Rocks are typically composed of an assortment of minerals, amorphous materials, or clasts. Iceland is composed mainly of igneous rock. Sedimentary rock accounts for only 8-10% of Iceland’s volume, and there is no true metamorphic rock on the island. Igneous rock is classified on the basis of its chemical and mineral composition. Igneous rock can be extrusive (volcanic), formed when magma is erupted onto the surface and solidifies there, or intrusive (plutonic), formed when magma solidifies below the surface. Extrusive volcanic rocks predominate in the exposed bedrock of Iceland.
A total of 25 types of igneous rock have been found in Iceland, the most common of which are tholeiite, olivine tholeiite, gabbro, and rhyolite. The main rock-forming minerals in tholeiite, olivine tholeiite, and gabbro are plagioclase, augite, olivine, magnetite, and apatite. Icelandic igneous rock forms three rock series: a tholeiitic series, an alkalic series, and a transitional alkalic series.