Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 50

mantle at the ocean trenches. It is still the most persuasive and fruit-
ful of hypotheses and, through the almost impossibly precise meas-
urements of movements of the Earth's crust provided by satellite-borne
lasers, it has essentially become science fact.
There are still a few small groups of 'expansionists' around the
world, holding on to their vision with all the tenacity of a tiny and
impassioned religious sect, and making elaborate models of a world
that they think has puffed out like a beach ball being blown up for a
day out at the seaside. And one has to remember that this would have
happened only in the last 200 million years (for that is the maximum
age of preserved ocean crust). It is still a striking, even outrageous
concept—and there are converts among the young and scientifically
romantic to be gained too, for the concept is now daringly anti-
establishment. Alas for this beautiful idea, measurements of the Earth
in the satellite age clearly show that the Earth is not expanding—or at
least not expanding by more than a fifth of a millimetre a year (the
current uncertainty of measurement). Plate tectonics has gone from
being a revolution to humdrum normality in one human generation.
That, though, is how the world works.
It is almost an equally shocking idea that the ocean plates—slabs of
crust and upper mantle about 100 kilometres thick—are continu-
ously inching their way across the Earth (almost literally inching, as
measured speeds range from less than 1 centimetre to more than 10
centimetres a year). Created as hot (and therefore relatively buoyant
and therefore higher) crust of the mid-ocean ridge, the ocean plates,
over time, separate ever more widely, cool, become denser, and sink
lower. Eventually they reach the end of the line.
Once an ocean plate reaches an ocean trench, it bends and sinks
downwards back into the mantle. Sliding one gigantic slab of rock
past another is not a gentle process. Part of the movement, it is now
known, does take place as slow 'creep', a process that does not cause