MODES OF ORIGIN 19long after this continental fragment began its
ocean voyaging. Similarly, changing sea level,
especially the reductions associated with major
periods of glaciation, can turn islands into
extensions of continents (such that for lengthy
periods during the Quaternary, Tasmania,
Australia, and New Guinea have formed one land
mass, called Sahul), while providing additional
‘stepping stone’ islands that will have assisted the
colonization of otherwise remote islands.
Debates between explanations for island biotas
that postulate vicariance (the breaking of a past
land connection) and those based on long-distance
dispersal continue to generate considerable
controversy and are sometimes not easily resolved.
Perhaps the most celebrated example is that ofNew Zealand. The case that New Zealand’s biota
constitutes a Gondwanan ‘time-capsule’ entirely
explicable through long isolation of a full
assemblage of species (a vicariance model) has been
contested on the basis of evidence indicating more
recently colonization of many lineages. As McGlone
(2005) has put it ‘... for New Zealanders in
particular, abandoning “Time Capsule of the South
Seas” for “Fly-paper of the Pacific” will be a
wrench. But it has to happen...‘ And indeed
many features of New Zealand’s biogeography are
comparable with those of true oceanic island
archipelagos such as Hawaii, strongly suggesting
that much of its biota has colonized after the
break-up of Gondwanaland, by long-distance
dispersal (Pole 1994; Cook and Crisp 2005).Table 2.2P. D. Nunn’s (1994) genetic classification of oceanic islands, with examples
Level 1 Level 2 Examples
Plate boundary islands Islands at divergent plate boundaries Iceland, St Paul (Indian Ocean)
Islands at convergent plate boundaries Antilles, South Sandwich (Atlantic)
Islands along transverse plate Cikobia and Clipperton (Pacific)
boundaries
Intra-plate islands Linear groups of islands Hawaii, Marquesas, Tuamotu
Clustered groups of islands Canaries, Galápagos, Cape Verde
Isolated islands St Helena, Christmas Island (Indian Ocean),
Easter Island
conditions necessary for their conversion from
seamounts to large islands. Iceland is the largest
such example (103 106 km^2 ), being a product of the
mid-Atlantic ridge and a hotspot that may have been
active for some 55 million years. Another context in
which mid-ocean ridge islands can be found is in
association with triple junctions in the plate system,
a notable example being the Azores, where the
North American, Eurasian, and African plates meet.
The second form of divergent plate boundary island,
exemplified by the Tongan island of Niuafo’ou, is
that sometimes formed in back-arc basins, which
develop as a result of plate convergence, but which
produce areas of sea-floor spreading (Nunn 1994).
Islands at convergent plate boundaries
Where two plates converge, one is subducted below
the other. This results in a trench in the ocean floor
at the point of subduction. Beyond the trench, a row
of volcanic islands develops parallel with the trench
axis on the surface of the upper of the two plates
(Fig. 2.4). This mechanism accounts for some of the
classic island arcs in the Pacific and Caribbean. Most
commonly, where islands are formed in connection
with subduction zones, the edges of the two con-
verging plates are formed exclusively of oceanic
crust. The magma involved in island arc volcanism
derives from the melting of the subducted ocean
crust, complete with its sedimentary load. Its com-
position thus owes much to the nature of the sub-
ducted crust. It is believed that the subduction of
basaltic crust and water-bearing sediment leads to
explosive andesitic volcanism. The Sunda island
arc, including the Indonesian islands, is predomi-
nantly of this form. Basaltic volcanism in island arcs
is less common, possibly because of a relative