EVSC 2800 Chapter Notes - Chapter 4: Volcanic Gas
12 May 2016
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Most rocks are solid aggregates of mineral grains, either mineral crystals or clasts (broken pieces) of mineral crystals and rocks. Exceptions: obsidian, rock made up of volcanic gas and coal, rock made up of plant fragments. Rock-forming materials come from earth"s mantle (magma), space (meteorites), organisms (parts of plants or animals), or the fragmentation and chemical decay of mineral crystals and other rocks. Igneous rocks formed when magma or lava cool to a solid form (either as glass or masses of tightly intergrown mineral crystals). The crystals are large if they had a long time to grow in a slowly cooling magma, and small if they formed quickly in rapidly cooling lava. Sedimentary rocks formed when mineral crystals and clasts (broken pieces, fragments) of plants, animals, mineral crystals, or rocks are compressed or cemented together. They also form when mineral crystals precipitate from water to form a rocky mass suck as rock salt or cave stalactites.
