Geologic Processes

The two most abundant igneous rocks in the Earth’s crust are of basaltic and granitic composition. There are intrusive and extrusive varieties of each: fine-grained extrusive basalt and coarse-grained intrusive gabbro; fine-grained extrusive rhyolite and coarse-grained intrusive granite. Basalts and gabbros have the same composition and differ only in grain size; the same is true for rhyolite and granite. The coarser grains in the intrusive rocks indicate slow cooling in large magma chambers with small surface/volume ratios. Rocks located at the Earth’s surface are subject to mechanical and chemical weathering. The rate of weathering is a function of the rock’s surface/volume ratio: under identical conditions, large rocks weather slowly, small rocks weather rapidly.