0198566123.pdf

16 ISLAND ENVIRONMENTS

thought to have been formed by both mid-ocean
ridge and hotspot activity.
Setting aside islands of continental origin for the
moment, a two-tier classification relating specifi-
cally to oceanic islands has been proposed by Nunn
(1994). At the first level are the plate boundary and
intraplate island types, each of which may also be
subdivided into a number of major types, based on
their geographical configuration in relation to the
plate boundaries (Table 2.2). The recognition of
these distinctive configurations of islands is a help-
ful starting point. However, Nunn (1994) warns
against an over-simplistic use of the classification,
pointing out that, whereas many islands with a
common origin may be found in proximity to each
other, islands of a particular geographical group do
not necessarily share a common origin (this is well
illustrated by the Mediterranean islands; Schüle
1993). A brief account of this classification (drawn
largely from Nunn 1994) now follows.

Plate boundary islands

Islands at divergent plate boundaries

Divergent plate boundaries produce islands in two

rather different circumstances, along mid-ocean

ridges and along the axes of back-arc or marginal

basins behind island arcs that are themselves

associated with convergent plate boundaries.

Although divergent plate boundaries are construc-

tive areas involving more magma output than

occurs in any other situation, seamounts, rather

than islands, tend to form in connection with these

boundaries. This is believed to be because of the rel-

ative youthfulness of the seamounts, the likelihood

that the supply of magma decreases as the

seamounts drift away from the plate boundary, and

the increasing depth of the ocean floor away from

these boundaries. In some cases, mid-ocean ridge

islands may also be associated with hotspots in

the mantle, providing the seemingly exceptional

Continental islands split from a

continent by seafloor spreading

Volcanic islands arising from

subduction zones as part of

an island archipelago

Volcanic islands arising from

hot spots and mid-ocean ridges

Easter island

Taumotu Ridge

Archipelago

Line Island

chain

MacDonald

Seamount

New

Caledonia

Norfolk

New Zealand

Stewart

Austral Se

chainamount

Marshall

Ellice

Island

chain

Emperor

Seamount

chain

Hawaiian ridge

Hawaii

Aleutian

islands

Kurils

Figure 2.3Map of the Pacific Ocean, depicting the major modes of origin of islands. NB: New Guinea and Tasmania are continental islands of
the Pleistocene Sahul Mainland, which included these islands plus Australia. The New Zealand land mass has been physically isolated from
Australia and Antarctica by up to 2000 km for 60 million years, as a result of Late Cretaceous to Late Palaeocene sea-floor spreading (Pole
1994). (Redrawn from Mielke 1989, Fig. 9.1., slightly modified)