Basin and Range Topography Overview

Basin and Range Topography Overview In geology, a basin is defined as a bounded area where the rock within the boundaries dips inward toward the center. By contrast, a range is a single line of mountains or hills forming a connected chain of land higher than the surrounding area. When combined, the two make up basin and range topography. A landscape comprised of basins and ranges is characterized as having a series of undulating mountain ranges sitting parallel to low, broad valleys (basins). Normally, each of these valleys is bounded on one or more sides by mountains and although the basins are relatively flat, the mountains can either rise abruptly out of them or slope upward gradually. The differences in elevations from the valley floors to the mountain peaks in most basin and range areas can range from several hundred feet to over 6,000 feet (1,828 meters). Causes of Basin and Range Topography The resulting faults are called normal faults and are characterized by rocks dropping down on one side and rising on the other. In these faults, there is a hanging wall and a footwall and the hanging wall is responsible for pushing down on the footwall. In basins and ranges, the hanging wall of the fault is what creates the range as they are the blocks of the Earths crust that are pushed upward during crustal extension. This upward movement occurs as the crust spreads apart. This portion of the rock is located on the margins of the fault line and moves up when the rock being moved in the extension gathers on the fault line. In geology, these ranges forming along fault lines are called horsts. Conversely, the rock below the fault line is down dropped because there is a space created by the divergence of lithospheric plates. As the crust continues to move, it stretches and becomes thinner, creating more faults and areas for rocks to drop into gaps. The results are the basins (also called grabens in geology) found in basin and range systems. One common feature to note in the worlds basins and ranges is the extreme amount of erosion that occurs on the peaks of the ranges. As they rise, they are immediately subject to weathering and erosion. The rocks are eroded by water, ice, and wind and particles are quickly stripped and washed down the mountainsides. This eroded material then fills the faults and collects as sediment in the valleys. The Basin and Range Province Within the Basin and Range Province, the relief is abrupt and the basins normally range from 4,000 to 5,000 feet (1,200- 1,500 m), while most of the mountain ranges climb 3,000 to 5,000 feet (900-1,500 m) above the basins. Death Valley, California is the lowest of the basins with its lowest elevation of -282 feet (-86 m). Conversely, Telescope Peak in the Panamint Range to the west of Death Valley has an elevation of 11,050 feet (3,368 m), showing the enormous topographic prominence within the province. In terms of the Basin and Range Provinces physiography, it features a dry climate with very few streams and internal drainage (a result of the basins). Although the area is arid, much of the rain that does fall accumulates in the lowest basins and forms pluvial lakes such as the Great Salt Lake in Utah and Pyramid Lake in Nevada. The valleys are mostly arid however and deserts such as the Sonoran dominate the region. This area also affected a significant portion of the United States’ history as it was a major barrier to westward migration because the combination of desert valleys, bounded by mountain ranges made any movement in the area difficult. Today, U.S. Highway 50 crosses the region and crosses five passes over 6,000 feet (1,900 m) and is considered The Loneliest Road in America. Worldwide Basin and Range Systems Western Turkey is also cut by an easterly trending basin and range landscape that extends into the Aegean Sea. It is also believed that many of the islands in that sea are portions of ranges between basins that have a high enough elevation to break the sea’s surface. Where ever basins and ranges occur, they represent an enormous amount of geologic history as it takes millions of years to form to the extent of those found in the Basin and Range Province.