What is a Gabbro, and Why is It Important?

Gabbro is a coarse-grained, mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism.

Composition

Gabbro is typically composed of the following minerals:

• Plagioclase feldspar (60-70%)
• Pyroxene (20-30%)
• Olivine (10%)
• Other common minerals in gabbro include amphibole, biotite, and magnetite.

Texture

Gabbro is typically coarse-grained, meaning that the individual crystals are visible to the naked eye. This is because gabbro cools slowly deep beneath the Earth's surface. In contrast, basalt is a fine-grained igneous rock that cools quickly at the surface.

Gabbro specimen with its mineral description

Formation

Gabbro is formed when magnesium-rich and iron-rich magma cools slowly deep beneath the Earth's surface. This type of magma is typically found in the mantle, the layer of rock that lies beneath the Earth's crust. As the magma cools, the minerals crystallize out of solution. The first minerals to crystallize are olivine and pyroxene. These minerals are denser than the magma, so they sink to the bottom of the magma chamber. As more and more minerals crystallize, the magma becomes more and more viscous. Eventually, the magma becomes so viscous that it cannot flow. This is when gabbro is formed.

Occurrence

Gabbro is a common rock in the Earth's crust. It is found in both oceanic and continental crust. In oceanic crust, gabbro is found in the upper mantle and in the lower crust. In continental crust, gabbro is found in large plutons, which are bodies of igneous rock that are larger than 100 square kilometers. Gabbro is also found in smaller bodies of igneous rock, such as dikes and sills.

Economic importance

Gabbro is an important rock for a variety of economic uses. It is used as a source of building materials, such as crushed stone and concrete aggregate. It is also used as a source of metals, such as iron, nickel, and chromium. Gabbro is also used in the production of industrial minerals, such as asbestos and talc.

Gabbro and the Earth's interior

Gabbro is an important rock for understanding the Earth's interior. It is the most abundant rock in the Earth's oceanic crust, and it is also found in the upper mantle. This suggests that gabbro is a major component of the Earth's lower crust and upper mantle. Gabbro is also important for understanding the Earth's evolution. It is thought that gabbro formed in the early Earth, when the Earth was much hotter than it is today. As the Earth cooled, gabbro was replaced by other types of rocks, such as granite. However, gabbro is still found in many parts of the Earth, and it provides us with a window into the Earth's past.