Lava, which comes to the surface from the very depths of
the earth in its molten state at temperatures between 700
and 1,250 °C, is a mixture of molten silicate and crystals.
As lava cools on the volcanoes, the minerals are
crystallized according to physical and chemical laws, and
together with quickly-hardened glass from volcanic
rock.
However, lava does not embody the most original
elements of the earth, but rather represents a rebirth of
matter. Since the condensation of the chemical elements
from the cosmos into a solid, rocklike crust, more than
four and a half thousand million years ago, heat has been
continuously produced by the radioactive decay of certain
elements. In zones of great heat concentration, at
temperatures between 1,250 and 1,500 °C, there begins a
slow, partial melting of the silicates in the earth's crust
formed of the minerals olivine, pyroxene and spinel. This
newly-formed molten matter is described as magma As it
is lighter than the surrounding rocks of the crusl it can get
up to the surface, where it then pours out as lava.
Generally, the magma formed within the earth's crust
has a basaltic composition, which means that basaltic
rock results therefrom, through the crystallization of
olivine, plagioclase, pyroxene and iron-titanium
oxide.
In the mid-ocean ridges, which run through the great
oceans at depths of between two and four thousand
metres, basaltic lava has been continuously pouring
through since the formation of the earth's crust,
unnoticeably for us. Through this process continents are
pushed apart and new oceans form. Thus basalt, as the
commonest volcanic rock, covers two-thirds of the entire
surface of the earth.
But basaltic lava also flows out of all volcanic islands,
which rise out of the depths of the seas as mighty
volcanoes, as for example in Hawaii, Santorin and
Stromboli. In contrast with the continuous pro
duction of magma within the mid-ocean ridges, the
life expectation of the big individual volcanoes is
short, generally limited to between a few hundred
thousand and a million years.
The variability of basaltic lava, and more particu-
larly its viscosity (or fluiditiy) and the temperature at
which it flows out, all depend on the chemical com-
position of the magma, and the gaseous substances
liberated therein, such as water, hydrogen, carbon
monoxide and sulphurated hydrogen. Higher pro-
portions of sodium and potash, and lower content in
silicon and aluminium, reduce the viscosity of
basaltic lava, and make a swift eruption possible
without a major explosion. This is especially so for
the basaltic lava on Hawaii, Stromboli, Etna and
Vesuvius.
Catastrophic volcanic explosions are mostly the
result of water coming into contact with magma
inside or underneath the volcano. Above all, magma
that is richer in silicon, non-basaltic and more vis-
cous can hinder the efflux of water and gases for
longer periods. There is no telling when a devas-
tating volcanic eruption will result. |
