27 August 2010

Montserrat and the Soufriere Hills volcano

Posted by Jessica Ball

This summer I was lucky enough to take an absolutely amazing field trip on the island of Montserrat, where my advisor spent two weeks showing us around her old stomping grounds. And let me tell you, I would have no problem doing volcanology on a Caribbean island for a while…

Montserrat is a beautiful island, but it’s small – and the fact that the Soufriere Hills volcano has made a bit more than half of it unlivable hasn’t helped any. Here’s a basic map of the island, with the approximate boundaries of the exclusion zone (which changes depending on activity):
From http://travel.state.gov/travel/cis_pa_tw/cis/cis_974.html 

Montserrat is part of the Lesser Antilles volcanic arc, which is formed by the subduction of the Atlantic tectonic plate below the Caribbean plate. Most of the islands in the arc have andesitic volcanoes, which tend to produce explosive eruptions and lava domes. Montserrat itself is composed of three different volcanic centers: the Silver Hills in the north (the oldest, where the rocks date to ~11-25 million years), the Centre Hills in the center (9.5 – 5.5 million years), and the Soufriere / South Soufriere Hills in the south (at least 1.7 million years to the present). (For more information on the argon-argon dates that produced these ages, see the Harford et al. reference below.)

Maps of the Antilles arc and the volcanic centers on Montserrat. From Smith et al. (2007), Prehistoric Stratigraphy of the Soufriere Hills–South Soufriere Hills Volcanic Complex, Montserrat, West Indies.

The island is very densely vegetated, with most towns clustered around the coast; this also means that the best (old) rock exposures are in sea cliffs. (Hence we spent a lot of mapping time on beaches. Not a bad deal, eh?)

Montserrat from the north, with the Silver Hills in the foreground and the Centre Hills just visible below the clouds.

When seismic activity (namely earthquake swarms) started happening in 1992, the island was still trying to recover from the damage caused by Hurricane Hugo in 1989. In 1995, phreatic explosions (involving heated groundwater, but no lava eruption) began opening vents in English’s Crater. Ash from larger explosions prompted the evacuation of southern Montserrat in August of 1995, and by November a lava dome began growing in the crater. Since then there have been a number of periods of dome growth and collapse, during which the volcano has built the original lava dome into a large edifice that completely covers the old crater. Dome growth has been accompanied by pyroclastic flows, lahars, and ash venting; occasional periods of explosive eruptions and phreatic activity have occurred.

The current lava dome viewed from Jack Boy Hill (east side of the island), with a large collapse scar visible to the right.  Because this is a tropical island, it’s pretty unusual to see the dome this clearly, so we got lucky. (The same thing happens in Central America – my field area is frequently clouded in.)
Looking at the west side of Soufriere Hills. The light gray slope on the left marks a point where pyroclastic flows and dome collapses have filled in old topography, as well as burying the former capital of Plymouth. The bluish cloud hanging over the volcano is a steam and gas plume, which gets its bluish tint from sulfur gases.

Dome collapses at Montserrat are impressive, often accompanied by explosions, and can involve anywhere from tens of thousands to millions of cubic meters of material. The most recent occurred in February of this year; you can see video (thermal and normal) at the Montserrat Volcano Observatory website, and I’ll be showing photos of our field trip to the collapse deposits in a later post.

Another Jack Boy Hill view, this time of the February 2010 collapse deposits. This is the site of the old Bramble Airport, and it’s been run over by various other pyroclastic flows in the past, but until February the old airport and some structures were still visible. Not anymore!

We had the opportunity to spend some time at the Montserrat Volcano Observatory, where the scientists graciously spend time showing us the ropes (and taking us on some great field trips).

The Montserrat Volcano Observatory perched above the Belham Valley. Swanky!
Examining friction marks on a boulder in the February 2010 deposits.

 In return, we gave lunchtime presentations on subjects relevant to andesitic dome eruptions, and helped map some of the older deposits on the west side of the island. In Smith et al. (2007) – mentioned above – the authors mapped cliffs on the east (old airport) side, but presumably got distracted by the new eruption before they could get to the rest of the island. As part of our class requirement (yes, we got credit for this trip!), we did a survey of sea cliffs on the west side of the island, which meant many long and grueling hours spent on beaches and in boats.

As field locations go, you really can’t beat one that’s steps away from the beach (and the beach bar).
How could anyone work in these conditions?

It was a fantastic trip, with great opportunities for seeing old and new products of explosive volcanism (sometimes side by side). The staff at the MVO were very gracious, as was everyone we met on the island. (Perhaps the only downside of the trip was the state of my legs after two weeks of being munched on by tropical mosquitoes; apparently I’m tastier than the majority of the folks who went on the trip.) More discussion to come of the February 2010 collapse, the older volcanics, and how to distinguish different kinds of deposits!

References

Harford, C.L., Pringle, M.S., Sparks, R.S.J., Young, S.R., 2002, The volcanic evolution of Montserrat using 40Ar/39Ar geochronology. In The Eruption of Soufriere Hills Volcano, Montserrat, from 1995-1999 (T.H. Druitt and B.P. Kokelaar, eds.), p. 93-113.
Smith, A.L., Roobol, M.J., Schellekens, J.H., Mattioli, G.S., 2007, Prehistoric Stratigraphy of the Soufriere Hills-South Soufriere Hills Volcanic Complex, Montserrat, West Indies. The Journal of Geology, v. 115, p.115-127.