Monserrat
A long time ago in the southern Caribbean (latitude 16.72 north and longitude 62.18 west, for those of you with maps) an undersea volcano erupted from the ocean's floor. Spewing its lava (molten rock), the volcano built an undersea mountain that grew higher and higher, eventually growing above the surface of the sea.
People may have worried but what could they do? Montserrat was their home. After each earthquake the people thought, "See, we are still in one piece. The volcano didn't blow!"
In mid July of 1995 the Montserrat volcano started to produce more earthquakes and a few days later, on July 18th the volcano's crater produced a phreatic explosion.
What's a phreatic explosion?
Phreatic (pronounced "free-a--tick") is from the Greek word for "well". Not "well" as in, "The eruption's going well.". It means "well" in the sense of water from the ground.
Some of the earlier earthquakes may have been caused by "hidden" phreatic explosions that the ground was able to shake off and contain. Some folks saw these explosions as the volcano just blowing off some steam. Perhaps it would calm down now. Wrong! Instead there were more phreatic explosions as the volcano started blowing off its steam frequently. These phreatic emissions became an everyday event.
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What's a phreatic explosion?
Phreatic (pronounced "free-a--tick") is from the Greek word for "well". Not "well" as in, "The eruption's going well.". It means "well" in the sense of water from the ground.
Some of the earlier earthquakes may have been caused by "hidden" phreatic explosions that the ground was able to shake off and contain. Some folks saw these explosions as the volcano just blowing off some steam. Perhaps it would calm down now. Wrong! Instead there were more phreatic explosions as the volcano started blowing off its steam frequently. These phreatic emissions became an everyday event.
A few months later a dome started to grow.
On March 29th, 1996 a magma dome collapsed releasing its lava and producing the first of many pyroclastic flows.
Soufriére Hills has been producing pyroclastic flows, on and off, for almost two years now and no one knows when it will stop or if it will get worse. On June 25th, 1997 a very large pyroclastic flow came down from Soufriére Hills. In a matter of minutes it dumped 4 to 5 million cubic meters of lava and covered 4 square kilometers on the east side of the island! It also killed at least nine people and destroyed more than a hundred homes. Anyone too close to a pyroclastic flow dies. The heat can ignite buildings and clothing. There are often poisonous gases mixed in too!
I once watched a TV show about volcanoes with a geologist friend of mine. Every time the fellow on TV said "ash" my friend would shout "TEPHRA!" (as if the guy on TV would correct his error). And where did geologists come up with this funny word? Tephra is Greek for "ash".
The tephra cloud from Montserrat's big (June 25th) explosion rose 10 kilometers and fell over the west side of the island covering the town of Plymouth.
As tephra falls to earth it cools down. The further away tephra falls from the volcano the more time it has to cool as it falls through the air. So a tephra cloud can be very hot if it hasn't had time to cool off.
In the year 79 (AD) a volcano in Italy called Vesuvius erupted dumping a great deal of tephra on the nearby cities of Pompeii and Herculaneum. It was still so hot that it covered the people in a cloud of hot tephra and they died a quick, painful death. Centuries later their bodies were discovered as hollow "statues"; sculptured rocks coating human bodies that had since decayed away. Some geologists called this stuff "hot tephra" but others insist on describing it as a pyroclastic flow of tephra. Take your pick.
Most tephra cools very quickly as it falls so unless you are very close to the source of the tephra it's only a nuisance. In 1980, when Mount St Helens exploded, I watched scenes on the TV of the tephra falling all over the place like black snow. It was the same with the evacuation of the USA Air Force base in the Philippines when Mount Pinatubo erupted in 1991. The tephra was everywhere. It was so thick it turned the noonday black like nighttime!
Another danger from volcanoes are lahars. Lahar (pronounce "la-har") is an Indonesian word for mud flows. (I guess the Greeks don't have a word for it!) Lahars are a common problem in the volcanic areas where there are large volumes of ice and snow. The volcano's lava can melt a huge amount of glacial ice and thus release a huge amount of water. That water rushes away carrying any soil along with it. The result is a wall of flowing mud, sometimes very large and moving very fast. You might think lahars are impossible on the tropical island of Montserrat but you'd be wrong.
Despite all the excitement the Montserrat volcano is a typical volcano. Technically it is called a "strato volcano" because it is built of layers from older eruptions. Strato is a word geologists like to use for layers. Another name for this kind of volcano is a "composite volcano" referring to the fact that when it erupts it produces both lava and pyroclastic flows. Strato (composite) volcanoes are the most common type of volcano.
Perhaps the most typical thing about the Montserrat volcano is its behavior. No one can predict when a volcano like Montserrat will explode, but geologists monitoring these kinds of volcanoes can at least tell you when things are getting "tense". Like most volcanoes the Montserrat volcano started warning of its fate. The phreatic activity and earthquakes are a sign that magma is on the move and the formation of a magma dome is a sure sign that magma is approaching the surface.
Geologists monitoring Montserrat, Mount St Helens, Mount Pinatubo and similar volcanoes, keep a careful watch on the potential hazards. When earthquakes become more common and show signs of magma movement, the geologists take notice. When the amount of phreatic activity increases, they worry. And they constantly monitor magma domes. The more the domes deform, and the faster they do so, the more likely it is that the dome will collapse, releasing lava streams and pyroclastic flows. The geologists who monitor volcanoes (vulcanologists) can't predict the exact time of an explosion or even say how likely it is to explode. It's all kind of relative. However, they can monitor the volcano's behavior (earthquakes, phreatic activity and magma dome distortions) in order to make an educated guess. Sometimes they warn that it might explode and they issue an alert, calling for people near the troubled area to leave immediately. Sometimes they are wrong, but when they are right these warnings have saved the lives of many people living around active volcanoes (Montserrat, Mount Pinatubo and Mount St Helens).
