The Devil's Marbles in Australia's Northern Territory. The surface of these boulders shows signs of weathering. It is caused by...Read More >>The Devil's Marbles in Australia's Northern Territory. The surface of these boulders shows signs of weathering. It is caused by water seeping through cracks in the rock. Rocks may appear to be solid and unchanging, but they are being slowly worn away. They are always under “attack” by the weather. Rainwater, frost or even just changes in the temperature can all break up rocks. This is called weathering. Rocks may also be ground down by erosion. This is the removal of rock fragments by water, ice or wind.
When water gets into cracks in rock (upper diagram) and freezes (lower diagram), its force of expansion is powerful enough to...Read More >>When water gets into cracks in rock (upper diagram) and freezes (lower diagram), its force of expansion is powerful enough to break up the rock into fragments.
How does the weather break down rocks? Rocks heat up and expand in the sunshine during the day. At night, they cool down and contract (get smaller). This expansion and contraction, repeated day after day, causes the rock’s surface to crack. Pieces flake off and are blown or washed away. Over many years, the rock disappears. Hard rocks take longer to break down than softer ones.
Rainwater seeps into cracks in the rock. If it later freezes, it expands with great force and splits off pieces of rock. On slopes or cliffs, rockfalls or slides are often the result. Piles of fallen rocks, called scree, build up at the bottom.
Sandstone cliffs created by weathering and wind erosion in the Sahara Desert
Deforestation (cutting down forest trees) and overgrazing (allowing cattle to graze the land for too long) can speed up the forces of erosion and strip the land of soils.
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