Aerial view of linear valleys and mountain ranges that characterize the Basin and Range Province. Photo by Marli Miller
Within the Basin and Range Province, the Earth's crust (and upper mantle) has been stretched up to 100% of its original width. The entire region has been subjected to extension that thinned and cracked the crust as it was pulled apart, creating large faults. Along these roughly north-south-trending faults mountains were uplifted and valleys down-dropped, producing the distinctive alternating pattern of linear mountain ranges and valleys of the Basin and Range province.
Although there are other types of faults in the Basin and Range province, the extension and crustal stretching that have shaped the present landscape produce mostly normal faults. The upthrown side of these faults form mountains that rise abruptly and steeply, and the down-dropped side creates low valleys. The fault plane, along which the two sides of the fault move, extends deep in the crust, usually an angle of 60 degrees. In places, the relief or vertical difference between the two sides is as much as 10,000 feet.
Hanaupah fault scarp, Death Valley National Park. This fault cuts like a knife across this alluvial fan, uplifting the mountain range relative to the valley floor. Many thousands of small uplifts and downdrops along faults like this are needed to create the basins and ranges characteristic of this province. Photo by Marli Miller
As the rocky ranges rise, they are immediately subject to weathering and erosion. The exposed bedrock is attacked by water, ice, wind and other erosional agents. Rock particles are stripped away and wash down the mountain sides, often covering young faults until they rupture again. Sediment collects in the adjacent valleys, in some places burying the bedrock under thousands of feet of rock debris.
The Basin and Range province's dynamic fault history has profoundly affected the region's water drainage system. Most precipitation in the Great Basin falls in the form of snow that melts in the spring. Rain that reaches the ground, or snow that melts, quickly evaporates in the dry desert environment. Some of the water that does not evaporate sinks into the ground to become ground water. The remaining water flows into streams and collects in short-lived lakes called playas on the valley floor and eventually evaporates. Any water that falls as rain or snow into this region does not escape out of it; not one of the streams that originate within this basin ever find an outlet to the ocean. The extent of internal drainage, the area in which surface water cannot reach the ocean, defines the geographic region we call the Great Basin.
The Great Basin's internal drainage results from blockage of water movement by high fault-created mountains and by lack of sufficient water flow to merge with larger drainages outside of the Great Basin. This internally-drained area occupies approximately 200,000 square miles, including most of Nevada, a large part of Utah, and portions of Idaho, California, and Oregon. Much of the present-day Great Basin would drain to the sea - just as it did in the recent Ice Ages - if there were more rain and snowfall. Great Basin - the geographical and hydrological region comprised of most of Nevada, southern Oregon and Idaho, western Utah, and a little of eastern California. Characterized by internal drainage, this region's surface water sources evaporate or percolate before the can flow to the ocean.