Question: What are the 3 main kinds of rocks?
The 3 kinds of rocks are igneous, sedimentary, and metamorphic.
Igneous rocks are either glassy, fine grained, coarse grained, or porphyritic. Volcanic rocks sometimes have holes in them from escaping gases (called vesicles). Sometimes the holes are filled with minerals, and then the rock is amygdaloidal. Rocks that have vesicles are usually fine grained like basalt. Volcanic rocks are fine grained, glassy, or porphyritic. Glassy texture is easy. The rock is usually black obsidian. Most volcanic lavas are fine grained or porphyritic. Porphyritic texture is the defining texture of volcanic rocks.
Porphyritic texture is when you have well formed visible crystals imbedded in a fine-grained groundmass. It is a result of a 2-stage cooling history. Deep below the surface, the molten rock can start to crystallize slowly so that some nice well-formed crystals grow. Then the molten rock and crystals are expelled from the volcano, and everything crystallizes.
The well-formed crystals are frozen or imbedded in the quickly cooled fine-grained rock. To identify a porphyritic volcanic rock you have to determine the composition of the bigger crystals. If they are quartz, then you probably have a rhyolite. If they are plagioclase, then the rock is an andesite or basalt.
Igneous rocks that consist of large and well-formed interlocking crystals are called intrusive or plutonic rocks. The crystalline interlocking texture helps to identify them as igneous...having formed slowly below the surface of the earth. Granite and gabbro are examples of this kind of rock. In the case of the granite the minerals quartz and orthoclase feldspar are easy to see. The quartz is gray and lumpy and the orthoclase is usually pink and has good cleavage surfaces.
Several things help in the identification of sedimentary rocks. They are the only ones that...