The Discovery of the Century - Four Revolutions in the Earth Sciences - page 133

the motion transmogrifying into the San Andreas Fault. 11 Using a technique called
“seismic tomography”—think of a CT scan of the Earth—scientists today can ac-
tually “see” the shattered remains of the vanished plate buried deep beneath west-
ern North America.
That same year, John Dewey and F. M. Bird explained how the process of plate
tectonics accounts for each major feature of mountain ranges. 12 For example, as
continents separate, wedges of sediment accumulate on their trailing edges. When
two diverging plates switch direction and converge, they squeeze and crumple the
now facing sediment wedges, throwing up mountains like the Appalachians. When
a trench descends under the margin of a continent, “Cordilleran” mountains form,
such as the Andes and the Sierras. Continents can impinge on island arcs to create
smaller mountains, as in New Guinea. And so on. Decades of work lay ahead, but
at last geologists had a theory that explained the building of mountains.
In 1968, Tuzo Wilson suggested that the margins of continents might contain
pieces broken off of older continents and carried along until the conveyor belt
turned downward and plastered the fragment onto some other continent. Today,
geologists think that as much as one-fourth of North America, for example, may
be composed of such welded microcontinents, thus turning the jigsaw puzzle into
a patchwork quilt. The author John McPhee has captured well how geologists of
the 1970s and 1980s regarded these “exotic terranes.” 13
Paraphrasing what Theodosius Dobzhansky said about evolution, little in geo-
logy makes sense except in light of plate tectonics. It is the global theory that gen-
erations of geologists had sought but lacked the data to perceive. But not surpris-
ingly, not all were convinced.