continents collided and closed an ancient ocean. Arcs of volcanic rock also
weave through central and eastern Canada down into the Dakotas.
A region between Canada’s Great Bear Lake and the Beaufort Sea holds
the roots of an ancient mountain range that formed by the collision of North
America and an unknown landmass between 1.2 and 0.9 billion years ago. A
continental collision possibly with South America created the Grenville
orogeny between 1.3 and 0.9 billion years ago, which formed a 3,000-mile-
long mountain belt in eastern North America from Texas to Labrador. Mean-
while, continental collisions continued to add a large area of new crust to the
growing proto–North American continent.
Most of the continental crust underlying the United States from Arizona
to the Great Lakes to Alabama formed in one great surge of crustal genera-
tion unequaled in North America since. The rapid buildup of new crust
resulted from possibly the most energetic period of tectonic activity than any
subsequent time in Earth history. The assembled North American continent
was stable enough to resist a billion years of jostling and rifting. It continued
to grow by adding bits and pieces of continents and island arcs to its edges.
Large masses of volcanic rock near the eastern margin of North Amer-
ica imply that the continent was the core of a larger supercontinent called
Rodinia formed during the late Proterozoic. The interior of this landmassFigure 43The
proto–North American
continent continued to
grow by adding
continental fragments to
its landmass.
Historical Geology
MazatzalGrenvilleSlave
H e a r nH e a r nWyomingCordilleraSuperiorMazatzalGrenvilleAppalachiansOuachitasTransHudson
TransHudsonNainRacklanRacklanRacklanWopmayWopmay