Volcanic activity at destructive margins

Overview

Eruption style at a destructive margin, lava dome and volcanic activity at constructive and at destructive margins.

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

   How is magma generated at a destructive margin?

   Model answer

   By flux melting rather than by decompression. The descending oceanic slab carries sea water locked into hydrated minerals in its crust and sediments. As it sinks, rising pressure and temperature drive that water out at depths of around 100 km, and it rises into the hot mantle wedge above the slab. Adding water to hot rock lowers its melting point sharply, so the mantle wedge partially melts even though it has not become any hotter.

2. 2

   If the mantle wedge melts to give basaltic magma, why do destructive margin volcanoes erupt andesite and rhyolite?

   Model answer

   Because the magma is modified on its way up. The basaltic melt is hot and buoyant and rises into the overlying plate, where it stalls in magma chambers. There it cools slowly and crystallises out silica-poor minerals first, which leaves the remaining liquid progressively richer in silica, a process called fractional crystallisation. It also melts and absorbs the rock through which it passes, and where that rock is silica-rich continental crust the magma becomes more silica-rich still. What began as basalt therefore reaches the surface as andesite, dacite or rhyolite.

3. 3

   Why is the magma at a destructive margin so rich in gas?

   Model answer

   Because the water that caused the melting stays in the melt. The whole process is triggered by water driven off the subducting slab, and much of that water, along with carbon dioxide and sulphur released from subducted sediments, remains dissolved in the magma under pressure. The magma is therefore both viscous and unusually gas-charged, the worst possible combination, because the gas cannot escape as the magma rises and instead builds pressure until the magma is blown apart.

4. 4

   What is the eruption style at a destructive margin, and why?

   Model answer

   Violently explosive. The magma is andesitic to rhyolitic, with roughly 55-70 per cent silica, cooler at around 800-1000 degrees Celsius, extremely viscous and gas-rich. Gas cannot bubble out freely, so pressure accumulates behind viscous magma and any solidified plug in the vent until the confining rock fails. The eruption then fragments the magma into ash and pumice, throwing an eruption column tens of kilometres into the atmosphere and generating pyroclastic flows down the flanks.

5. 5

   What is a lava dome, and why is it so dangerous?

   Model answer

   A lava dome is a steep-sided mound of extremely viscous lava squeezed out over a vent, too stiff to flow away, which grows by inflating from within and by adding new lobes. It is dangerous because it is inherently unstable: it grows on a steep slope, it is hot and gas-charged inside, and when part of it collapses the sudden release of pressure causes the interior to explode, generating pyroclastic flows and surges. Domes therefore produce repeated, unpredictable collapses over months or years rather than a single eruption.

6. 6

   What is a lahar, and why is it so closely associated with destructive margin volcanoes?

   Model answer

   A lahar is a fast-moving flow of volcanic ash and debris mixed with water, with the consistency of wet concrete, that follows river valleys and can travel tens of kilometres from the volcano. Explosive volcanoes produce enormous volumes of loose ash on steep slopes, and destructive margins are common in humid tropical regions and in ranges with snow and ice at the summit. Heavy rain, a crater lake bursting, or melting snow and ice therefore mobilises that ash easily, and lahars can occur years after the eruption itself.

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