Why would intrusive rocks chemically weather more quickly than extrusive rocks? Why would mafic rocks chemically weather more easily than felsic rocks?
What will be an ideal response?
Answer: There are several reasons that intrusive rocks would chemically weather more quickly than extrusive rocks. The place they form is the first reason: by definition, extrusive igneous rocks form at Earth's surface, which is also where weathering happens. For intrusive igneous rocks to experience weathering at Earth's surface, they must first go through an additional step: uplift sufficient to cause exposure. Second is the issue of grain size. Minerals weather from the outside in. Smaller mineral grains have a greater amount of surface per unit of volume, while relatively coarse crystals in plutons have less reactive surface area. Basaltic (mafic) rocks have a greater proportion of dark silicate minerals, which are enriched in the elements iron and magnesium, both of which are susceptible to oxidation. Felsic rocks have a higher proportion of quartz, which is stable at Earth surface conditions. The higher the temperature of formation of the minerals in the rock, the quicker that rock will weather and breakdown. If you look at Bowen's, the higher the temperature regime, the order is predicted in this sequence.
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