquakes to great depth. As it sinks, increasing pressure and temperature squeeze water out of hydrated minerals within it, and this water rises into the mantle above. Eventually the slab warms, loses its brittleness and is absorbed into the mantle at around 700 km depth.

4. 4

   Why do earthquake foci get deeper away from the trench?

   Model answer

   Because the earthquakes are generated within and along the top of the subducting slab, and the slab descends at an angle. Directly at the trench the slab is at the surface, so foci there are shallow; a horizontal distance of several hundred kilometres inland corresponds to a slab depth of hundreds of kilometres, so foci there are deep. Plotting focal depth against distance therefore reveals the inclined Benioff zone and the angle of subduction.

5. 5

   Why do earthquakes stop at around 700 km depth?

   Model answer

   Earthquakes require brittle failure, and brittle failure requires rock that is cool and rigid enough to fracture rather than flow. By around 700 km the slab has been in the hot mantle long enough to warm through, and the surrounding pressure is so great that rock deforms plastically instead of snapping. Below that depth the slab can still be traced by other means but it no longer generates earthquakes.

6. 6

   Describe a deep sea trench.

   Model answer

   A deep sea trench is a long, narrow, steep-sided depression in the ocean floor marking the line where the subducting plate bends and starts to descend. Trenches are the deepest features on the Earth's surface, reaching more than 10 km below sea level in the deepest cases, are typically only tens of kilometres wide but thousands of kilometres long, and are markedly asymmetric, with a steeper wall on the overriding plate side. They curve in plan view, being arcuate rather than straight.

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