Basin and Range

The Basin and Range Province is a vast geographic region in the west United States and northwest Mexico. Its topography has abrupt changes in height. It has narrow faulted mountain chains and flat arid valleys or basins. There are many ecoregions, but the lower part is mostly desert.
The physical geography (physiography) of the province was caused by extension and thinning of the lithosphere, which is composed of crust and upper mantle. What caused this thinning is still being investigated. The thinning began around 17 million years ago (mya) in early Miocene time.
The highest point fully within the Province is White Mountain Peak in California, while the lowest point is Badwater Basin in Death Valley. Other high points are at the boundary of the Basin and Range province.
The numerous ranges within the Province in the United States are collectively referred to as the Great Basin Ranges, although many are not actually in the Great Basin. The Great Basin is only the northern part of the greater Basin and Range region. The Great Basin is defined by its watershed (internal drainage).
Clarence Dutton compared the many narrow mountain ranges of the Basin and Range to an "army of caterpillars crawling northward", which is a helpful way to visualize the overall appearance of the region.
Volcanics.
Before the Eocene epoch (55.8 ±0.2 to 33.9 ±0.1 mya) the Farallon and North American plates were converging. During the Eocene the many volcanos in the Basin and Range Province flared up. The Farallon plate continued to be underthrust until about 19 mya. By then it was consumed and volcanic activity ceased.