Geoscience Reference
In-Depth Information
2
The Geography, Science and Allure
of Volcanoes
Looking at an old painted canvas, one can sometimes find dis-
tinctly outlined areas of cracking, or 'craquelure', as it is known
in the art trade. The cracks make irregular and varied patterns
across the paint surface, creating distinct divides from one area
to the next. As the paint dries, so the gaps between these areas
widen, until all movement ceases.
So, to a very minor extent, with the surface of the Earth,
the lithosphere. In its irregular cooling over thousands of
millions of years the lithosphere, land and sea, has cracked into
what has become an established pattern - seven large plates
that enwrap the globe: the North American, Eurasian, Pacific,
South American, African, Indo-Australian and Antarctic plates.
These have travelled the globe for many millions of years, since
the crust first developed a rigid surface roughly 2.5 billion years
ago. In addition, seven smaller plates range in area from the
largest, the Nazca plate in the southeast Pacific, to smaller ones
such as the Cocos and Juan de Fuca plates. These are at the
Pacific coasts of Central America and parallel to the Cascades
range in North America respectively. The lithosphere lies like
a skin of paint on top of the asthenosphere, an inner layer of
the earth that is comparatively weaker than the lithosphere
and can flow over time, allowing the lithosphere some degree
of surface movement. It is this freedom that causes the litho-
sphere to adjust itself continually in relation to the earth's
inner energy, as a coat jacket will adjust itself in response to
the movement of the shoulders of its wearer. Plato described
San Andreas Fault,
California, 1992
