The composition of igneous rocks falls into four main categories. They are determined by the amount of silica that the rocks contain. The four categories are acidic, intermediate, basic, and ultramafic. Acidic rocks have a high silica content (65% or more) along with a relatively high amount of sodium and potassium. These rocks are composed of the minerals quartz and feldspar. Rhyolite and granite are the two most common types of acidic rock. Intermediate rocks contain between 53% and 65% silica. They also contain potassium and plagioclase feldspar with a small amount of quartz. Diorite and Andesite are the two most common types of intermediate rock. Click on the "Next" button to continue!

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