Geoscience Reference
In-Depth Information
Plinean eruptions
The nature of a volcanic eruption depends on the viscosity, or stickiness, of the magma and
also on the amount of dissolved gas and water it contains. Early in an eruption any ground-
water will be flashed explosively to steam. As the gas comes out of solution when the pres-
sure is released, it will expand rapidly, sometimes explosively. Moderate amounts of gas
in runny basalt produces the fire fountains of Hawaii. Greater quantities of gas will carry
with it finely divided solid material such as ash and cinders. Eruptions tend to be more vi-
olent early on when the magma still contains a lot of gas. If it has had time to settle in a
comparatively shallow magma chamber, it will be better behaved. Sometimes the gas and
ash rise so high into the air that they spread widely before the ash falls. This was the sort
of eruption witnessed on Vesuvius in 79
AD
and has been called a Plinian eruption after the
account by Pliny of his uncle's death.
Many different volcanic rocks can be produced in such eruptions. Where the ash and
cinders are hard before they land, layers of loose tuff will build up. If the fragments are still
molten, it would be a welded tuff. Near the vents, larger lumps of magma will be thrown
out. If they are still molten when they hit the ground, they will form splatter bombs that
look a bit like cow pats. If a solid crust forms in the air around the still-expanding lava
bomb, it will form a bread-crust bomb, looking rather like a loaf of risen bread. Lava that
is rapidly quenched can form volcanic glass called obsidian. If the lava solidifies with gas
bubbles still in it, these are known as vesicles. Sometimes a foam of gas bubbles in lava can
form, creating pumice of such low density that it floats on water. The surface of a lava flow
can be very rough and cinder-like, in Hawaii called 'aa' lava. (This is a Hawaiian word,
not just what you say when you try to walk over it!) Where a thin skin forms on a fluid
lava flow, it can crinkle into flow lines, creating ropy or 'pahoehoe' lava. Occasionally, fine
strands of lava become drawn out, an effect sometimes known as Pelée's hair.
Ring of fire
As long as you keep clear of the fire fountains and the fast-flowing lava, Hawaiian erup-
tions are reasonably safe. But that is not true of most volcanoes. Much of the Pacific is
circled by a ring of fire - volcanoes of a much more temperamental nature. Where an ocean
plate dives down in a subduction zone beneath a continent or an island arc, so-called stra-
tovolcanoes develop. These can be picture-postcard volcanoes. Mount Fuji in Japan is one,
with steep, conical slopes and a snow-capped peak topped by a smoking crater. But the
beauty of such mountains can mask their sinister behaviour. They are notorious for earth-
quakes and sudden, violent eruptions, as Mount Unzen in Japan and Mount Pinatubo in the
Philippines both showed in 1991. They are called stratovolcanoes because of their strati-
