resetting of the biological clock has occurred, with
renewed colonization from other islands. Santorini
in the Aegean Sea, whose collapse in 1628BCmay
have destroyed the Minoan culture on Crete, or,
more recently, Martinique and Montserrat in the
Caribbean, provide further examples of the destruc-
tiveness of island volcanoes.
It has been established that volcanic islands are
typically active over an extensive period, often
spanning many millions of years, and that there is a
variety of distinctive tectonic situations in which
volcanic islands are built (above). A corollary of this
is that a number of major types of volcanic eruption
can be identified. In increasing degrees of explosive-
ness (Decker and Decker 1991) they are as follows:
●Icelandic eruptionsare fluid outpourings from
lengthy fissures, and they build flat plateaux of
lava, such as typify much of Iceland itself.
●Hawaiian eruptionsare similar, but occur more
as summit eruptions than as rift eruptions, thereby
buildingshield volcanoes.
●Strombolian eruptionstake their name from a
small island off Sicily that produces small explo-
sions of bursting gas that throw clots of incandes-
cent lava into the air.
●Vulcanian eruptions, named after the nearby
island of Vulcano, involve the output of dark ash
clouds preceding the extrusion of viscous lava flows,
thus building a stratovolcanoorcomposite cone.
●Peléan eruptions produce pyroclastic flows
termednuées ardentes, high-speed avalanches
of hot ash mobilized by expanding gases and trav-
elling at speeds in excess of 100 km/h.
●Plinian eruptions are extremely explosive,
involving the sustained projection of volcanic ash
into a high cloud. They can be so violent, and
involve so much movement of magma from beneath
the volcano, that the summit area collapses, forming
a great circular basin, termed a caldera.
Like all generalizations, this classification is over-
simplistic; for example, calderas have formed by
collapse in Iceland and Hawaii as well as through
major explosive volcanoes. However, this classifica-
tion serves to illustrate that there are great differ-
ences in the nature of volcanism, both between
islands and within a single island over time.
The eruptive action within Hawaii over the past
few centuries has been both more consistent and less
ecologically destructive than that of Krakatau, which
in 1883 not only sterilized itself but, through caldera
collapse, created a tsunami killing an estimated
36 000 inhabitants of the coastlines of Java and
Sumatra. This death toll was tragically surpassed by
an order of magnitude on 26 December 2004, by a
tsunami that swept across coastal settlements in an
arc from north Sumatra, in this case generated not by
caldera collapse but by submarine faulting. Even
fairly small volcanic eruptions can have very impor-
tant consequences for island ecosystems. For
instance, eruptions on Tristan de Cunha in 1962 cov-
ered only a few hectares in ejecta, but toxic gases
affected one-quarter of the island area. The evacua-
tion of the human population left behind uncon-
trolled domestic and feral animals, which
transformed the effects of the eruption and produced
a lasting ecological impact (Stoddart and Walsh 1992).Mega-landslides
The progressive accumulation of volcanic material
upon young oceanic islands tends to build up rela-
tively steep slopes within a short geologic time span,
and at a faster rate than ‘normal’ subaerial erosion
processes can bring into dynamic equilibrium (cf. Le
Friantet al. 2004). Exceeding critical slope values pro-
duces gravitational instabilities that lead to the col-
lapse of the slopes through debris avalanches that
transfer hundreds of cubic kilometres into the sea
(Carracedo and Tilling 2003, Whelan and Kelletat
2003). Such collapses may occur unexpectedly and
suddenly and may lead to the disappearance of a sig-
nificant part of an island, as much as a quarter of it,
in just minutes (Carracedo and Tilling 2003), achiev-
ing mass movements with velocities that have been
estimated as over 100 km/h (Lipman et al. 1988).
Although they were introduced into modern vul-
canology by Telesforo Bravo (1962) almost half a
century ago to explain the origin of the Las
Cañadas caldera in Tenerife, our knowledge of cat-
astrophic landslides has developed relatively
recently, with improvements in the three-dimen-
sional bathymetric analysis of the ocean floor. Such
analyses have led to the discovery of the blocks and
debris avalanches resulting from the landslides col-
lapses, often extending over areas of hundreds of42 ISLAND ENVIRONMENTS