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USGS CMG InfoBank: Water and Moisture

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Comment: 06:42 - 08:06 (01:24)
Source: Annenberg/CPB Resources - Earth Revealed - 15. Weathering and Soils
Keywords: "chemical weathering", weathering, water, moisture, "chemical decomposition", mineral, rainfall, "carbon dioxide", atmosphere, "carbonic acid", vegetation, soil, limestone, "rock composition", iron, oxygen, "iron oxide", limonite, geothite, hematite, rust

Our transcription: The principal agents of chemical weathering are water in the ground and moisture in the air.
Usually these solutions are weakly acidic, and are capable of causing the slow chemical decomposition of most rock-making minerals.
As rain falls, for example, it dissolves small quantities of carbon dioxide in the atmosphere producing weak carbonic acid.
As this solution percolates through decaying vegetation in soil, it picks up more acids and thus becomes more effective in decomposing underlying rocks, especially limestone.
This time lapse sequence shows how rainfall gradually dissolves limestone.
Thus, unlike mechanical weathering, chemical weathering actually destroys or changes the composition of the rock.
The rusting of metal is a good example.
The red rust on this nail is a different substance from the metallic iron at its core.
One substance has changed into another.
The iron of the nail has combined with oxygen from the atmosphere to form rust, iron oxides, the minerals, limonite and geothite or hematite.

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