The Cenozoic

Figure 1.12: (A) North America 50 million years ago—the entire west coast of North America is a subduction zone. (B) North America 30 million years ago—the East Pacific Rise is approaching the subduction zone. (C) North America 5 million years ago—by this time California is a transform boundary and the Basin and Range begins stretching.

The geography of the Western States became progressively more recognizable during the Cenozoic. Around 66 million years ago, the entire west coast of North America was a convergent boundary, where subduction created volcanic mountain ranges. Although some of the volcanic ranges from this time span remain active today, all of the earliest ranges have ceased to erupt, often being replaced by newer ranges farther seaward (see Figure 1.5). As more land accreted to the edge of the continent, subduction became increasingly more difficult, until it finally stopped, forming a new convergent boundary farther seaward. When this happened, the source of magma for the old volcanic arcs was cut off, and a new arc formed closer to the new boundary. This accretion caused the western edge of North America to expand outward, towards its present configuration (Figure 1.12).

A major change in the plate boundary along California also contributed to the extinction of the older Cenozoic volcanic arcs. Beginning about 30 million years ago, a mid-ocean ridge called the East Pacific Rise collided with the North American continent. As this ridge subducted, the plate boundary adjacent to California gradually changed, becoming a transform or sideways-moving boundary. As a result, subduction ceased, as did the last of the volcanic activity in what we now know as the Sierra Nevada. An important byproduct of this change in plate motion was that Nevada and southern Oregon began to pull apart, forming the collection of north-south trending mountains and valleys known as the Basin and Range.

Much of the present-day Sierra Nevada is a large exposure of granite called a batholith—the remains of the magma chambers that previously fueled volcanic mountain ranges that are exposed once the ancestral ranges have been eroded away. In the case of the Sierra Nevada, the stretching of the Basin and Range uplifted the solidified magma chambers of the ancestral range as rivers, landslides, and glaciers eroded away the overlying rock. The continued erosion of this rock is the source of much of the gold-laden gravel found in the rivers of California’s Central Valley.

The present-day Cascade Range, which extends all the way from southern British Columbia to northern California, is made up of a series of volcanoes that have built up a large platform of volcanic debris. These volcanoes, the Cascade Volcanic Arc, began to arise 36 million years ago due to the subduction of the Pacific plate beneath North America. However, the major volcanic peaks that make up what we call the High Cascades today formed more recently, within the Pleistocene. As long as subduction continues, the Cascades will remain volcanically active, although there is evidence that the rate of subduction is slowing, and as a result, volcanism in the Cascades will eventually cease.

Farther north, accretion and volcanism continued to add more land to Alaska’s coastline. As mountain ranges rose and eroded, sediment washed from the land, mixed with the remains of microscopic marine life, and settled onto the continental shelf fringing the state. As this organic material was buried by even more sediment, it was slowly converted into the large petroleum deposits on which much of Alaska’s economy now relies.

Fossils found in Alaska tell us that during parts of the Cenozoic, its climate was temperate, perhaps even subtropical. Although plate motion can account for some of this change—North America has been slowly moving north throughout the Cenozoic—other important factors also played a role in Alaska’s changing climate history. The majority of the Cenozoic was characterized by greenhouse conditions, during which sea levels are generally higher and glaciers diminish. During periods of changing global climate, polar areas tend to see a greater shift in climate than do areas close to the equator. During a shift to greenhouse conditions, tropical climate zones could have moved into areas that are now temperate or even subpolar.