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known as
accretion tectonics.
One side believes that Western British Colum-
bia has always been in its present-day position. The other believes that all of
this vast region drifted northward during the Age of Dinosaurs, finally to col-
lide with the then-westernmost coastline of North America, producing the
Coast Ranges of Canada and the North Cascades of Washington State.
Plate tectonics gave us the overall view of how the largest pieces of the
earth's crust, the continents and ocean basins, have interacted through time.
Accretional
tectonics tells us about continental assembly and how coastal
mountain belts form. The controversy often arises from the simple fact that
mountains are the most complex geological units on the planet. Because
mountains are composed of rocks thrust and shattered, folded and heated, pres-
surized and flung high into the sky, they are usually impervious to simple struc-
tural analysis of their component parts. To put it more simply, the characteris-
tic trauma of mountain building destroys the evidence about their formation.
Mountains are formed by several processes, including compression, ex-
tension, and the eruption of volcanoes forming in response to subduction.
But when these processes are melded with accretional events—such as an is-
land the size of Vancouver Island colliding with an already-forming coastal
mountain range—things become much more complex. Today, the most re-
searched aspects of continental drift are related to the importance of such
collisions between continents and the smallest crustal fragments accreting
onto continental edges. These ephemeral and active bits of land are called
suspect terranes,
because more often than not their formation in this manner
is hypothetical—and usually controversial as well.
John McPhee, perhaps the greatest living writer on things geological,
has visited the topic (as well as the real estate) of suspect terranes in several
of his books. They are wonderful subjects: regions where large pieces of
mountain chains are seemingly exotic, perhaps far-wanderers coalesced into
more banal country rock. They come in various shapes and sizes, and to dis-
cuss them we need some specialized terminology.
The first important pieces of such puzzles is
microplates.
Most plates
(the
plate
of plate tectonics) are enormous affairs, covering significant por-
tions of the earth's crust. It is one of the great geological mysteries
why
there
