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Movement Of Rock Debris:
The flow of water in rivers is turbulent, meaning that water particles move in more or less erratic fashion; although the general path is downstream, a particle may move forward, backward, and sideways. Turbulence is of great importance because it makes possible erosion and movement of rock debris. Such debris (gravel, sand, mud) is called sediment.The movement of the earth's crust may carry the rock as much as 700km (454 miles) below the surface. Here the temperature and pressure will be even higher and the rock will begin to melt. Molten rock is lighter than solid rock and it will begin to rise up through the overlying rock towards the surface. If it reaches the surface as a lava flow it will immediately be ready for weathering and erosion and the start of a new cycle. More often the molten rock solidifies underground and then all the rock above it must be eroded away before it can begin the cycle again.
Mountains are sculptured and destroyed by the climatic forces of frost, water (in the form of snow, ice and rain) and wind. Frost may shatter and break up rocks to form screes (masses of debris at a cliff base) and snow and glaciers gouge out rock debris and transport it down the mountainside, leaving the debris as an elongated moraine at the tip of the glacier. Lower down, rivers cut into the mountainside and form zigzag valleys with interlocking spurs. These spurs may in turn be sliced off by glaciers making their way to lower levels down the mountain. In short the erosion of mountains is the continuing story of the breakup of rocks and their gradual descent under the influence of gravity.
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