LAURASIA
During the Silurian, all northern continents collided to form Laurasia, which
included the interior of North America, Greenland, and Northern Europe.The
ancestral North American continent called Laurentia assembled from several
microcontinents that collided beginning about 1.8 billion years ago. Most of the
continent evolved in a relatively brief period of only 150 million years.
A major part of the continental crust underlying the United States from
Arizona to the Great Lakes to Alabama formed in one great surge of crustal
generation that has no known equal. This was possibly the most energetic
period of tectonic activity and crustal generation in Earth history, when more
than 80 percent of all continental mass was created.The best exposure of these
Precambrian metamorphic rocks is the Vishnu Schist on the floor of the
Grand Canyon (Fig. 85).
Laurentia was stable enough to resist another billion years of jostling and
rifting. It continued to grow by plastering bits and pieces of continents and
island arcs to its edges. The presence of large amounts of volcanic rock near
the eastern edge of Laurentia implies the continent was once the core of a
larger supercontinent.The central portion of the supercontinent was far
removed from the cooling effects of subducting plates, where Earth’s crust
sinks into the mantle.As a result, the interior of the supercontinent heated and
erupted with volcanism.Figure 85Precambrian
Vishnu Schist,Grand
Canyon National Park,
Arizona.
(Photo by R. M.Turner,
courtesy USGS)
Historical Geology