Kamis, 18 Februari 2010

THE GEOLOGY OF ZION NATIONAL PARK
Zion national Park is a showcase of geology. geologic processes have played an important role in shaping Zion.The arid climate and sparse vegetation allow the exposure of large expenses of bare rock and reveal the park’s geologic history.Zion is located along the edge of a region called the Colorado plateau. The rock layers have been aplified, tilted, and croded, forming a led the Grand Staircase,a series of colorful cliffs stretching between Bryce Canyon and the Grand Canyon. The bottom layer of rock at Bryce Canyon is the top layer at Zion, and the bottom layer at Zion is the top layer at the Grand Canyon.
Sedimentation
Zion was a relatively flat basin near sea level 240 milion years ago. as sands, gravels, and mud eroded from surrounding mountains, streams,caried these material into basin and deposited them in layers. The sheer weight of these accumulated layers caused the basin to sink, so that the top surface always remained near sea level. As the land roseand fell and as the climated change, the depositional environment fluctuated from shallow seas to coastal plains to a desert of the massive windblown sand. This process of sedimentation continued until over 10,000 feet of material accumulated.
Lithification
Mineral-laden waters slowly filtered trough the compacted sediments. Iron oxide, calcium carbonate and silica acted as cementing agents, ad with pressure from overlying layers over long periods of time, transformed the deposites into stone. Ancient seabeds became limestone; mud and clay became mudstone and shale; and desert sand became sandstone. Each layer originated from a distinct source and so differs in thickness, mineral content, color, and eroded appearance.
Uplift
In area from Zion to the Rocky Mountains, forces deep within the earth started to push the surface up. This was not chaotic uplift. but very slow vertical hoisting of huge blocks of the crust. Zion’s elevation rose from near sea level to as high as 10,000 feet above sea level. Uplift is still occuring. In 1992 a magnitude 5.8 earthquake caused a landslide visible just outside the south entrance of the park in Springdale.
Erosion
This uplift gave the streams greater cutting force in their descent to the sea. Zion’s location on the western edge of this uplift caused the streams to tumble off the plateau, flowing rapidly down a steep gradient. a fast-moving stream carries more sediment and larger boulders than a slow-moving river. These streams began eroding and cutting into rock layer, forming deep and narrow canyons. Since the uplift began, the North Fork of the Virgin river has carried away several thousand feet of rock that once lay above the highest layers visible today. The Virgin river is still excavating. Upstream from the Temple of Sinawava the river cuts through Navajo Sandstone, creating a slot canyon. At the temple, the river has reached the softer Kayenta Formation below. Water erodes the shale, undermining the overlaying sandstone and causing it to colapse, widening the canyon.
Geology in Action
A landslide once dammed the Virgin River forming a lake. Sediments settled out of the quite waters, covering the lake bottom. When the river breached the dam and the lake drained, it left behind a flat-bottomed valley. This change in the character of vthe canyon can be seen from the scenic drive south of the Zion Lodge near the Sentinel Slide. This slide was active again in 1995, damaging the road. Flash floods occur when sudden thunderstorms dump water on exposed rock. With little soil to absorb the rain, water runs down-hill, gathering volume as it goes. These floods often occur without warning and can increase water flow by ove 100 times. In 1998 a flash flood increased the volume of the Virgin River from 200 cubic feet per second to 4.500 cubic feet per second, again damaging the scenic drive at the Sentinel Slide.

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