Geoscience Reference
In-Depth Information
even smaller variations of a few billionths of a second. Some may be due to atmospheric
circulation blowing on mountain ranges like wind on a sail. But there is another component
which seems to be caused by circulation in the outer core pushing on ridges in the base of
the mantle like ocean currents pushing on the keel of a ship. So there may be ridges and
valleys like upside-down mountain ranges on the base of the mantle. There seems to be a
great depression in the core beneath the Philippines that is 10 kilometres deep, twice the
depth of the Grand Canyon. Bulging up beneath the Gulf of Alaska is a high spot on the
core; a liquid mountain taller than Everest. Maybe sinking cold material indents the core,
while hotspots bulge up.
Super plumes
Although much hotter, the perovskite rock of the lower mantle is much more viscous than
upper mantle rocks. Estimates suggest that it is 30 times more resistant to flow. As a res-
ult, material rises from the base of the mantle in a much slower, broader column than the
plumes which characterize the upper mantle. It behaves, in very, very slow motion, rather
like the blobs of gloop in a lava lamp. It may well be true that, although some material cir-
culates through the entire mantle, there are also smaller convection cells that are confined
to the upper mantle. Convection cells in experimental systems tend to be about the same
width as they are deep and, in some parts of the world at least, the spacing of plumes of
mantle material seem to match the 660-kilometre depth of the upper mantle.
How the Earth melts
What goes down must come back up again. As plumes of hot mantle rock slowly rise to-
wards the crust, the pressure on them drops and they begin to melt. Scientists can recreate
what happens using great hydraulic presses to squeeze samples of artificial rock, heated in
furnaces. It's not the entire rock that melts, only a few per cent; producing magma that is
less dense than the rest of the mantle and so is able to rise up rapidly to the surface and
erupt as basalt lava. How it flows through the remaining rock was another great mystery.
It turns out to be down to the microscopic structure of the rock. If the angles at the corners
of the little pockets of melt that form between rock grains were large, the rock would be
like a Swiss cheese; the pockets would not interconnect and the melt couldn't flow out. But
those angles are small and the rock is like a sponge, with all the pockets interconnecting.
Squeeze the sponge and the liquid flows. Squeeze the mantle and the magma erupts.
