CASE STUDY- Monserrat

Geological Background

Monserrat is in the northern part of the lesser Antilles volcanic island arc marking the destructive plate boundary margin resulting from the subduction of the North American plate below the Carribbean plate. 4 volcanic centres but 3 are dorminant (sources of magma removed) but soufriere hills are active.

Soufriere Hills

Composite volcanic cones made of andesite (acid) magma with some basalt (basic) intrusions. There are springs and fumaroles on the slopes and densely populated areas.

In 1995

Earthquakes above background levels + small eruptions. By the end of the year magma had reached the surface and a laval dome bgan to grow.

In June 1997

Series of small earthquakes followed by pyroclastic flows (100kmh) partially buried the community of spanish point. By sept this community was completely covered.

In sept-oct 1997

74 magmatic explosive eruptions. Large metre sized rocks thrown 1.5km. Vertically ejected material runs out of kinetic energy and falls back 300m in an eruption called a fountain collapse.

Main eruption

4-5 million m3 of material released in a 20 min period. Pyroclastic flow detroyed plymouth. 100 million m3 of debris and ash have been remobilised by rains to swamp the town further in flash floods and lahars. (The 1998 hurricane season rains resulted in many lahars. Parts of the town were buried under 10 metres of debris). Ash deposites place considerable strains on buildings causing them to collapse. Ash contains fine silica materials which cause complications when breathed in. 30 cm of ash fell on plymouth blown by the prevailing wind. Pyroclastic surges are the most dangerous effects of volcanic activity. Buildings are knocked down…