Here's why:
* Chemical weathering involves chemical reactions that break down the minerals within a rock. This can happen through:
* Dissolution: Water, especially acidic rainwater, can dissolve certain minerals like calcite (found in limestone).
* Oxidation: Iron in minerals reacts with oxygen, creating iron oxides (like rust) and changing the mineral structure.
* Hydrolysis: Water reacts with minerals, breaking them down into different compounds.
* Biological weathering: Organisms, like lichens and bacteria, can release acids that break down minerals.
Physical weathering, on the other hand, breaks rocks down into smaller pieces without changing their chemical composition. Examples include:
* Frost wedging: Water freezes in cracks, expands, and breaks the rock apart.
* Abrasion: Rocks are rubbed against each other, wearing them down.
* Thermal expansion and contraction: Temperature changes cause rocks to expand and contract, leading to fracturing.
So, while both physical and chemical weathering can change the appearance of a rock, only chemical weathering actually alters the mineral composition within the rock.
