St. Kitts - Volcanism
Mt. Liamuiga rises to a height of 1155 m (3792 ft) and has a summit crater
~900 m wide and 244 m deep. The summit of Mt. Liamuiga exposes remnant lava flows
and/or domes but the most common deposits identified on the lower flanks of Mt.
Liamuiga are pyroclastic deposits. Cliff exposures along the coastline reveal 5-30
m thick successions of pyroclastic deposits (fall, flow and surge deposits), debris
avalanche deposits and lahar deposits. Lava domes are prominent on the flanks of
the volcano at Brimstone Hill, Sandy Point Hill and Farm Flat. There are also apparently
two small craters located on Bourke’s Estate (Baker, 1965) although these could
not be located during this study and their origin is unknown. Age dates on deposits
interpreted to have been erupted from Mt. Liamuiga range from 1620 to > 41,000
yrs BP . These are the youngest known deposits on the island. The lava domes on
the flanks of the volcano have not yet been dated and it is not known what the relative
ages of these features are with respect to the dated pyroclastic deposits from Mt.
Mt. Liamuiga principally formed by effusive lava-dome forming eruptions. Periodically
the lava dome collapsed producing pyroclastic flows, surges and airfall. During
and after periods of heavy rainfall it is likely that lahars or mudflows were also
common. A final phase of explosive magmatic activity is interpreted to have formed
the steep-walled, deep crater that is present at the summit of Mt. Liamuiga and
to have produced the thick succession of airfall deposits that blanket the NW part
of the island.
Fumaroles are present within the crater of Mt. Liamuiga as well asand on the flanks
of the volcano along the SW base of Brimstone Hill and along the coast below Brimstone
Hill. Volcanic earthquakes have also been associated with Mt. Liamuiga and are discussed
in more detail below (see Volcanic Earthquakes).
There are two unsubstantiated reports of historic eruptions of Mt. Liamuiga in 1692
and 1843. The first report was by a Franciscan friar (Sloane 1694) who describes
the island as being troubled by earthquakes and mentions an eruption “of a Great
Mountain of Combustable Matter”. The second report comes from Capadose (1845) who
describes a white spiral cloud of smoke and bubbling sulphurous springs from the
crater of Mt. Liamuiga. There are no other historical reports to support the occurrence
of either eruption and both of these alleged eruptions occurred immediately after
major earthquakes. It is possible that the effects of the earthquakes were confused
with genuine eruptions or that the earthquakes could have triggered minor eruptions.
Baker (1985) references S. Skerrit (pers. comm.) as describing a Carib legend about
Brimstone Hill suggesting that the hill grew out of the lower slopes of Mt. Liamuiga.
This suggests that the pre-Columbian Indians possibly witnessed the growth of Brimstone
Present and past studies indicate that the northern part of St. Kitts near to Mt.
Liamuiga is the most likely location for future eruptions. The most likely style
of eruption is an effusive lava dome-forming eruption. There is also evidence of
past occurrences of explosive magmatic eruptions resulting in pyroclastic , fall
deposits; however, in the past dome growth and collapse have been the dominant processes.
Future effusive eruptions may occur either on the flanks of Mt. Liamuiga (e.g. as
at Brimstone Hill) or within the crater itself.
1 Pyroclastic flows and surges. A pyroclastic flow is a hot (100-600
0C), fast-moving (>100km/hr) mixture of ash, rock fragments and gas. They usually
travel down valleys and cause total destruction of the area over which they flow.
Pyroclastic flows have been the main cause of destruction and loss of life in Montserrat
since 1995. A pyroclastic surge is a dilute turbulent cloud of gases and rock debris
that moves above the ground surface at great speeds. These form in a similar way
to pyroclastic flows, but their effects are more widespread. Pyroclastic surges
can be either hot or cold.
2 Effusive eruption. Effusive eruptions occur when molten rock (lava)
reaches the Earth's surface and erupts passively. The products of these eruptions
are lava flows and lava domes. They generally occur when the gas content of the
magma is low.