Gabbro

Gabbro is a dark, coarse-grained plutonic rock of calcium-rich plagioclase and pyroxene, the intrusive equivalent of basalt. It forms a major part of the lower oceanic crust that underlies the ocean basins.

🪨 Geology Guide Specimen: Open Gabbro in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.

Beneath mid-ocean ridges, basaltic magma stalls in chambers and crystallizes slowly, building thick gabbro layers under the sheeted dikes and pillow lavas above. Slices of ancient oceanic crust called ophiolites, as in Oman and Cyprus, are lifted onto continents and expose the whole sequence.

Gabbro is dominated by calcic plagioclase (labradorite to bytownite) and the pyroxene augite, sometimes with olivine, and it contains little or no quartz. Its plagioclase is more calcium-rich than the andesine of diorite, and rocks with orthopyroxene instead of augite are called norite.

Large gabbroic bodies such as the Skaergaard in Greenland, the Bushveld in South Africa and the Stillwater complex in Montana show repeating layers of different minerals, built as crystals settled and accumulated in huge magma chambers. Such intrusions can hold major deposits of chromium, nickel, copper and platinum-group metals.