Dynamic Pulsating Eruptions

Dynamic Pulsating Eruptions

The eruption of the Soufriere Hills, Montserrat in the Caribbean began in July 1995. Over 0.3 cubic kilometres of lava erupted over the next 3 years, causing widespread disruption to the island community. This eruption became the focus of intense international scientific study and some major advances in understanding how volcanoes work have emerged as a result. By March 1998 the volcanic activity had diminished. The volcano continued to rumble with occasional explosions and emissions of ash, but the activity was sufficiently low for the islanders to rebuild their lives and the scientists monitoring the eruption could start to concentrate on analysing the huge amounts of data. Here I describe some of the exciting new results that are emerging as this research begins to pay off. New ideas stimulated by Soufriere Hills and others similar volcanoes like Mount St Helens, USA are beginning to transform our understanding of how these highly dangerous volcanoes work.

Scientists have long imagined that a large body of magma in a chamber exists permanently a few kilometres beneath a volcano. It was believed that in the periods of dormancy this magma body slowly cooled and crystallised, and a mixture of crystals and molten rock erupted from time to time. When a larger eruption began this magma moved to the surface and either exploded or erupted as lava. The processes that occur during the journey of the magma to the surface were not really considered in any great detail. However the observations and ideas emerging from work on the Soufriere Hills and other volcanoes indicate that this kind of simplistic model may be wrong.

Volcanologists now believe that a large magma chamber didn’t exist at all under the Soufriere Hills. Its lavas contain features that suggest they were formed from a mixture of old solidifying magma invaded by an upsurge of fresh hot magma from much deeper in the Earth. They contain small blobs of dark basaltic rock that represents the telltale sign of deep, hot magma rising up into the volcano to remelt the in situ crystalline rocks and trigger a new eruption. Similar features have also been observed on many other volcanoes such as Mount Unzen in Japan and various volcanoes in the Andes. It seems that hot basaltic magma rapidly rises from great depth and invades the hot core of the volcanoes just a few kilometres beneath the surface. Here it transfers its heat and some of its gas, mostly water vapour, into the surrounding rocks. They then remelt, producing a crystal rich magma and an immediate eruption.