Geoscience Reference
In-Depth Information
2
AT GROS MORNE
National Park, on the west coast of Newfoundland, a great chunk of the Earth's innards is ex-
posed on the surface. This mélange of oceanic crust and underlying mantle—a complex known as ophiolite—is
completely infertile laden as it is with heavy concentrations of nickel and iron and, in the words of nature writer
Wayne Grady, most resembles the “tailings from some colossal iron ore mine.” It has proved impervious to
erosion, having not broken down into soil in the past half billion years. This slab from deep within the Earth
now stands nakedly on the surface, an enduring monument to the process that has shaped and is continuing to
shape the planet.
It has been said that Gros Morne is to geology what the Galápagos is to biology. Just as the Galápagos Is-
lands, with their famous finches and tortoises, seeded and supported Darwin's theory of evolution by natural se-
lection, so Gros Morne's rocks are testament to the unifying theory of the evolution of Earth's continents and
oceans—the theory of continental drift first proposed by the German meteorologist Alfred Wegener in his sem-
inal topic, The Origin of Continents and Oceans, published in English in 1924, and later refined as the theory of
plate tectonics.
Wegener, a German-born Arctic explorer and meteorologist, noticed that the coastlines of Africa and South
America seemed to dovetail, “as if we were to refit the torn pieces of a newspaper by matching their edges.” He
surmised that they had been a single landmass and, over the eons, had somehow drifted apart. He set about com-
piling other evidence for his theory, matching coal deposits and fossils found on either side of the oceans where
the continents had presumably been joined. One such fossil was of the fish-eating freshwater reptile Mesosaurus
which lived only in Brazil and southern Africa about 270 million years ago. But the scientific community vehe-
mently rejected Wegener's radical theory of continental drift before his death on an expedition to Greenland in
1930, and it would be nearly half a century before it was vindicated. What was missing was a mechanism to ex-
plain how continents could drift and collide and how oceans could open and close.
The new theory of plate tectonics, first put forward by a University of Toronto professor, J. Tuzo Wilson, in
1965, solved that problem. It described a dynamic view of the Earth in which a series of rigid plates—some
twenty of them on the planet's surface—are in constant slow motion, migrating at the rate one's fingernails
grow. Over millions of years, they do eventually bump into each other: the results can vary but are always dra-
matic—earthquakes, volcanic eruptions, the creation of islands, or a monumental crumpling of the landmasses
at their leading edges, which pushes up mountains. And then they drift apart again, creating separate continents
and oceans in between. A record of this process is uniquely preserved at Gros Morne in the form of ophiolite.
To understand plate tectonics, it is first necessary to examine the structure of the Earth, from the inside out.
The Earth has been compared to a series of nested balls. At the center is a solid core, 2,740 kilometers (1,700
miles) in diameter, surrounded by a 2,000-kilometer (1,200-mile) thick outer core of molten nickel and iron. The
core, in turn, is surrounded by the mantle, a 3,000-kilometer-thick (1,900-mile) layer of dark, coarsely crystal-
line rock called peridotite, which is rich in both magnesium and iron. Floating atop the mantle and core is the re-
latively thin crust, which varies in thickness from 6 to 35 kilometers (4 to 22 miles), being thicker under moun-
tain ranges than under oceans.
The crust is formed of lighter minerals that floated to the top of the mantle early in the Earth's formation.
There are two types of crust: oceanic and continental. Oceanic crust is similar in composition to the mantle, con-
