Geoscience Reference
In-Depth Information
likely to have been present
somewhere
. It doesn't mean that there were
necessarily oceans at the surface, although conditions may have
allowed them to exist,
27
but it does suggest that, kilometres below the
surface, water was circulating. At this stage, such a finding may be
even more significant than pools of surface water, for it is a hint that
the engine room of this planet was becoming primed to function.
The Engines of the Earth
In a world without water things work differently—and with more
difficulty—than on a world with oceans. The hot, high-pressure iron
and magnesium silicate minerals at hundreds of kilometres depth in
a dry planet's interior are stiff and unyielding, and cannot easily give
way, even as heat builds up and those minerals begin to melt. Eventu-
ally magma rushes out on to the surface in great floods, after which
release of heat and pressure the interior stagnates again, until the next
great outburst. That currently dry planet, Venus (see Chapter 9), seems
to behave in this way. This kind of volcanism is not good for any
organisms that might try to get a toehold on such a world.
Mix in just a little water, though, and the minerals soften and
weaken. These rocks, deep down, can therefore flow more easily. Cru-
cially, if magma comes up in one place causing the Earth's rigid litho-
sphere (the crust together with the uppermost part of the mantle) to
stretch and break apart, then other sections of crust can begin to
founder and sink through into the mantle. As these sink, they pull on
the adjacent lithosphere, and that pulls apart yet farther, allowing
more magma to the surface.
This is the beginning of plate tectonics, in which the plates are
separate and independently moving sections of the Earth's litho-
sphere. In Francis Albarède's view, the water that arrived in a shower
of water-bearing meteorites (and, as we now suspect, comets) began,
over many millions of years, to get mixed in with the crust and upper
Search WWH ::
Custom Search