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Rocks and Minerals

How are rocks formed?




Your question is so broad it is difficult to begin. How do rocks form? Well, there are three classes of rocks: igneous, metamorphic, and sedimentary. Each of these rock types forms by distinctly different processes, and within each type there are subtypes that form by different, but related processes. This is a big topic in geology.

Igneous rocks are those that form as melts. The rocks get so hot, at great depth in the earth, that they melt. Then they can come to the surface as extrusive or volcanic rocks or stay within the crust and upper mantle as intrusive or plutonic rocks. Metamorphic rocks are rocks that are transformed (metamorphosed) from some other rock type, igneous, sedimentary, or metamorphic by heat and pressure and fluids, so that new crystals form and the rocks change texture. Sedimentary rocks are the rocks that form by deposition of rock particles that form from erosion of other rocks by flowing water, wind, and ice. Depending on the size of the rock particles the sediment that is deposited and hardens into rock can be a mudrock or shale (very, very fine grained) or fine to medium grained (sandstone) or very, very coarse (conglomerate).

If you want to introduce these concepts to your elementary school students, I suggest you check your local library for books on "Rocks and Minerals". There are some excellent geology books written for children and educators that give exercises on making synthetic "rocks" using things in the kitchen, or provide methods for collecting rocks around your school and classifying them and describing how they form.

Bob Burruss, United States Geological Survey

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    Last Modified: 2004-12-10