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paucity of sedimentary load in the subducted crust.
The Sandwich arc in the south Atlantic involves
both basaltic and andesitic volcanism.

Islands along transverse plate boundaries
This is a fairly rare context for island formation as,
by definition, less divergence or convergence of
plates is involved. However, strike–slip movement,
compression, or both, can occur between adjacent
parts of plates. An example of an island believed to
have been produced by these forces is the Fijian
island of Cikobia, in the south-west Pacific.

Islands in intraplate locations

This class of islands contains some of the most bio-
geographically fascinating islands of all, notably
the Hawaiian group. They are equally fascinating
geologically.

Linear island groups
The Hawaiian chain is the classic example of a
series of islands with a significant age sequence
arranged linearly across a plate (Table 2.3, Fig. 2.3).
Among the larger islands, Kauai is the oldest at
more than 5 million years, whereas Hawaii itself is
less than 1 million years old. Islands older than

20 ISLAND ENVIRONMENTS

Sea level

Volcanic

islands

Trench

PLATE A PLATE B

Figure 2.4Simplified representation of the formation of an island arc, by the subduction of one plate under another (from an original in
Williamson 1981).

Table 2.3Approximate ages of selected islands and of the Meiji

seamount. This table illustrates the wide variation in the ages of

oceanic islands, the simple age sequence of members moving

north-west along the Hawaii–Emperor chain, and the lack of a

similarly simple sequence moving south in the Austral–Cook cluster.

For fuller listings, error margins, and original sources see Nunn

(1994); for Hawaii, in addition see Wagner and Funk (1995)

Island group Age (Ma)

Hawaii–Emperor chain

Mauna Kea, Hawaii (the Big Island) 0.38

Kauai 5.1

Laysan 19.9

Midway 27.7

Meiji Seamount 74.0

Austral–Cook cluster

Aitutaki 0.7

Rarotonga 1.1–2.3

Mitiaro 12.3

Rimatara 4.8–28.6

Rurutu 0.6–12.3

Isolated intra-plate islands

Ascension 1.5

St Helena 14.6

Christmas Island (Indian Ocean) 37.5

Kauai stretch away to the north-west, but through

combinations of erosion, subsidence, and coral

formation, they are now reduced to forming coral