Historical Geology Understanding Our Planet\'s Past

by following volcanic rocks for 400 miles across Idaho’s Snake River Plain.

During the last 2 million years, it was responsible for three major episodes of

volcanic activity in the vicinity of Yellowstone National Park in Wyoming,

which are counted among the greatest catastrophes of nature.

CENOZOIC MOUNTAIN BUILDING

The Cenozoic is known for its intense mountain building. The spurt in

mountain growth over the past 5 million years might have triggered the

Pleistocene ice ages. The Rocky Mountains (Fig. 181), extending from

Mexico to Canada, heaved upward during the Laramide orogeny from about

80 million to 40 million years ago. During the Miocene, beginning some 25

million years ago, a large portion of western North America uplifted. The

entire Rocky Mountain region rose about a mile above sea level. Great

blocks of granite soared high above the surrounding terrain. To the west in

the Basin and Range Province, the crust stretched and thinned out, in some

places dropping below sea level.

Arizona’s Grand Canyon (Figs. 182 and 183) lies at the southwest end of

the Colorado plateau, a generally mountain-free expanse that stretches from

Figure 181Granitic
dikes cutting through
horizontal sedimentary
rocks, west Spanish Peak,
Colorado.
(Photo by G.W. Stose,
courtesy USGS)

Historical Geology