Figure 5. Close-up view of one of the dark-colored dikes where it is not covered by the brown weathering rind. It is a very fine-grained igneous rock, almost microscopic, which contains some larger mineral grains. This texture of two dominant grain sizes, is called porphyritic. A porphyritic texture may be produced by a two-stage crystallization process, in which the larger crystals began to form first, at a deeper level in the earth, and then the rest of the rock crystallized rapidly into very fine crystals at the time the dike was emplaced into colder rocks.

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Last updated on October 4, 2012

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