Geoscience Reference
In-Depth Information
9.7 Oceanic islands
Oceanic island chains represent anomalies in the oceanic lithosphere, being
locations away from the plate boundaries where considerable volcanic and
microearthquake activity is taking place. However, because of this they have
been very useful in advancing an understanding of the physical properties of the
plates and the underlying mantle.
Many of the details of seamount chains and oceanic islands have been dis-
cussed elsewhere in this topic: dating of seamount chains and the hotspot reference
frame (Chapter 2), gravity and flexure of the oceanic lithosphere due to the load-
ing of the Hawaiian Ridge (Section 5.7) and possible origins for oceanic-island
basalts (Chapter 8). In this section some details of the seismicity beneath Hawaii
and the seismic structure of the crust and upper mantle are presented.
Hawaii is the best-studied active oceanic island. Work by the staff at the Hawaii
Volcano Observatory, located on the rim of the Kilauea crater, and others has built
up a detailed picture of the processes taking place beneath the island. Figure 9.64
shows the P-wave velocity and density structure of the island of Hawaii on a
profile crossing the Mauna Loa volcano, which, together with the neighbouring
volcano Kilauea and seamount Loihi, marks the present location of the Hawai-
ian hotspot. The density values for the layers defined by the P-wave velocity
were calculated by assuming a relation between velocity and density similar to
that shown in Fig. 4.2. Some small adjustments to the seismic layering were
necessary in order to fit the gravity data well. The seismic-velocity and density
models, taken together, show that the crust beneath Mauna Loa thickens to some
18 km and that most of this material is of high density and velocity. The oceanic
crust on which the volcano has formed is bent downwards by the load, in accor-
dance with the flexural models. A schematic geological interpretation of these
models is shown in Fig. 9.64(d). The high-velocity, high-density intrusive core
of the volcano is interpreted as a sequence of densely packed dykes similar to
the sheeted-dyke complex proposed for the upper portions of oceanic layer 3
(see Fig. 9.5(b)).
Figure 9.65 shows the seismic activity occurring down to depths of 60 km
along two profiles across the island of Hawaii. In Fig. 9.65(a) the epicentres are
for all earthquakes from 1970 to 1983; in Fig. 9.65(b) the epicentres are for all
long-period earthquakes from 1972 to 1984. Both sets of data show an extensive
shallow (less than about 13 km depth) zone of activity. On the basis of seismic and
density models, these shallow earthquakes, which often occurred in swarms, were
all in the crust and are judged to be of volcanic origin. The deeper earthquakes are
larger in magnitude and appear to be tectonic. The Kilauea magma conduit can
be traced down to about 30 km. Deeper than 30 km the events merge into a broad
zone. The magma-transport systems for Mauna Loa, Kilauea and the seamount
Loihi appear to be connected to this deep zone.
The chemistry and dynamics of the Hawaiian magma supply have been stud-
ied in great detail. The top of the magma-source zone beneath Hawaii is at about
