El Hierro) (Carracedo et al. 1998). It seems that
neither idea on its own can explain the origins of
this particular archipelago, and that the roles of a
residual hotspot and the Atlas tectonics must be con-
sidered simultaneously (Anguita and Hernán 2000).
The Cape Verde islands are also of uncertain ori-
gin, but Nunn (1994) favours the tectonic-control
model. The Galápagos deserve a mention in this
section as being a group of great significance to the
development of island evolutionary models. They
lie in the western Pacific just south of the divergent
plate boundary separating the Nazca and Cocos
plates. Again, Nunn regards them as an intraplate
cluster rather than mid-ocean ridge islands.
Isolated islands
As knowledge of the bathymetry of the sea floor has
improved, a number of islands believed to be iso-
lated have been shown rather to be part of a
seamount-island chain or cluster. In general, it can be
taken that truly isolated intraplate islands form only
at or close to mid-ocean ridges. They result from a
single volcano breaking off the ridge with part of the
sub-ridge magma chamber beneath it. Nunn (1994)
identifies this condition as essential if the island is to
continue to grow once it is no longer associated with
the ridge crest. Examples include Ascension, Gough,
and St Helena in the Atlantic, Christmas Island in the
Indian Ocean, and Guadalupe in the Pacific. Isolated
intraplate islands or island clusters may also, in cer-
tain cases, be the product of small continental frag-
ments being separated from the main continental
mass. The granitic Seychelles provide the best exam-
ple, where there is good evidence of a continental
basement affiliated to the Madagascar–India part of
Gondwanaland (Box 2.2).
2.3 Environmental changes over long timescales
In the case of these islands we see the importance of tak-
ing account of past conditions of sea and land and past
changes of climate, in order to explain the relations of the
peculiar or endemic species of their fauna and flora.
(Wallace 1902, p. 291)The separation of environmental changes from
island origin is artificial. Islands may be built up
by volcanism over millions of years; in such cases,
active volcanism is thus a major part of the envi-
ronment within which island biotas develop and
evolve. The preceding section may give the
impression that all islands have been formed by
volcanism, or at least fairly directly by the action
of plate-tectonic processes. However, the forma-
tion of an island may come about either by
connecting tracts of land disappearing under
rising water, or by land appearing above the
water’s surface, either by depositional action or by
some other process, of which tectonic forms are a
large but not exclusive class. Indeed, Nunn (1994)
offers the observation that changes of sea level are
perhaps the most important reason why islands
appear and disappear. This section therefore deals
with long-term changes in the environments of
islands, focusing particularly on changes in sea
level, but also on locational shifts and changes in
climate.Changes in relative sea level—reefs, atolls, and guyots
As already noted, islands may come and go as a
consequence of sea-level changes. Some of these
sea-level changes are eustatic (i.e. they are due to
the changing volume of water in the sea), and oth-
ers are due to relative adjustment of the elevation
of the land surface (isostacy). This can be brought
about by the removal of mass from the land caus-
ing uplift, as when an icecap melts, or by tectonic
uplift. Subsidence of the lithosphere can be due to
increased mass (e.g. increased ice, water, or rock
loading) or may be due to the movement of the
island away from mid-ocean ridges and other
areas that can support anomalous mass. In the
right environment, coral reefs build around sub-
siding volcanoes, eventually forming atolls—an
important category of tropical island.
Darwin (1842) distinguished three main reef
types: fringing reefs, barrier reefs, and atolls. He
explained atolls by invoking a developmental
series from one type into the next as a result of sub-
sidence of volcanic islands: fringing reefs are coral
reefs around the shore of an island, barrier reefs fea-
ture an expanse of water between reef and island,22 ISLAND ENVIRONMENTS