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.
Last updated on October 4, 2012