Geoscience Reference
In-Depth Information
fied structure of alternate layers of lava and ash or cinders, which can spew over an area far
wider than the peak itself.
The reason for their violent behaviour compared to Hawaiian volcanoes is that they are not
fed by clean, fresh magma from the mantle. The material sinking down beneath them is old
ocean crust. It is saturated with water, both as liquid in pores and fissures but also bound
into hydrous minerals. As the slab descends, it heats, due to its depth and also possibly due
to friction. The presence of water lowers the melting point, so partial melting occurs. The
pressure is so great that the water easily dissolves in the melt, lubricating it, and this magma
squeezes up through the continental crust above. As it nears the surface, the pressure drops
and the water starts to escape as steam. It can do so very rapidly and violently, in much the
same way as gas escapes when a well-shaken bottle of champagne is uncorked.
On its way up, the magma may accumulate in chambers until there is enough pressure to
erupt. During this time, dense minerals can solidify and fall to the floor of the chamber.
These minerals, particularly iron compounds, are what make basalts dark and dense. The
melt that remains for the eruption is lighter-coloured and richer in silica - up to 70 or 80%
silica in some cases, compared to 50% or less in basalt. It forms rocks known as rhyolites
and andesites, typical of places like Japan and the Andes. Their eruption is violent, not only
due to the high water content but also because such silica-rich lavas are much more vis-
cous and sticky. They do not flow easily, and bubbles can't easily escape. Such lava cannot
fountain like a Hawaiian eruption but is flung out of the way explosively.
Mount St Helens
One of the most famous eruptions of recent times was of a volcano of this type. Mount
St Helens is in Washington State in the northwestern USA, where the Pacific plate is sub-
ducting. At the beginning of 1980 it was a beautiful mountain surrounded by pine forests
and lakes, a popular holiday retreat. It had shown little activity since 1857. Then, on 20
March 1980, in a series of small earth tremors building up to a magnitude 4.2 quake, it
reawakened. Earth tremors continued to increase and trigger minor avalanches until 27
March, when a big explosion occurred in the summit crater and Mount St Helens began to
spew ash and steam. The prevailing wind blew the darker ash to one side, leaving white
snow on the other. The mountain took on a two-tone appearance.
So far, no lava had erupted. All that escaped from the crater was steam, blowing ash with
it. But it signalled the rise of hot magma beneath the volcano. The earthquake activity con-
tinued, but the seismographs also began to record a continuous rhythmic ground-shaking
different from the sharp shocks of the earthquakes. This so-called harmonic tremor is be-
lieved to have been generated by magma rising beneath the volcano. By mid-May, 10,000
earthquakes had been recorded and a prominent bulge had developed in the north flank of
