Montserrat is made up of four volcanic centres – Silver Hills, Centre Hills, Soufrière Hills and South Soufrière Hills. These range in age from about 2.5 million years to the present. The Soufrière Hills Volcano is the youngest and the only one that is active. Garibaldi Hill and St. George’s Hill are not volcanic centres and were created by localised uplift. The ages of these volcanic centres, determined from radiometric dating of rocks, are given in the table below.
|Volcanic Centre||Age in thousands of years|
|Silver Hills||2580 ± 60 to 1160 ± 46|
|Centre Hills||954 ± 12 to 550 ±23|
|Soufrière Hills||174 ± 3 to present|
|South Soufrière Hills||128 ± 27|
The island is mainly composed of andesitic lavas and volcaniclastic rocks produced by dome-forming eruptions. although the South Soufrière Hills are of basaltic to basaltic-andesite composition. The main rocks formations are the remnants of andesite lava domes; andesitic breccias representing the talus of previous lava domes; pyroclastic flow deposits formed by the collapse of lava domes; lahar and debris avalanche deposits; and subordinate tephra fall deposits. There are zones of hydrothermally altered rocks and active fumarole fields occur on the Soufrière Hills Volcano.
The Silver Hills centre was active between 2.6 and 1.2 million years ago. It is made up of andesitic lava, in the form of eroded lava domes and sequences of volcaniclastic beds (formed from pyroclastic fan deposits around the lava domes and debris avalanche deposits). Extensive areas of the Silver Hills have been hydrothermally altered, a notable example being Yellow Hole (a bay on the east coast). The highest point is on Silver Hill itself, 403 metres above sea level. The submarine shelf is particularly well developed around Silver Hills. It is believed that this represents the eroded remains of what was once a much more substantial volcanic complex, of a size comparable to that of Centre Hills or Soufrière Hills.
The interior mountains of Centre Hills was an active volcanic complex between 950 and 550 thousand years ago. They are formed of andesitic lava: the remnants of lava domes that collapsed to produce the pyroclastic deposits on the flanks. The highest peak in the complex is Katy Hill at 741 metres above sea level. Centre Hills has moderately sloping flanks (typically less than 10 degrees) formed of andesitic volcaniclastic deposits. The material is predominantly pyroclastic flow deposit, with lesser amounts of pumice-and-ash-flow, pumice-fall, lahar, debris-avalanche and fluvial deposits. Ages have been determined from 954 to 550 ka.
South Soufrière Hills
South Soufrière Hills was active for a relatively short period, about 130 thousand years ago. The slopes are formed of lava flows and volcaniclastic beds (mainly breccias, with some scoria-fall deposits). In contrast to the other volcanic centres on Montserrat, the lavas and pyroclastic deposits from South Soufrière Hills are of basaltic to basaltic-andesite composition. The summit area has at least four horseshoe shaped structures, the remnants of dome collapse scars. The highest point is 756 metres above sea level.
Soufrière Hills Volcano is a Pelean-type volcano (explosive outbursts that generate pyroclastic flows, dense mixtures of hot volcanic fragments and gas), and the only active volcanic centre on the island. It became active about 175 thousand years ago. Before 1995 the summit consisted of five steep-sided andesitic lava domes: Gages Mountain (the oldest), Chances Peak (the highest at [xxx] metres above sea level), Galway’s Mountain, Perches Mountain and Castle Peak (the youngest). The last of these was inside the horseshoe-shaped English’s Crater, which was created by a large dome collapse.
Other Geological Features on Montserrat
The bathymetry of the ocean floor around Montserrat shows a wide shelf around more than half of the island. This marks the extent of the Montserrat volcanic deposits and suggests that the Centre Hills and Silver Hills volcanic complexes were much larger than they are today, possible larger than the Soufrière Hills Volcano. These have been heavily eroded above the sea surface, but not below.
There are also very extensive deposits from the Soufrière Hills Volcano extending up to ten kilometres out from the Tar River Valley.
Belham Valley Fault System
The major drainage system on Montserrat, the Belham Valley, reflects a fault system crossing southern Montserrat, marked BVF in the map above. This is part of a larger fracture system in the Earth’s crust, but does not appear to be active as there are no earthquakes associated with movement on the Belham Valley Fault.
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