Rock resistance to weathering

Rocks react differently to weathering due to the differences in mineral content and structure. Some minerals are unstable under surface conditions and are readily soluble. Others are stable and resist the agents of weathering. Some rock-forming minerals are physically soft, being easily crushed and split while harder minerals are less easily broken apart. The arrangement and size of mineral grains control weathering processes. Water has a difficult time penetrating intricately locked and closely spaced mineral grains to promote weathering. Larger, loosely cemented minerals disintegrate and decompose more readily. Minerals in the form of poorly joined sheets readily break apart.

Figure 17. 4 Granite, a predominate rock type that composes the continental land masses.

Granite is a coarse grained rock composed of quartz  and feldspar. Both quartz and feldspar are hard minerals, but feldspar is less stable under surface conditions than quartz. The feldspar readily weathers to become clay in humid conditions. The feldspar will weather in dry climates as the somewhat porous granite allows moisture to penetrate. As the feldspar decomposes it weakens the bonds holding the rock together and it disintegrates

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