Geoscience Reference
In-Depth Information
in each case the sediments helped to form the basement complex—the foundation—of the North American
Plate.
Another supercontinent, Rodinia, coalesced approximately 1.2 billion years ago and resulted in yet another
mountain-building episode. These ancient rocks now form part of the Canadian or Precambrian Shield and un-
derlie much of eastern Quebec, bordering the Gulf of St. Lawrence and Labrador. The Long Range Mountains,
which are the backbone of Gros Morne National Park, form the shield's eastern edge, and the Adirondacks are
also part of this ancient complex of rocks known as the Grenville Province.
In the Late Precambrian, 570 million years ago, Rodinia began to break apart to form a new ocean, the Ia-
petus, named after the father of Atlas, who would become the Atlantic Ocean's namesake. Newfoundland was
then near the equator, and the rocks that would become Gros Morne were deposited at the edge of a new con-
tinent, forming a continental shelf that extended 100 kilometers (60 miles) out beneath this new sea, which
was rich with tropical sea life. Coral-like algae, trilobites, primitive corals, brachiopods, molluscs, and other
marine organisms flourished in the warm waters. Eventually the skeletons and shelly remains of these
creatures built up a carbonate bank that hardened into limestone. Seamounts also formed far offshore where
magma pushed up from 100 kilometers (62 miles) below, which is how the Hawaiian Island volcanoes are now
being formed.
Then, about 500 million years ago, the Iapetus Ocean began to close. As it did so, the eastern continental
plate (Eurasia and Africa) was on a collision course with the North American Plate. What happened next was
unusual: as the continental plates inched toward each other, the ocean crust cracked far out to sea, and the
ocean floor of the eastern plate slid over the undersea portion of the western plate, forcing the edge of the
North American Plate down toward the mantle, where it melted, then welled up to form an arc of volcanic is-
lands several hundred kilometers from the shore. As the continents pressed more closely together, the new
oceanic lithosphere, or crust, mixed with underlying upper mantle to form the rocks we call ophiolites, which
rode up and over the continental plate. As they did they stripped off slabs of the deep-sea sediments, which
stuck to the bottom, according to one geologist's account, “like snow caked under a slow-moving toboggan.”
Eventually all of this material—a so-called mélange—was plowed up on shore, where the 15-by-8-kilometer
(9-by-5-mile) slab of utterly barren ophiolites remain exposed on the surface, hard evidence of the Earth-chan-
ging process of plate tectonics.
PTYCHOPARIA
When Old Mountains Were New
As we have seen, each tectonic plate has a leading edge and a trailing one. Today the North American contin-
ent forms the western part of the North American Plate, which is drifting toward the northwest. The Atlantic
coast is, therefore, on the trailing edge; the Pacific coast, on the leading edge. The latter is colliding with the
continental crust of the Pacific Plate, which, being heavier, is sliding under the continental plate, causing all
kinds of geological mayhem: volcanoes, earthquakes, and mountain building. On the trailing edge, the Atlantic
