Montserrat’s ‘invisible’ eruptions

Pyroclastic surge entering the sea offshore Montserrat

HOT, gas and sediment rich mixtures called pyroclastic surges, that are generated by volcanic island eruptions, may be leaving little to no trace in the marine record.

When the Caribbean island of Montserrat’s Soufrière Hills volcano underwent dome collapse  in 1997, it sent 40 km3 of hot gas and rock into the atmosphere which fell under its own weight and barrelled into the sea. From the Greek ‘pyros’ meaning ‘fire’ and ‘klastos’ meaning ‘broken in pieces’, surges are a force to be reckoned with. They can travel up to 450 mph and at temperatures of 1000 °C making them one of the most dangerous products of volcanic eruptions.

In February 2013, a team of researchers onboard the RRS James Cook, research vessel of the National Environmental Research Council (NERC),  gathered sediment cores from around the island, and then brought them back to be analysed at the National Oceanography Centre (NOC), Southampton, UK. The deposits were found to be mainly made up of coarse, black, volcanic fragments and shards of glass, sitting atop >1000 years’ worth of plankton rich muds.

But it was the limited thickness of the layers which lifted eyebrows. Did some of it get removed? Possibly, by submarine landslides. The island is surrounded by the crumbled remnants of its rocky past. Some of the megablocks found in the Atlantic rival London’s Wembley Stadium!

These studies into surges are key for our understanding of the geohazards they pose.  One of these deadly eruptions sourced from Mt Vesuvius caused the devastation in the Roman city of Pompeii in AD79, and the blast from Mt Krakatau in 1883 travelled 40km across the sea to the island of Sumatra, where it not only asphyxiated its victims, but it is thought to be the cause of a very large tsunami wave which reached South Africa.

The marine record is a much more complete record of volcanic island eruptions, however, this record may not be showing the whole story. If the result from the Lesser Antilles is not an isolated occurrence, then some active volcanoes around the globe may just be a little more active than we actually think.

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Martine Coates

Marketing Executive at Scarab4
Geologist hailing from the University of Southampton with a specialism in marine volcanic sedimentology and the subaqueous emplacement of subaerial volcanic mass flows. Now working for the LLW Repository Ltd in Cumbria, UK as part of the Environmental Monitoring and Site Characterisation team.

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