Montserrat is one of the youngest volcanic islands that make up the Lesser Antilles Island arc, the chain of islands that stretches some 800 km from Grenada in the south to Saba in the north. The arc is the result of slow subduction of Atlantic crust (on the North and South American plates) beneath the Caribbean tectonic plate. This subduction generates different igneous melts that rise up through the upper parts of the Earth’s mantle and crust to generate the volcanism that has constructed Montserrat and all the other volcanic islands in the chain.

The island of Montserrat measures 16 km N-S and 10 km E-W, and its mountainous slopes are covered with lush tropical rainforest. The topography of the island is dominated by four volcanic centres: from the Silver Hills in the north through the Centre Hills and Soufrière Hills to the South Soufrière Hills, the most southerly. Of these, the Soufrière Hills is the only active volcanic centre on island.

Prior to the activity at the Soufrière Hills volcano, the highest peak on the island was Chance’s Peak at 909 m above sea level. However, the lava dome that currently forms the summit of the Soufrière Hills volcano is now the highest point on the island, reaching 1089 m above sea level, although at times during the 1995-2010 eruption, the summit of the dome was more than 1100 m above sea level.

The island is surrounded by a shallow submarine shelf that extends down to about 100 m below sea level. The width of the shelf varies considerably around the island. It is widest in the north, extending to 5 km offshore of the Silver Hills, an indication that the present Silver Hills are the eroded remnants of a much more substantial volcanic centre. Around the southern coast of Montserrat, the shelf is considerably narrower, testament to the youthfulness of the Soufrière Hills and South Soufrière Hills volcanic centres.

The southern part of Montserrat is cut by a series of faults trending WNW-ESE across the island. These on-island faults, known as the Belham Valley Graben, link up with two offshore faults systems: the Montserrat-Havers Fault System to the west and the Bouillante‐Montserrat Fault System to the east. These regional fault systems accommodate both extension and slip from the oblique convergence of the Caribbean and North American tectonic plates. The Belham Valley Fault System also controls the alignment of the uplifted areas and the pre-1995 lava domes that constitute the Soufriere Hills and South Soufriere Hills volcanic centres.

Volcanism on Montserrat

The four volcanic centres on Montserrat decrease in age from north to south and show some variation in the eruptive styles and eruptive products.

The Silver Hills (~2.17 to 1.04 Ma) is a deeply eroded andesitic volcanic centre consisting of volcaniclastic sequences, debris avalanche deposits and extensive areas of hydrothermal alteration. Volcanism at the Silver Hills was dominated by episodic andesite lava dome growth and collapse, produced Vulcanian style eruptions, and experienced periodic sector collapse events. The Silver Hills is surrounded by a shallow submarine shelf that extends up to 5 km offshore.

The Centre Hills (~1.14 to 0.38 Ma) is the largest volcanic centre on the island with its lava domes and volcaniclastic deposits occupying most of the centre of the island. Eruptive products associated with the Centre Hills consist of andesitic block-and-ash flows, pumice-and-ash flows, pumice fall, lahars, fluvial and debris avalanche deposits. Abundant pumiceous deposits indicate that the Centre Hills was the most explosive volcanic centre on island and produced the largest known explosive eruptions on Montserrat.

The Soufrière Hills (~0.45 Ma to the present) consists of a predominantly andesitic dome complex (Chance’s Peak, Gage’s Mountain, Galway’s Mountain, Perches Mountain and the present lava dome) flanked by volcaniclastic deposits. Before the 1995 eruption, Castle Peak (formed 1630 AD) partially filled English’s Crater, but was initially buried and then later destroyed by the growth and collapse of multiple lava domes during the 1995-2010 eruption. Located to the northwest of Soufriere Hills, Garibaldi Hill, Richmond Hill and St George’s Hill are tectonically uplifted areas consisting of pyroclastic and epiclastic deposits.

The South Soufrière Hills differs from its northern counterparts in comprising basaltic to basaltic andesite lava flows, with flow collapse breccias and scoria-fall deposits. The activity that formed these eruptive products occurred over a very short (geologically speaking) interval of approximately 10,000 years between 131–128 ka ago. In addition to the basaltic eruption products, there is a small area of uplifted carbonate rocks exposed in the cliffs along the southeast coast of the island within the South Soufriere Hills volcanic centre.