Gabbro is an intrusive igneous rock with a black or dark-green appearance. The term “Gabbro” was first used in the 1760s to name a newly discovered rock found in ophiolites of the Apennine Mountains in Italy. The rock was named after Gabbro, a hamlet near Rosignano Marittimo in Tuscany, Italy.
A famous German geologist, Christian Leopold von Buch, used the term to describe Italian ophiolitic rocks.
Gabbro comprises calcium-rich plagioclase feldspar (labradorite or bytownite) and pyroxenes (augite). Olivine may also be present in trace amounts. The mineral makeup in Gabbro is responsible for its black and dark-green color. Gabbro contains very little quartz compared to other igneous rocks.
Gabbro and basalt are similar in composition but have different grain sizes. Basalt is an intrusive igneous rate with quick cooling, and gabbro is intrusive with slow cooling. Basalt has fine-grained crystals, and gabbro has coarse-grained crystals.
Gabbros are divided based on mineral content. Gabbro rocks containing less than 35% mafic mineral content are known as leucogabbros, 35-65% mafic mineral content are called mesogabbros, and those with more than 65% mafic mineral content as melagabbros. Gabbros with more than 90% mafic mineral content and less than 10% are known as ultramafic rock and anorthosite, respectively.
Petrologists study detailed classification based on plagioclase, pyroxene, hornblende, and olivine percentages found in the rock. These 4 categories are explained below.
Normal gabbro: mainly plagioclase and clinopyroxene and less than 35% of hornblende, olivine, or orthopyroxene.
Norite: mainly plagioclase and orthopyroxene, less than 5% of hornblende, olivine, or orthopyroxene.
Troctolite: mainly plagioclase and olivine, and less than 5% of pyroxene or hornblende.
Hornblende gabbro: mainly plagioclase and hornblende, and less than 5% each of pyroxene or olivine.
Gabbroids are a family of coarse-grained igneous rocks that are similar to gabbro. These rocks contain trace amounts of iron-titanium oxides such as ilmenite, magnetite, and ulvospinel. Accessory minerals like apatite, zircon, and biotite may also be present in gabbroids.
Gabbro Rock Formation
Gabbro is formed by the cooling and crystallization of magma underneath the earth. Its cooling time is long; hence, it has larger crystals than rocks that grow in extrusive settings like basalt.
Where is Gabbro Found?
Gabbro is abundantly found in the deep oceanic crust. The top layer of the oceanic’s crust is made up of basalt, and the bottom layers are gabbro. Because gabbro takes more time to cool than basalt, it is often found deeper in the oceanic crust.
Gabbro is found in thick basaltic lava flows on the continents. Gabbro is also present in the deep plutons, formed when basaltic magma chambers crystallize.
Gabbro is found in large quantities in the following areas
- Bushveld complex, South Africa
- Muskox intrusion, Northwest Canada
- Rum layered intrusion, Scotland
- Stillwater complex, Montana
- Stavanger, Norway
- Zuma Rock, Nigeria
Gabbro is difficult to find, but available sources are polished to make cemetery markers, kitchen counters, floor tiles, facing stones, and as a dimension stone. Gabbro is used to make curbing, paving stones, and masonry. Many people refer to gabbro as black granite.
Some gabbro mines contain large amounts of rare metals like titanium, nickel, chromium, or platinum. The Merensky Reef is the world’s most valuable source of platinum.