Sunday, 11 December 2011

What happens when a volcano meets a subduction zone?

This week a reader in New Zealand asked me what happens when a volcano meets a subduction zone? This is a slightly complex question, as there are different answers for different types of volcano, so I've written a full article to answer the question in full.

The question was asked with reference to the Kermadec Islands, which are an island arc created as one section of ocean plate is subducted beneath another. As the underlying Pacific Plate disappears beneath the overlying Australian Plate it is partially melted. The lighter minerals from the underlying plate then rise up through the overlying plate, forming a row of volcanoes, the island arc at the surface.

How volcanoes are formed in the Kermadec Islands.

Eventually all of the underlying plate will be consumed, and the subduction zone will encounter the far side of the plate, which may have another subduction zone with an island arc, or possibly a continent. Neither of these can be easily consumed, so the subduction zone closes, forming a suture. This starves the volcanoes of new material and they cease activity. Where two ocean subduction zones meet then one of the subduction zones may survive, with the two sutured island arcs sitting on top of the remaining subduction zone. Some scientists think that the Earth's earliest continents formed in this way, by the accretion of large numbers of suture zones. Suture zones tend to be mineral rich, and are often exploited by mining companies for their natural wealth.
The formation of a suture zone, in what is now part of the Himalayas.

The next common source of volcanic activity are mid-ocean ridges, where new crust is being created by convection currents within the Earth's mantle. Mid Ocean ridges underly the volcanoes of Iceland and St. Helena; the East African Rift volcanoes are thought to occur where a new ocean ridge is forming, breaking the continent apart.

New ocean crust forming at a mid-ocean ridge.

These structures can be subducted if they are overrun by a subduction zone. This disrupts the convection currents that from the ridge, stopping the volcanic activity, but does not form a suture, so that there is a break in the subducting material, between what are effectively two plates. The San Andreas Fault in California is formed this way. The overlying surface that people build cities upon is all part of the North American Plate, but beneath the surface the ancient Farallon Spreading Ridge is being subducted, forming a deep fault that occasionally causes spectacular and devastating earthquakes.

At the San Andreas Fault the subduction of a spreading ridge has lead to spectacular faulting at the surface, but volcanic activity has stopped.

The third type of volcanoes found on Earth are hotspot volcanoes. These occur away from plate margins, and drift across the surface of the Earth apparently independently of the movement of the continents. The volcanoes of Hawaii, the Canaries and the Seychelles are hotspot volcanoes. Many scientists believe that these volcanoes sit on top of mantle plumes that originate deep within the Earth; deeper than the convection currents that cause the movement of the continents, and therefore independent of them.

The Hawaiian Islands depicted sitting on top of a mantle plume.

Under this model a mantle plume is largely independent of the movement of the plates. It's origin is deep bellow the forces that cause ocean spreading and subduction zones, and it ought to be able to pass under these structures without major effect. It is thought that the Hawaiian Hotspot may have crossed Siberia during the Mesozoic, and that the Seychelles Hotspot once lay beneath India.

The movement of volcanic hotspots across the Pacific Ocean.

However not all scientists agree with this interpretation of volcanic hotspots. Some scientists believe in a shallow origin for these structures. Under this model a volcanic hotspot is a spreading ridge that has failed to expand into a new area of ocean creation. Such hotspots would have origins with shallow convection currents in the Earth's mantle, and would be disrupted by an encounter with a subduction zone, with the volcanoes dying and being sutured to the overlying plate. The hotspot might then persist for a while as a localized centre of earthquake activity.

4 comments:

  1. It's too bad these images are so small, and the text so hard to read on them.

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  2. Y'know you completely left out the one geological feature on the planet that spawned his question - the Mariana Islands, which from what I understand are volcanoes slowly falling into the Mariana's Trench a subduction zone.

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