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Rocks are natural materials made from minerals, fragments, or other geological constituents. They form much of the solid ground beneath soil, water, and vegetation. A rock can look uniform at first glance while containing grains, layers, or structures that reveal a more complicated history. Differences in these features help explain why rocks respond differently to water, weathering, pressure, and the passage of time. One broad way of describing rocks is by how they formed. Igneous rocks develop from cooling molten material. Sedimentary rocks form through the accumulation and later alteration of material deposited at the surface or in water. Metamorphic rocks [...]