Physical weathering refers to the mechanical processes that break down rocks without altering their chemical composition. Based on this definition, the statements that describe physical weathering are:
- Two rocks hit against each other in a fast flowing stream and break apart.
The other statements describe processes that involve chemical weathering or biological weathering:
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Smal rocks are pushed together when a mole digs an underground den. (This could involve physical weathering, but it is not as direct an example as the statement about rocks in a stream.)
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Mosses grow on a rock and produce acids that wear away the rock over time. (This is a form of biological weathering, which involves chemical processes due to the acids produced by the moss.)
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Oxygen and water change the composition of the minerals in a rock. (This describes chemical weathering, not physical weathering.)
Thus, the only statement that clearly describes physical weathering is:
- Two rocks hit against each other in a fast flowing stream and break apart.