Geoscience Reference
In-Depth Information
the earth beautifully in
Phaedo
; he might have been writing of
tectonic plates:
The earth, viewed from above, looks like one of those balls
made of twelve pieces of leather, painted in various colours . . .
Up there the whole earth displays such colours, and indeed far
brighter and purer ones than these. One part is marvellously
beautiful purple, another golden: the white is whiter than
chalk or snow . . . more beautiful than any we have beheld.¹
Thus the movement of the lithosphere continues, as the earth
cools and as plates change their dimensions relative to each other.
The size and number of plates has adjusted over the millennia,
and continues to change: the Indo-Australian plate is understood
to be very slowly coming apart to form the distinct Indian and
Australian plates. Other much smaller plates, like flakes of
damaged paint on a canvas, lie unstable beside larger ones, in
particular in the area around Indonesia and the Philippines. And
how slow the changes are: the rates of movement of the plates
across the globe vary from the speed of growth of the human
fingernail, to that of human hair. Very slow indeed; though one
might reasonably feel that it is not slow enough.
In most cases volcanoes emerge along the lines of the
meeting of the plates. Thus commuters travelling to work in
Birmingham, England, or Alice Springs, Australia, will not en -
counter a volcano, as they are thousands of miles from a tectonic
plate boundary. Commuters in Naples, Reykjavík or Mexico
City, however, are at a statistical disadvantage here, while nearby
plate boundaries move together or draw apart millimetre by
millimetre.
The earth's volcanoes, principally, run along the lines
known as the Pacific Ring of Fire, which edges the Pacific and
Nazca plates, and the mid-Atlantic oceanic ridge. This is a crack
in the earth's surface running north-south from beyond Iceland
to Antarctica, with vents all along it like a line of buttons on
that jacket: as the wearer of the jacket puts on weight, so the
buttons begin to pull and fly off. In the case of the earth, it is not
