Chemical Weathering of Rocks
The chemical weathering of rocks is part of Earth’s carbon cycle. In chemical weathering, when carbon dioxide (CO2) in the atmosphere combines with rainwater, it forms a weak acid called carbonic acid. The slightly acidic rainwater dissolves the minerals in rocks, which may contain calcium, magnesium, and potassium. Rivers carry these minerals into the ocean. There, reactions produce calcium carbonate, a key building block of the shells of sea organisms. When these organisms die, they sink to the seafloor. Over millions of years, the layers of calcium carbonate from their shells turn into rock, storing the carbon and keeping it out of the atmosphere.
The reaction rates in chemical weathering depend on temperature. Because CO2 is a greenhouse gas, more atmospheric CO2 means that more heat is trapped, resulting in a higher global temperature. When the global temperature rises, the rate of chemical weathering increases—more CO2 is removed from the atmosphere and becomes “locked” in Earth’s surface. The storing of carbon in rocks reduces the amount of CO2 in the atmosphere, which reduces the warming from CO2.
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Conversely, if global temperatures decrease, reaction rates decrease, keeping more CO2 in the atmosphere, which increases the temperature. In this way, chemical weathering serves as Earth’s “natural thermostat” that helps control the planet’s global temperature. However, it acts only on very long time scales of hundreds of thousands to millions of years.
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