What are the three main classes of rock?
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Of the three main rock groups, igneous rocks reside deepest within the earth. These rocks require magma, a catalyst for growth. Magma produces igneous rocks as it cools and hardens. Common rocks in this category include granite, basalt and obsidian. read more
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Sedimentary rocks are called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are three main types of sedimentary rocks: Clastic: your basic sedimentary rock. read more
Source: jersey.uoregon.edu
