Tuesday 14 August 2012

Pumice raft linked to Havre Seamount, not Monowai.

When the HMNZS Canterbury encountered a floating raft of pumice stone last week, scientists at New Zealand’s GNS Science assumed it had come from the Monowai Seamount, a submarine volcano in the Kermadec Islands noted for regular eruptions, which has been active this month. However a report of the raft made on 1 August by an airline pilot has subsequently emerged, a report made before the start of the Monowai eruption. Scientists have now examined satellite images of the Kermadec region, and concluded the rafts started to form on or around 19 July, and that they appear to be connected to a number of Earthquakes detected in the region on 17-18 July, and a thermal anomaly on Havre Seamount on 18 July.
Thermal image of the Kermadec region taken on 18 July 2012 by the MODIS Satellite System. Université Libre de Bruxelles.

Havre is much less well known than Monowai; its summit is 1100 m bellow the sea surface, and it has never been known to erupt before, and had not previously been identified as an active volcano. Like Monowai it is located on the Kermadec Ridge, a chain of (mostly) submarine volcanoes fed by the subduction of the Pacific Plate beneath the Australian Plate, which produces liquid magma as the underlying Pacific Plate is melted by friction and the heat of the Earth's interior.

Map showing the location of Havre Seamount. Google Maps.

Seamount is a term applied to any mountain entirely submerged beneath the ocean (mountains that stick out of the ocean are islands). Most of these are presumed to be extinct volcanoes, with a minority known to be still active (non-volcanic fold mountains are thought to be a feature of continental plates, not found in the oceans. Seamounts often form unique ecosystems, where the seabed is thousands of meters higher than the surrounding abyssal plains and thus forms a completely different environment. Seamounts that rise close to the ocean surface are often host to coral reefs. Unfortunately the rich ecosystems of seamounts has made them a prime target for the fishing industry, and many have been damaged by unregulated trawling.

Pumice rafts are the equivalent of volcanic plumes produced by submarine volcanoes. When a volcano under the sea produces an ejection rich in gas and liquid lava, the lava is cooled rapidly by contact with the cold ocean water, forming pumice, a light volcanic rock containing bubbles of trapped gas. This typically lighter than water, and major eruptions can produce large rafts of pumice, covering thousands of square kilometers. This in itself forms a unique ecosystem, carrying large colonies of algae and encrusting organisms.

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