Geoscience Reference
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every continent. Examples are the Columbia River country of Ore-
gon and Washington, the Parana basin of Brazil, the South African
Karroo, and the Siberian traps. Before the advent of plate tectonics,
their origin was a mystery, but Tuzo Wilson, tolerated but unheeded
in my graduate school seminar, provided the key insight.
Wilson was a master at providing an innovative interpretation of
facts that had stumped others, in this case, that the Hawaiian Islands
lie along a straight line. Wilson noted, as had many, that the islands
to the southeast, where Kilauea and the other great volcanoes are
active today, are youngest and that they grow steadily older in a line
to the northwest. Beyond the most northwesterly island lies a linear
chain of submarine seamounts that become progressively deeper
and older, also to the northwest. Unlike the many others who knew
these facts, Wilson deduced their meaning. Deep in the earth's man-
tle, beneath the present active volcanoes, lies a
hot spot,
a zone that
melts periodically, sending jets of less-dense magma up to be ex-
truded onto the surface. But plate tectonics tells us that the rigid,
uppermost surface layer of the earth moves horizontally over the
fluid mantle, so that as time passes different sections of the crust lie
over a given spot in the mantle deep below. Now we can begin to
see what Wilson envisioned: A fixed hot spot deep beneath the cen-
tral Pacific crust episodically spurts magma toward the surface, but
by the time each spurt arrives, the crust has moved laterally so that
the new eruption occurs at a different point on the surface, produc-
ing a new volcano. The former volcanic sites move progressively fur-
ther and further away from the point directly above the hot spot,
becoming older, eroding, and finally disappearing beneath the sea.
From this, Wilson deduced that the crust in the mid-Pacific has been
moving steadily to the northwest, over the fixed position of the
deep hot spot. Since the ages of the Hawaiian volcanoes and some of
the seamounts are known precisely, we can calculate the speed at
which the crust there is moving; the rate checks exactly with results
obtained using the magnetic reversal time scale.
Jason Morgan of Princeton extended Wilson's idea to explain the
origin of flood basalts. When he reconstructed the past positions of
tectonic plates, he noticed that the basalt provinces lie directly over
present-day active volcanoes. For example, the hot spot that is now
under Yellowstone National Park once produced the Columbia River
basalts. Morgan thought that hot spots at the base of the earth's man-
tle sometimes produce huge, bulbous masses of hot, low-density,
low-viscosity basaltic magma, that then "float" to the top of the man-
tle, like a hot air balloon rising through colder, denser air. There these
