Not all biological weathering occurs visibly. Many microorganisms in the soil and on the surface of rock can contribute. Some bacteria derive nutrition by taking a combination of nitrogen from the air and minerals -- such as silica, phosphorous and calcium -- from rock. By removing these minerals, the rock is weakened and is further subject to other weathering forces such as wind and water. Lichens, symbiotic colonies of fungi and microscopic algae that grow on rock, also contribute to weathering. The fungi in a lichen produce chemicals that break down the minerals in the rock. The algae, like the bacteria, use these minerals for nutrition.
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