Geoscience Reference
In-Depth Information
Think of Canada, say: its flat central expanse of the 'Canadian Shield'
consists of the eroded roots of mountain belts that are 2-3 billion
(and more) years old, while the youthful Rocky Mountains rise in
the west.
The Rockies—simplifying greatly—may be thought of as repre-
senting sediment washed into ocean trench systems at the continen-
tal margin, sediment that was then scrunched by tectonic pressures
and plastered on to the side of the continent. The oceanic slab, inch-
ing its way down to the mantle beneath the Rockies, also causes melt-
ing of deep-lying rocks: the ascending magma, whether erupted at the
surface or cooling at depth as granite bodies, also adds to the bulk of
the newly forming mountains. By such processes, continents, over
time, will grow at their outer edges, to slowly reduce the proportion
of the Earth's surface that is made up of oceanic crust.
However, it is not quite this simple. It is now becoming clear that
plate subduction does not always create continental crust. It can also
destroy it, via a process termed subduction erosion. Here, the descend-
ing oceanic plate acts as a giant rasping file, wearing away fragments
of the overlying continent and dragging them down to destruction
with it in the mantle. In any one place, the balance between subduc-
tion accretion (i.e. continental building processes) and subduction
erosion varies depending on a number of factors, such as the rough-
ness of the oceanic crust (i.e. how abrasive it is), the angle at which it
descends, and so forth. Quite how important subduction erosion is
remains unclear—and almost certainly its importance has changed
over time.
There have also been attempts to chart the growth of continents
in the deep geological past by such means as trying to work out the
areas or volumes of crust of different ages, or by looking at the
spread of ages of magmatic rocks associated with mountain build-
ing. This is easier said than done, because the ancient cores of
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