Geoscience Reference
In-Depth Information
Free-fall
When he saw an apple fall, Isaac Newton realized that the force of gravity was pulling ob-
jects towards the centre of the Earth. What he did not know was that apples fall slightly
faster in some parts of the world than others - not that it is a difference you normally notice,
nor could you easily measure it with apples. But you can with spacecraft. The secret of fly-
ing, according to Douglas Adams' Hitchhiker's Guide to the Galaxy , is to fall but forget to
hit the ground. That's roughly what a satellite does. It's falling freely, but its speed keeps it
in orbit. The stronger gravitational pull of a region of dense rock will make satellites speed
up. Over a region of lower gravity, they will slow down. By tracking the orbits of low satel-
lites, geologists can build up gravitational maps of the Earth beneath.
When geophysicists compared gravity maps of the surface of the Earth with seismic tomo-
graphy scans of its interior, they had a surprise. You might expect that cold, dense slabs of
ocean crust would result in an excessive gravitational pull because of their higher density,
whilst a plume of hot mantle rock rising upwards would be less dense and cause a grav-
ity low. That reality is the opposite way around. The effect is especially pronounced over
southern Africa, where a huge plume of hot mantle appears to be rising, and around Indone-
sia, where cold slabs are sinking. Brad Hager of the Massachusetts Institute of Technology
came up with an explanation. The mantle super-plume under southern Africa is causing a
huge part of the continent to rise up, higher than you would expect were it simply floating
on a static mantle. Southern Africa, he estimates, is elevated by about 1,000 metres above
where it would naturally float on the mantle, and this excess uplift of rock causes the grav-
ity high. Similarly, the subducting ocean lithosphere beneath Indonesia is dragging the sur-
rounding surface down behind it, creating a gravity low and resulting in a general rise of
sea level compared to the land. Clement Chase, now at the University of Arizona, realized
that other broad gravity anomalies corresponded to the ghosts of past subduction. A long
band of low gravity that passes from Hudson Bay in Canada, over the North Pole, through
Siberia and India, and on to the Antarctic seems to mark a series of subduction zones where
ancient sea floor has plunged back into the mantle over the last 125 million years. What
was thought to be a rise in sea level which submerged most of the eastern half of Australia
about 90 million years ago may have been caused by the continent drifting over an ancient
subduction zone that tugged at the region as it passed over, lowering land by more than 600
metres.
The core
We have no direct experience or samples of the Earth's core. But we do know from seismic
waves that the outer part of it is liquid and only the inner core is solid. We also know that
the core has a much higher density than the mantle. The only material that is dense enough
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