Gabbro: Composition, Types, Properties, Uses

Gabbro is a coarse-grained, dark-colored intrusive igneous rock that forms from the slow cooling and crystallization of mafic magma beneath the Earth's surface. It is composed primarily of calcium-rich plagioclase feldspar and pyroxene minerals (such as augite), with smaller amounts of olivine and amphibole. Gabbro is the intrusive equivalent of basalt, which is its extrusive counterpart formed from the same type of magma but cooled rapidly at the surface.

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. Chemically, gabbro is classified as a mafic rock. This means it's relatively low in silica (45-55%) and rich in iron, magnesium, and calcium. This composition contributes to its dark color and high density.

The term "gabbro" originates from the Italian town of Gabbro, near Rosignano Marittimo, Tuscany. In the 18th century, German geologist Abraham Gottlob Werner described a dark-colored, coarse-grained rock found in the area and named it "Gabbro verde" (green gabbro) due to its greenish-gray color. This name eventually shortened to simply "gabbro" and became widely adopted for similar rocks elsewhere.

Gabbro Key Features

Category: Intrusive igneous rock
Chemical Composition: Mafic
Color: Dark gray to black, sometimes lighter gray or greenish.
Texture: Coarse-grained, with crystals visible to the naked eye.
Hardness: Hard and strong, can scratch glass.
Mineral composition: Primarily plagioclase feldspar, pyroxene, and olivine.

Gabbro Composition

Chemical Composition

Silica: Gabbro has a relatively low silica content (45-55%) compared to other igneous rocks like granite. This makes it a "mafic" rock, meaning it is rich in iron, magnesium, and calcium.

Major oxides: After silica, the most abundant oxides in gabbro are typically aluminum oxide (Al₂O₃), iron oxide (FeO), magnesium oxide (MgO), and calcium oxide (CaO).

Trace elements: Gabbro also contains small amounts of various trace elements, such as chromium, nickel, and titanium.

Mineral Composition

Gabbro is typically composed of the following minerals:

Primary Mineralogy:

Plagioclase feldspar: 50-65% - This is the most abundant mineral in gabbro, typically ranging from labradorite to bytownite (calcium-rich varieties). It often appears as lighter-colored grains in the rock.

Pyroxene: 25-40% - Clinopyroxene, particularly augite, is the most common type. Orthopyroxene like hypersthene might be present in smaller amounts (0-10%) in some gabbros. Pyroxene contributes to the dark color and coarse-grained texture of the rock.

Secondary Mineralogy:

Olivine: When present in significant quantities (>10%), olivine can lead to the classification of "olivine gabbro."

Hornblende: Though less common, hornblende can contribute to the rock's composition, particularly as rims around pyroxene crystals or as poikilitic grains enclosing other minerals.

Accessory Minerals: Trace amounts of minerals like magnetite, apatite, and zircon may be present.

Gabbro Texture

Gabbro is typically coarse-grained (phaneritic), 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 composition.

How is Gabbro Formed

Gabbro is formed when magnesium-rich and iron-rich magma cools slowly deep beneath the Earth's surface. The formation process of gabbro involves several key steps:

Magma Generation: Gabbro typically forms from mafic (magnesium- and iron-rich) magma, which is often generated in the Earth's mantle. This magma can be derived from partial melting of the mantle, often in tectonic settings such as mid-ocean ridges, subduction zones, or continental rifts.

Magma Ascent and Intrusion: The mafic magma ascends through the Earth's crust due to its lower density compared to the surrounding solid rock. As it rises, it can accumulate in magma chambers at various depths. If the magma cools slowly in these chambers, it can form gabbro.

Cooling and Crystallization: The slow cooling of the magma allows for the formation of large, well-formed mineral crystals. The primary minerals in gabbro are plagioclase feldspar and pyroxene, with smaller amounts of olivine, amphibole, and other accessory minerals. The slow cooling rate is crucial for the development of these large crystals, which is a characteristic feature of gabbro.

Textural Development: The texture of gabbro is typically coarse-grained, with visible crystals. The slow cooling also allows for the segregation of different minerals, which can lead to the formation of distinct layers or zones within the rock.

Solidification: As the magma continues to cool and solidify, the remaining liquid magma may become enriched in certain elements, leading to the formation of more complex mineral assemblages. This process can result in a variety of gabbroic rocks with different compositions and textures.

Geological Settings: Gabbro is commonly found in:

Oceanic crust (as part of the lower crust).
Layered intrusions (e.g., the Bushveld Complex in South Africa).
Continental rift zones.

It is often associated with other mafic like basalt (its extrusive equivalent) and ultramafic rocks like peridotite.

Types of Gabbro

Troctolite: Distinguished by its high olivine content (usually exceeding 50%). This gives the rock a greenish-yellow hue and makes it an ultramafic type, meaning it has an even lower silica content than typical gabbro.

Norite: Characterized by the predominance of orthopyroxene (exceeding 50%). It often has a slightly lighter color than gabbro due to the presence of pale green or brown orthopyroxene crystals.

Leucogabbro: Rich in plagioclase feldspar (plagioclase content > 70%). Light-colored, coarse-grained, with minor pyroxene and olivine.

Mesogabbro: The most common type, with 50-65% plagioclase feldspar and varying proportions of pyroxene and olivine. Its color ranges from light to dark gray, and the texture is typically coarse-grained.

Melagabbro: Rich in pyroxene (pyroxene content > 70%). Dark-colored, coarse-grained, with minor plagioclase and olivine.

Gabbronorite: This term encompasses gabbro with significant amounts of both clinopyroxene and orthopyroxene (typically 10-50% each). It bridges the gap between gabbro and norite in terms of composition and appearance.

Olivine Gabbro: Contains more than 5% olivine but doesn't meet the threshold for troctolite classification.

Hornblende Gabbro: Features substantial hornblende (greater than 5%) alongside pyroxene.

Quartz Gabbro: Incorporates a minor amount of quartz, indicating more evolved magma compared to typical gabbro.

Gabbro Properties

Color: Typically dark gray to black, due to its high content of dark-colored minerals like pyroxene and amphibole.

Hardness: Gabbro is a hard and strong rock, with a Mohs hardness of 6-7. This means it can scratch glass, but not steel.

Mineral composition: Primarily composed of pyroxene (augite is most common), plagioclase feldspar (labradorite or bytownite), and sometimes olivine or hornblende.

Density: Gabbro is a dense rock, with a specific gravity of around 2.9-3.1. This means it is about three times heavier than water.

Weathering resistance: Gabbro is generally resistant to weathering due to its dense and tightly interlocking crystals. However, it can weather over time, especially in humid environments, where the minerals can break down and release iron oxides, giving the rock a rusty appearance.

Magnetism: Gabbro can be slightly magnetic due to the presence of iron-bearing minerals like magnetite and ilmenite. However, it is not as strongly magnetic as some other rocks, such as basalt.

Gabbro Occurrence

Gabbro, a coarse-grained igneous rock, can be found in various locations around the globe, though it isn't as widespread as some other rock types. Here are some key areas where you're likely to encounter gabbro:

Intrusive Bodies

Batholiths: These large, underground magma chambers can solidify and form gabbro masses, often exposed through erosion. Examples include the Sierra Nevada batholith in California and the Skaergaard intrusion in Greenland.

Dikes: Sheet-like intrusions of magma that cool and solidify into gabbro, often cutting through other rock formations. Examples include the Whin Sill in England and the Palisades Sill in New Jersey.

Laccoliths: Dome-shaped intrusions of magma that solidify and uplift overlying rocks, sometimes exposing the gabbro core. An example is the Spanish Peaks in Colorado.

Oceanic Crust

Mid-ocean ridges: The gabbroic layer forms the lower part of the oceanic crust, created by the solidification of magma at these underwater mountain ranges.

Ophiolites: These are sections of the oceanic crust that have been obducted (pushed onto land) and expose the underlying gabbro layer. Examples include the Oman Ophiolite and the Trinity Ophiolite in California.

Global distribution

Gabbro is found on all continents and beneath the world's oceans. Some notable localities include:

North America: Sierra Nevada Batholith (USA), Duluth Complex (USA), Sudbury Igneous Complex (Canada)
South America: Bushveld Complex (South Africa)
Europe: Scandinavian Caledonides, Scottish Highlands
Asia: Ural Mountains, Indian Deccan Traps
Oceania: Great Dividing Range (Australia), New Zealand (West Coast)

Gabbro Uses

Gabbro, a coarse-grained igneous rock, offers a variety of uses thanks to its strength, durability, and diverse properties. Here's a breakdown of its key applications:

Construction and Infrastructure

Crushed stone: The most common use of gabbro is as crushed aggregate for roads, highways, and other construction projects. Its strength and resistance to wear and tear make it ideal for road bases, foundations, and drainage systems.

Riprap: Larger gabbro pieces are used as riprap to protect shorelines, embankments, and other structures from erosion by waves or currents.

Dimension stone: Polished gabbro, often called "black granite," is used for countertops, flooring, building facades, and other architectural applications. Its dark color and coarse texture provide a unique aesthetic appeal.

Gabbro from (Salem Neck, Massachusetts, USA)

Other Applications

Abrasives: Crushed gabbro can be used as an abrasive material for grinding wheels, sandblasting, and other industrial applications.

Soil amendment: Crushed gabbro can be used to improve soil drainage and add essential minerals to agricultural land.

Fiber production: Basalt fibers, derived from molten gabbro, are used in high-performance composites for aerospace, automotive, and other applications due to their strength, lightweight nature, and fire resistance.

Scientific research: Studying gabbro provides valuable insights into the composition and evolution of the Earth's crust and mantle, aiding in understanding geological processes and mineral exploration.

Gabbro 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.