0198566123.pdf

other, or moving past each other laterally. Relatively
few islands are ancient biogeographically; some
have experienced a complex history of both lateral
movements and alternating emergence and submer-
gence, with profound importance to the resulting
biogeography, not just of the particular island in
question, but also of the wider region (Keast and
Miller 1996).
Plate tectonic processes give rise to islands by
three main means. First, the breaking away of

pieces of continent by sea-floor spreading has

carried New Zealand, Madagascar, and a few other

ancient islands off into isolation (Box 2.2). Second,

in connection with plate boundaries, volcanic

islands may arise to form an archipelago of islands,

as in the case of the Greater and Lesser Sundas—the

many islands of the Indonesian region. Third, vol-

canic islands may arise from hotspots (e.g. the

Hawaiian islands) and certain parts of mid-ocean

ridges. Hawaii is of hotspot origin, and Iceland is

MODES OF ORIGIN 13

Trench

Sea level

Asthenosphere

Trench

(a)

mantle

Volcanic arc

Convergent

plate

boundary

Divergent

plate boundary

(mid-ocean ridge)

Convergent

plate boundary

(b)

Upper

Eurasian

plate

Pacific

plate Cocos

Nazca

plate

Antarctic plate

South

Eurasian

plate

African

plate

North

American

plate

90 °N

90 °E 120 °E 150°E 180 ° 150 °W 120°W90°W60°W30°W30 0 ° °E60°E90°E

60 °N

30 °N

30 °S

60 °S

75 °S

0 °

Indo-

Aust.

plate

American

plate

Continental

crust

Indo-Aust.

plate

Upper mantle

Oceanic crust

Oceanic

crust

Figure 2.1Model and map of crustal plates. (a) The basic plate tectonics model, exemplified by the South Pacific. New oceanic crust and upper
mantle are created along the divergent plate boundary and spread east and west from here. The eastward moving plate (the Nazca plate) is
subducted beneath the continental lithosphere of South America. The westward movement of the much larger Pacific plate ends with subduction
beneath the oceanic lithosphere of the Indo-Australian plate. At both trenches, the subducted plate begins to melt when it reaches the
asthenosphere. (b) The major plates of the world. Divergent plate boundaries (mid-ocean ridges), at which plates move apart, are represented by
parallel lines. Convergent plate boundaries (mostly marked by trenches) are represented by lines with teeth along one side: the teeth point from
the downward moving plate towards the overriding or buoyant plate. Transverse plate boundaries are shown by solid lines. Broken lines indicate
boundaries of an uncertain nature. (Redrawn from Nunn 1994, Fig. 2.4.)