Day 3 (February 18, 2017): Road Cuts and Lookouts
We started off another beautiful day in paradise by visiting a road cut near the Montserrat Cultural Center. Road cuts around Montserrat provide unique insight into the fascinating geology of the area.
Our examination revealed a lahar flow channel. It originated from volcanic activity of the “Centre Hills” zone of the island. The flow channel was deposited over the older Silver Hills andesite.
Farther up the road we were also lucky enough to encounter a southwest trending normal fault!
The next outcrop we visited was a road cut to the east of Carr’s Bay and directly south of Government Hill. This outcrop contained several units of the distal sequence from the Silver Hills. Highlights of this outcrop were block & ash flows and weathered laharic flows. Both originated from the Centre Hills zone of Montserrat. The laharic flow deposits displayed a unique weathering feature, an orange colour, which was termed “orangification” in the field. Further up the outcrop was a fault which separated the block & ash flow from the “orangified” laharic deposits. This fault, which cut through the lahar deposit, suggested that the weathering of deposits pre-dated the faulting. Otherwise, we would have seen the same orangification on both sides of the fault.
Afterwards, we headed off to the famous Hilltop Coffee House owned by David Lea and his wife. After all, no field work is complete without a quick waffle break!David Lea, provided us with his first hand experience of the 1995 eruption of the Soufriere Hills Volcano. Rock samples and funky memorabilia made the Hilltop Coffee House as much a museum as it was a waffle stop!
To gain some perspective on the knowledge we had gathered so far, we went to the top of Lookout Hill. Here, we could see the Silver Hills to the north, the oldest portion of the island, and the steep terrain of the Centre Hills to the south. Lookout Hill is also a great vantage point for viewing the changed orientation of the Caribbean Island Arc. Guadalupe, visible to the south, represents the current orientation of the island arc. In contrast, Antigua, to the east, represents the old arc trend. The leading hypothesis for the two arcs is that there was a change in subduction angle of the North American tectonic plate underneath the Caribbean tectonic plate. The angle may have shallowed, which migrated the volcanic activity west towards the current island arc.
Our last road cut for the day was south of Lookout Hill, titled NE 1 from the 2013 GSA Field Guide. The outcrop was made up of 2 different volcanic debris deposits separated by a thin sediment layer.The bottom unit was a high velocity lahar flow, characterized by poor sorting, angular rock fragments, and boulder sized clasts. It was also the oldest unit at the outcrop. The middle unit was a thin, fined grain sediment unit deposited by water flowing on top of the volcanic debris. The third unit was a slurry, high viscosity flow deposit characterized by a coarse, sandy matrix, and angular clasts. These clasts were noticeably smaller than the older lahar deposit. All three units originated from the Centre Hills.
After our last outcrop, we headed to Jack Boy Hill. This spot gave us a great vantage point to see the buried Trants Settlement. This used to be an old archaeological site and even housed the old international airport. It was completely buried by previous pyroclastic flows from the 2010 eruption. The pyroclastic flows greatly extended the coast line, leaving no trace of any building or structures, save for the top of a sugar cane tower.
Finally, we finished off our long day of learning with a visit to Woodlands Bay. Our group tested the buoyancy of pumice samples to make sure they really floated (spoiler: they did!).
Phew. Another awesome day in the books. Tomorrow we take off on a hike to Yellow Hole to view the extremely hydrothermally altered units of Silver Hills. Stay tuned!
~Written by Jason Hinde and Michael Lopez