Gabbro

Gabbro is a coarse-grained intrusive igneous rock composed mainly of plagioclase feldspar and pyroxene. It forms when mafic magma cools slowly beneath Earth’s surface.

Plagioclase: A group of feldspar minerals forming a continuous series from sodium-rich albite to calcium-rich anorthite. Plagioclase is typically gabbro’s dominant light-colored mineral.
Magma: Molten or partially molten rock beneath Earth’s surface, containing dissolved gases and crystals. Gabbro crystallizes from mafic magma trapped below the surface.
Oceanic crust: The basaltic, gabbroic outer layer of Earth beneath the oceans. Gabbro makes up much of the lower part of normal oceanic crust.
Bushveld Complex: A vast layered igneous complex in South Africa containing major deposits of platinum-group metals and chromite. Its layered mafic rocks include extensive gabbroic units.
Pyroxene: A group of chain-silicate minerals common in igneous and metamorphic rocks. Pyroxene supplies much of gabbro’s dark color and mafic mineral content.
Intrusive rock: Igneous rock formed when magma solidifies beneath Earth’s surface. Gabbro is an intrusive rock, cooled slowly underground.
Mid-ocean ridge: A submarine mountain chain where tectonic plates diverge and new oceanic crust forms. Ridge magmatism produces gabbro beneath newly formed basaltic crust.
Stillwater Complex: A layered mafic intrusion in Montana, United States, known for chromium and platinum-group metal deposits. Its gabbroic layers illustrate crystal accumulation in a large magma chamber.
Mafic rock: An igneous rock rich in magnesium and iron and relatively low in silica. Gabbro belongs to this compositional class.
Fractional crystallization: The chemical evolution of magma as early-forming crystals separate from the remaining melt. Crystal separation can produce gabbroic rocks and more evolved residual magmas.

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