Travel Reference
In-Depth Information
can yield highly accurate information about both polarity and geomagnetic
field directions (which are themselves dictated by latitude). These two bits
of information became crucial lines of evidence of continental drift. In areas
where new sea floor is being created at the mid-ocean spreading centers, a se-
ries of linear magnetic stripes of opposing polarity have been measured by
ships and planes towing sensitive air- or waterborne magnetometers. Second,
the common application of magnetic coring from continental rocks showed
that different continents seemed to exhibit very different directions of the
ancient magnetic poles. Successively older rocks in North America, for in-
stance, seemed to suggest that the position of the earth's magnetic pole has
been migrating. Of course, it was not the earth's magnetic dipole positions
that were wandering but
the
continents
themselves.
By tracing them back
through time, via paleomagnetism, detailed histories of drift could be ob-
tained.
Such was the up-side of paleomagnetic analyses. The down-side is that
rock units that have been reheated gives spurious results. Unfortunately, no
easily observable sign alerts the investigator that this has happened. The
magnetic resetting of mineral grains occurs most frequently in those parts of
the earth where regional heating and high pressure are most intense—in
mountainous regions. Because these are the regions that are usually of great-
est interest for deciphering a continent's history, paleomagnetic work rarely
yields a simple interpretation. The mountainous regions harbor the various
slivers and shards of crust formed by collisions of continents with other rock
units, and it is in the mountains that paleomagnetic studies are most ur-
gently needed—and most perilous.
Within a decade of victorious overthrow of all preceding theories,
global tectonics and continental drift theory had revealed the path of North
America's drift over the last 500 million years (although many details still re-
main to be discovered). But much more mysterious is the history not of
North America's
drift
but of its tectonic formation, and no region remains
more obscure than its west coast. Stretching from Alaska to southern Mex-
ico, virtually the entire western margin of North America is mountainous.
How did these mountains form? Were they the product of subduction only,
