BACK-BACK IN MONTSERRAT
Dr. Lester CN Simon
In July 2003, a unique volcanic eruption took place in Montserrat. It is being hailed as a geological finding that has never been documented anywhere before. In a remarkable article in the March 26-April 1, 2005 edition of New Scientist, Christina Reed outlined the scientific path to this discovery with the opening remark, “When lava flows uphill, you know you’re in deep trouble.”
The Soufriere Hill volcano in Montserrat has seen off about 8,000 of the 12,000 inhabitants and invited a number of scientists to work at the Volcano Observatory. Christina Reed reported that in July 2003, the volcano staged its largest eruption since it became active in 1995. As the pyroclastic flow made its way towards the Atlantic Ocean, the scientists crowded the Volcano Observatory. During the 18 hours long drama, one of their volcanic sensors took a direct hit from the pyroclastic flow. A pyroclastic flow comprises ash, blocks of rocks and superheated steam.
After the tsunami a few months ago, we wondered about the possibility of one occurring in the Caribbean. Back in July 2003, as the pyroclastic flow in Montserrat discharged its contents into the Atlantic Ocean, Guadeloupe issued a radio report to say that a small tsunami had smashed 15 fishing boats. Meanwhile, more than 40 animals died on Montserrat in an area considered safe for animals.
The report in New Scientist is an excellent reminder of the power of keen observation, a fundamental requirement of any scientist. After the pyroclastic flow ended and it was safe, volcanologists Marie Edmonds and Richard Herd left their instrument panels in the Observatory and ventured out to get a real view of the damaged area.
According to Christina Reed, the dome at the top of the volcano had entirely collapsed and dumped around 200 million cubic meters of pyroclastic material into the Tar River valley. The Tar River valley is downhill of the volcano. It flows towards the Atlantic Ocean.
Something about the July 2003 volcanic eruption seemed odd. Near the Atlantic coastline, trees facing the ocean had rocks embedded in them only on their coastal side, which is the side away from the volcano. It was as if the pyroclastic down-flow from the volcano had passed the trees with relatively little damage, turned around when it reached the ocean and went back uphill with a greater force!
Force does make water go uphill and it does the same to pyroclastic flow. But what was this force? As the pyroclastic flow “back-back” or turned around, it traveled uphill for about 3.5 kilometers. It reached valleys that farmers had been told were safe for animals. Dozens of animals died.
A volcanologist must spend many hours inside and outside an Observatory recording and analyzing data. However, like all good scientists, volcanologists spends most of the time looking inside their head, thinking, trying to make sense of the reams of data collected.
What had caused the back flow of pyroclastic material? The first and obvious possibility was that the turbulent, downhill pyroclastic flow had hit an object and shot back up the hill. This has been known to happen and the back shot can be sideways or even back up the same hill from whence it came. This happened in 1902 when Mount Pelee erupted on Martinique. The only problem with this explanation was that in Montserrat, there were no such obstacles between the down flow, the Tar River and the Atlantic Ocean, to produce such a turbulent back flow.
The second possibility was that the flat Tar River delta had caused the pyroclastic flow to build and to back up. This was discounted when the volcanologists found large chunks of rapidly cooled volcanic rocks as far as 2 to 3 kilometers away from the parent, pyroclastic down flow. This separation of new, offspring volcanic rocks uphill from the parent rocks of the pyroclastic down flow was too great and wide to result from a mere splash back.
The only explanation the volcanologists were left with was as follows: When the down-flowing pyroclastic material hit the seawater, it caused a secondary eruption, a hydrovolcanic eruption.
The report by Edmonds and Herd of this first-time eruption was accepted for publication in the journal, Geology. They also presented their findings at the Western Pacific Geophysics meeting in Hawaii in 2004. Hydrovolcanic eruptions have been reported before. It was first observed in 1965 when lake water entered a volcano crater in the Philippines. But the discovery in Montserrat marked the first time this was documented in a river delta.
Normally, when a pyroclastic flow meets the sea, there are only small-scale interactions by way of local boiling of sea water and jets of dust and rock fragments (called tephra) shooting up into the air. Never before have such an explosion column and base surge been documented to occur and to thunder back onto the land. A base surge is the backward surge or discharge from the secondary explosions at the coast caused by the deadly mixture of pyroclast and sea water.
There is a lesson to be heeded here. There are many costal volcanoes all over the world. There should be larger buffer zones around coastal volcanoes to avoid the perils of a base surge after a secondary, hydrovolcanic explosion. More than 550,000 people live in the dangerous “red zone” between the sea and Vesuvius, the most dangerous, active volcano, in Italy.
In her excellent article in New Scientist, Christian Reed makes the tipping point that the death of dozens of animals in Montserrat in valleys that were considered safe for them is a forewarning to people worldwide who are living around coastal volcanoes.
Sunday, April 17, 2005
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