0198566123.pdf

1 Volcanoes form over the hotspot and are sub-
sequently removed from it by the movement of the
Pacific plate, producing a linear array of volcanic
summits increasing in age to the north-west.
2 After achieving their maximum heights perhaps
0.5 Ma after their emergence, volcanoes rapidly
subside as they move away from the hotspot
(Moore and Clague 1992).
3 Erosive processes reduce volcanic peaks to sea
level over several million years.
4 Small atolls may remain while conditions are
suitable for coral growth, or they are reduced to
guyots if they drown and sink below the sea
surface, carried down by the spreading and
subsiding plate (Jenkyns and Wilson 1999).

Recently, sonar surveys of the sea floor have
enabled the reconstruction of past island
configurations (Moore et al. 1994, Clague 1996). As
Hawaiian volcanoes grow, the lava deposited
below the sea level forms a steeper slope than that
deposited subaerially. The location of the now

submerged breaks-in-slope enables the estimation

of the maximum shorelines and thus areas of

islands throughout the archipelago (Fig. 2.12).

Furthermore, the maximum altitude achieved by

an island can also be reconstructed assuming a

7 angle for subaerial lava deposits. Knowing the

age, the original altitude of rocky outcrops and the

subsidence rate, it is possible to estimate the rate of

erosion of each particular island. This is, of course,

a fairly approximate science, as erosion can vary

through time as a result of climatic fluctuations,

the occurrence of mega-landslides, and enhanced

precipitation around higher peaks associated with

orographic cloud formation.

Using all this information, Price and Clague

(2002) have reconstructed the history of the

Hawaiian Chain, from 32 Ma, which marks the

appearance of the first Hawaiian island (the earlier

islands of the Emperor seamount chain were either

submerged or just atolls by this time). The island

configurations at 5 Ma intervals for the Hawaiian

chain are given in Fig. 2.13a, and Fig. 2.13b

summarizes the reconstructed distributions of

island altitudinal range. During the first half of

Hawaiian chain history (c.32–18 Ma) a few volca-

noes (Kure, Midway, Lisianski, Laysan), briefly

exceeded 1000 m above sea level (ASL) although

at any given moment most islands have been small.

The most important island that pre-dated the

present high islands was Gardner, which formed

around 16 Ma and was likely comparable in size

(10 000 km^2 ) and height ( 4000 m) to today’s

Big Island (also known simply as Hawaii). After its -

formation, a series of mid-sized volcanoes formed

(French Frigate Shoals, La Perouse Pinnacle), culmi-

nating with Necker, c.11 Ma. The period between 18

and 8 Ma, is described by Price and Clague (2002) as

the ‘first peak period’, because of the existence of

multiple peaks over 1000 and some over 2000 m,

contributing to a substantial archipelago, with

several zonal (coast to summit) ecosystems.

After the formation of Necker, there was a

period when only smaller islands formed, leading

to an archipelago diminished in height and area

and thus impoverished in habitats. The emergence

of Kauai (5 Ma) constituted the beginning of the

‘second peak period’, which continues today, with

30 ISLAND ENVIRONMENTS

Josephine Ormonde

Gettysburg

Ampere

Seine

Porto Santo

Desertas

Salvages

Canary

Islands

Dacia

Mogador

Concepción

Madeira

AFRICA

0 50 100 150 200 250 km

Figure 2.11A reconstruction of PalaeoMacaronesia at the time of
the sea-level minimum of the Last Glacial period (Redrawn from
García-Talavera 1999).