where do most rocks have a basaltic composition
Basalt By definition, basalt is an aphanitic (fine-grained) igneous rock with generally 45-53% silica (SiO2)[2] and less than 10% feldspathoid by volume, and where at least 65% of the rock is feldspar in the form of plagioclase. This is as per definition of the International Union of Geological Sciences (IUGS) classification scheme.[3][4][5] It is the most common volcanic rock type on Earth, being a key component of oceanic crust as well as the principal volcanic rock in many mid-oceanic islands, including Iceland, the Faroe Islands, Réunion and the islands of Hawaiʻi. Basalt commonly features a very fine-grained or glassy matrix interspersed with visible mineral grains. The average density is 3.0 g/cm3.
Mineral There are over 5,300 known mineral species; as of March 2017[update], over 5,230 of these had been approved by the International Mineralogical Association (IMA).[2] The silicate minerals compose over 90% of the Earth's crust. Silicon and oxygen constitute approximately 75% of the Earth's crust, which translates directly into the predominance of silicate minerals.