Anorthosite is a light-colored igneous rock defined by its unique composition, consisting of 90% or more plagioclase feldspar. This rock is generally whitish-grey, though varieties containing the iridescent mineral labradorite can display striking blue or green flashes. Igneous rocks are classified based on where they cool and solidify, which dictates the final texture and raises the question of where this feldspar-rich rock forms.
Intrusive Versus Extrusive Rocks
Intrusive igneous rocks, also termed plutonic rocks, form when magma cools slowly beneath the Earth’s surface. The insulating effect of surrounding rock layers allows for a gradual cooling process over thousands or millions of years. This prolonged cooling provides ample opportunity for mineral crystals to grow large enough to be seen with the naked eye, resulting in a coarse-grained texture.
Extrusive igneous rocks, also known as volcanic rocks, form when molten material erupts onto the Earth’s surface as lava. Contact with the atmosphere or water causes very rapid cooling, which prevents the formation of large mineral grains. This results in a fine-grained, or aphanitic, texture, or sometimes a glassy texture with no crystalline structure at all.
Anorthosite as a Plutonic Rock
Anorthosite is classified as an intrusive or plutonic rock, forming deep within the Earth’s crust. This is confirmed by its characteristic coarse-grained texture, with plagioclase crystals often measuring several centimeters in length, indicative of slow cooling. The rock’s nearly monomineralic composition, dominated by plagioclase feldspar, is achieved through fractional crystallization within a large magma chamber.
During fractional crystallization, plagioclase crystals form early and float upward because they are less dense than the surrounding iron- and magnesium-rich magma. This buoyancy concentrates the plagioclase into a massive layer, which then solidifies slowly underground to form the anorthosite body. While an extrusive equivalent is theoretically possible, no significant lava flows of anorthositic composition have ever been observed on Earth. Therefore, naturally occurring deposits of anorthosite are recognized as exclusively plutonic.
Where Anorthosite is Found
The vast majority of anorthosite on Earth exists in large, ancient geological structures known as Massive Anorthosite Complexes (MACs). These batholith-sized intrusions are primarily found in the Precambrian shields of continental crust. These terrestrial deposits can cover vast areas, such as the 155,000 square kilometers found in eastern Canada.
Notable Terrestrial Locations
- Adirondack Mountains in New York
- Grenville Province in eastern Canada
- Norway
- Scandinavia
Anorthosite is the dominant rock type found in the Moon’s bright, heavily cratered highlands. Samples returned by the Apollo missions confirmed that the Moon’s ancient crust is largely composed of this feldspar-rich material. Lunar anorthosite is believed to have formed early when plagioclase crystals floated to the top of a global magma ocean. This rock type is also found on Mars and in certain meteorites, underscoring its role as a widespread, primordial component of rocky bodies in the solar system.