Geoscience Reference
In-Depth Information
killed his uncle, Pliny the Elder, and destroyed Pompeii and Herculaneum. But for a long
time no one understood the cause of volcanic eruptions. Often they were thought to be the
work of a fire god or goddess, such as the Hawaiian goddess Pelée. In medieval Europe,
volcanoes were thought of as the chimneys of hell. Later, it was suggested that the Earth
was a cooling star with remnants of the stellar fire within, linked through a system of fis-
sures. In the 19th century, what we now know to be volcanic rock was widely thought to
be deposited from oceans, the Neptunist theory, as opposed to the Plutonist idea that it had
once been molten. After the Plutonist view gained ground, many thought that the interior
of the Earth must be molten, an idea that did not get ruled out until the dawn of seismology.
One of the mysteries was that volcanic rocks can have different compositions; sometimes
even when erupted from the same volcano. Charles Darwin was one of the first to suggest
that the composition of the melt could change as a result of dense minerals crystallizing out
and sinking in the magma, something that he backed up by observations of volcanic rocks
in the Galapagos islands. As with his ideas on continental drift, it was Arthur Holmes in the
mid-20th century who was the first to come close to the truth with his ideas of convection
within the solid Earth's mantle.
How rocks melt
The key to understanding volcanoes comes from understanding how rocks melt. For a start,
they don't have to melt completely, so the bulk of the mantle remains solid even though
it gives rise to a fluid, molten magma. That means that the melt does not have the same
composition as the bulk of the mantle. As long as the so-called dihedral angles, the angles
at which the mineral grains in mantle rock meet, are large enough, the rock behaves like
a porous sponge and the melt can be squeezed out. Calculations show how it will tend to
flow together and rise quite rapidly in a sort of wave, producing lava at the surface in the
sort of quantities seen in typical eruptions.
Melting does not necessarily involve increasing the temperature. It can result from decreas-
ing the pressure. So a plume of hot, solid mantle material will begin to melt as it rises and
the pressure upon it reduces. In the case of a mantle plume, that can happen at considerable
depths. Helium isotope ratios in the basalt erupted on Hawaii suggest that it originates at
around 150 kilometres depth. The mantle there is composed mainly of peridotite, rich in
the mineral olivine. Compared to that, the magma that erupts contains less magnesium and
more aluminium. It is estimated that as little as 4% of the rock melts to produce Hawaiian
basalts.
Beneath the mid-ocean ridge system, the melting takes place at much shallower depths.
Here there is little or no mantle lithosphere and the hot asthenosphere comes close to the
surface. The lower pressures here can result in a larger proportion of the rock melting, per-
