NaAlSi3O8S 9H2Ol 2H2CO3aq Al2Si2O5 OH 4s 2Na aq 2HCOaq 4H4SiO4qeqn 414

We might conclude that the dominant weathering mechanism of the upper crust is acid hydrolysis, resulting in a partially degraded and hydrated residue, with silicic acid and metal bicarbonate dissolved in water. We should, however, remember that much of the Earth's continental area is mantled by younger sedimentary rocks, including relatively soluble ones, for example limestone. Limestones are common in young mountain belts, such as the European Alps and the Himalayas, where rates of physical weathering are high. It is therefore probable that average weathering reactions are really biased toward the weathering of the sediment cover, rather than average continental crust. Studies of Alpine weathering seem to confirm this. Many Alpine streamwaters are low in dissolved sodium and H4SiO4 but high in calcium and HCO-. These results suggest that the dissolution of limestone—not feldspar—is the locally important weathering reaction.

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