To learn some geology,
try pouring a cool cocktail.
Plate tectonics may be about to be
turned on its head. Instead of being heated from below
like water on a stove, the Earth's interior could have
more in common with a neat gin on the rocks.
While geologists agree that the plates
of the Earth's crust are constantly moving, nobody is
quite sure why. Some believe that heat from within the
Earth causes convection currents in the molten interior
that drive the plates. But geophysicist Don Anderson
of the California Institute of Technology in Pasadena
thinks the convection could be triggered by the plates
themselves. Efforts to model mantle convection based
on heat from below have failed.
"Those convection calculations
have never come up with plate tectonics or anything
resembling the present situation on Earth," Anderson
says.
Thinking about the problem from the
top down may be the key. The new view is that where
the hot mantle comes into contact with the relatively
chilly crust above, it cools and sinks. The same thing
happens when you order a drink on the rocks, says Anderson.
"If you put two ice cubes in a
glass of gin, the gin next to the ice cubes will get
cold and sink."
By studying the pattern of the plates
today and in the past, Anderson concludes that the Earth
is in a transitional state between so-called "ideal
configurations" – low-energy, stable states
seen in other spherical systems such as buckyballs and
viruses.
"If things ever settle down to
an equilibrium state, then the ideal number of plates
seems to be somewhere around 12," he adds.
Although Earth does have roughly 12
plates, they have widely different shapes and sizes.
It's an unstable configuration but may eventually even
out into patterns where three similar plates meet at
triple junctions. This is typical of systems in nature
that are controlled by surface processes, and Anderson
speculates that the Earth's plates are controlling the
interior, not vice versa.
"It shows that plate tectonics
is much more powerful than people thought," he
says.
If Anderson is right, geologists may
have to reconsider why some volcanoes appear where they
do. Volcanic activity where two plates collide can easily
be blamed on plates melting as one is forced under the
other. But when volcanoes emerge in the middle of a
plate, geologists usually invoke a narrow plume of hot
mantle rising up from deep within the Earth and bursting
through the crust.
Anderson, however, believes that weak
spots in the plates rather than plumes control where
the hot magma erupts onto the surface.
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