LATERAL SPREADS: Lateral spreads are distinctive because they usually occur on very gentle slopes or flat terrain (fig. 3J). The dominant mode of movement is lateral extension accompanied by shear or tensile fractures. The failure is caused by liquefaction, the process whereby saturated, loose, cohesionless sediments (usually sands and silts) are transformed from a solid into a liquefied state. Failure is usually triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced. When coherent material, either bedrock or soil, rests on materials that liquefy, the upper units may undergo fracturing and extension and may then subside, translate, rotate, disintegrate, or liquefy and flow. Lateral spreading in fine-grained materials on shallow slopes is usually progressive. The failure starts suddenly in a small area and spreads rapidly. Often the initial failure is a slump, but in some materials movement occurs for no apparent reason. Combination of two or more of the above types is known as a complex landslide.
Giant Sand Valley Of Rain Rapidshare Downloads
Hoodoos typically form in areas where a thick layer of a relatively soft rock, such as mudstone, poorly cemented sandstone, or tuff (consolidated volcanic ash), is covered by a thin layer of hard rock, such as well-cemented sandstone, limestone, or basalt. In glaciated mountainous valleys the soft eroded material may be glacial till with the protective capstones being large boulders in the till. Over time, cracks in the resistant layer allow the much softer rock beneath to be eroded and washed away. Hoodoos form where a small cap of the resistant layer remains, and protects a cone of the underlying softer layer from erosion. The heavy cap pressing downward gives the pedestal of the hoodoo its strength to resist erosion.[12] With time, erosion of the soft layer causes the cap to be undercut, eventually falling off, and the remaining cone is then quickly eroded.[13][14]
Millions of years ago fissures in the earth belched forth massive volumes of basalt, a dark, fine grained volcanic rock. In fact, the Pacific Northwest is home to one of the largest basalt provinces anywhere on earth, with basalt covering much of eastern Washington, northern Oregon, and western Idaho. This basalt can be thousands of feet thick, with a weight so heavy that some areas of the Columbia Basin are actually below sea level, creating a low desert environment. 2ff7e9595c
Comments