Geoscience Reference
In-Depth Information
haps 20 or 25%, supplying magma at about the right rate to sustain sea floor spreading
and produce an ocean crust 7 kilometres thick. Most of the ocean ridge eruptions pass un-
noticed as they take place more than 2,000 metres underwater as rapidly quenched pillow
lavas. But seismic studies have revealed magma chambers a few kilometres beneath the sea
floor in parts of the ridges, particularly in the Pacific and Indian oceans, though there is
also some evidence of magma chambers beneath the mid-Atlantic ridge. Where a mantle
plume coincides with an ocean ridge system, as in the case of Iceland, more magma is gen-
erated and the ocean crust is thicker, in this case rising above the sea to form Iceland.
Hawaii
The Big Island of Hawaii has welcoming people and friendly volcanoes. The town of Hilo
is probably more at risk from tsunamis triggered by distant earthquakes than from the great
4,000-metre volcano of Mauna Loa that looms behind it. To the north and west lie the other
Hawaiian islands and the Emperor seamount chain, tracing the long journey of the Pacific
plate across the hot spot of an underlying mantle plume. To the south of the Big Island of
Hawaii is Loihi, the newest of the Hawaiian volcanoes. As yet it has not broken the sur-
face of the Pacific, but it has already built a high mountain of basalt on the ocean floor and
will almost certainly become an island above water before long. Because Hawaiian lava is
very fluid, it can spread over a wide area and does not tend to form very steep slopes. Such
volcanoes are sometimes known as shield volcanoes, and they can flood basalt over a wide
area. Often, a particular flow will develop a tunnel around it as the outer crust solidifies but
the lava continues to flow inside. When the supply of lava ceases, the tunnel can drain and
be left hollow.
The last big volcano to finish erupting on Hawaii, Mauna Kea, is home to an international
astronomical observatory beneath some of the clearest skies on Earth. It was whilst visiting
there one night that I saw, through binoculars, a distant fiery eruption on the flanks of
Mauna Loa, in the Puu Oo crater. The next day I was able to take a helicopter flight low
over the freshly erupted lava flows. Through the open door I could feel the radiant heat
from the glowing, and in places still moving, lava and smell traces of sulphur in the air.
But it all seemed quite safe, even hovering in the crater itself, though avoiding the plume
of smoke and steam. The nearby Kilauea caldera, where many of the current eruptions take
place, boasts an observatory and viewing platforms. Every few weeks, visitors can wit-
ness a new eruption, often beginning with a curtain of fire with many fountains of glowing
magma along a rift. Nothing is burning in the fire, but the release of volcanic gases through
the hot, runny lava causes incandescent streams to fountain tens or even hundreds of metres
into the sky. The eruption may last only a few hours. In spite of the high fountains, erup-
tions are not particularly explosive due to the very runny nature of the lava. This allows
