Day 6 (February 21, 2017): A visit to Trants on our last day in Montserrat.
Today we were lucky enough to be invited by MVO volcanologist Dr. Adam Stinton to enter Trants. The settlement of Trants lies within the exclusion zone of Montserrat. It was completely buried by pyroclastic flows during the 2010 eruption. A dome of the Soufriere Hills Volcano collapsed, which caused a block and ash flow hurtling toward the evacuated city. The entire event occurred within forty five minutes, with most of the material being deposited in 15 minutes.
While walking around the area we noticed some very unique features. First, we looked at an outcrop with two distinct layers. Usually two layers are classified as two separate events, especially if the contact between the two is sharp. However in this case, the two layers are related to the same 2010 pyroclastic flow. The bottom layer was the pyroclastic flow deposit, consisting of several poorly sorted angular fragments. The top layer is finer material. Adam explained to us that the top layer is re-worked material that formed from rain water settling in the hot ash. The ash superheated the water causing phreatic-like eruptions. Walking further, we noticed a series of craters. These were the locations where water and material jetted out of the ground. This was a perfect example of two different layers can from the same event.
Traversing further into Trants, we were greeted by a large, gray, ash covered, pyroclastic flow deposit. An isolated smoke stack stood ominously as the last piece of human habitation. Upon closer inspection, the roof of a sugar mill next to the smoke stack was just barely visible under the deposited material. The fact that this final evidence of habitation exists gives useful information to the characteristics of the flow itself. The pyroclastic flow is believed to have had a velocity of ~50–70 m/s near the top of the volcano, but slowed significantly, to ~10 m/s, by the time it hit the sugar mill. Therefore, the smoke stack and sugar remained intact, but the sheer volume of material buried most of Trants. It left an eerie, gray, almost barren landscape in its wake.
This desolate terrain was a result of the destructive energy by the volcano. Kinematic energy ripped through the town of Trants, not only burying buildings, but extending the shore line up to 600 m. When the pyroclastic flows hit the original shoreline, the lower block component sank into the water, but the nuee ardente (hot, fast travelling pumice flows) travelled up to 2 km out to sea. The deposits on land were so hot, that it can take up to 10 years to cool to normal ground temperatures. For example, 4 years after the dome collapse, the deposit 5 m down was still 70–80 degrees Celsius. Large boulders were buried by the flow, which help insulate the deposits.
The boulders in Trants were up to 25 m in diameter and 64,000 tonnes. Thus, the boulders originated from the Soufriere Hills volcano. One of these boulders had a rare feature. It was a fracture that was filled with glassy materials, similar to obsidian, and had a high concentration of metals. The prevailing theory is that a seismic event opens and reopen filled fractures. This will generate enough heat around the fracture to melt the wallrock and cause it to recrystallize. In this particular case, the opening of the fracture also introduced fluids, which cooled the melt quick enough to produce glass.
Afterwards, we boarded the ferry to Antigua. We said goodbye to Mr. Charles Daly who was an excellent driver during our stay. In Antigua, we will be looking at evidence for extinct volcanism of the old island arc. We are going to miss Montserrat but are looking forward to contrasting the differences between the two islands.
~ written by: Danielle Shirriff, William McNeice, Colin Roth, and Joshua Nguyen.
