North America

The tectonic story begun on the western border of North America around 340 million years ago continues in the present day. Land masses created far away from North America began to collide with the continent. Off the western coast, the tectonic forces began moving in a new direction, and a long quiet interval came to an end. These are some of the phenomena that resulted:

Around 340 million years ago, an offshore island arc, called the Antler Arc, struck the shores about where Nevada and Idaho now are (then the westernmost part of the continent), extending the shoreline of North America a hundred miles westward.

By 245 million years ago, the beginning of the age of dinosaurs, another island arc had run into the American west. The Golconda Arc added a Sumatra-sized piece of land to North America, and the continent bulged out to present-day northern California.

After the Golconda Arc piled onto the West Coast of that time, the crust broke beneath the continent's border, and the ocean's plate ran under North America's west coast like a speeding low-slung sport coupe might run under the rear bumper of a tractor-trailer. A continental arc was born around 230 million years ago in western North America, and its volcanos have been erupting frequently from the dawn of the age of dinosaurs (the Mesozoic era) until today.

Several more island arcs struck western North America since the middle Jurassic period. The granite mountains of the Sierra Nevada are the roots of one of these island arcs. Landmasses created on the Pacific Plate have been scraped off it like the roof of the sports coupe mentioned earlier would be scraped off as it crammed itself under the rear bumper of a tractor-trailer. This mechanism is the origin of the west coast's ranges, the Cascades, and much of British Columbia and Alaska's southern coast.

A range of fault block mountains rose far inland as the continent was squeezed from west to east. The Sevier mountains stood west of the Cretaceous period's interior seaway, in what is now Montana, Idaho, Nevada, and Utah. The dinosaurs of that time (80-130 million years ago) left their tracks and remains in the mud and sand worn off these mountains.

In the same manner as large island chains were carried to North America on moving plates of oceanic crust, small pieces of land came to the coasts in this way as well. Numerous "exotic terrains," impacting on the western coasts during the Mesozoic and Cenozoic eras, added large areas now covered by British Columbia, Washington, Oregon, California, and Mexico. These little rafts of continental crust were formed far from their present location, for the fossils in them are of creatures that lived halfway around the world-but never in North America. A sizeable piece of continental crust-southern Mexico as far south as the Isthmus of Tehuantepec-joined northern Mexico between 180-140 million years ago.