Gabbro () is a
phaneritic
A phanerite is an igneous rock whose microstructure is made up of crystals large enough to be distinguished with the unaided human eye. In contrast, the crystals in an aphanitic rock are too fine-grained to be identifiable. Phaneritic texture fo ...
(coarse-grained),
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
intrusive igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
formed from the slow cooling of magnesium-rich and iron-rich
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
into a
holocrystalline
Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion ...
mass deep beneath the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
. Much of the Earth's
oceanic crust
Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...
is made of gabbro, formed at
mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
s. Gabbro is also found as
pluton
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
s associated with continental
volcanism
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called ...
. Due to its variant nature, the term ''gabbro'' may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as
granite
Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
is to
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
.
Etymology
The term "gabbro" was used in the 1760s to name a set of rock types that were found in the
ophiolite
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ''ophis'' (''snake'') is found i ...
s of the
Apennine Mountains
The Apennines or Apennine Mountains (; grc-gre, links=no, Ἀπέννινα ὄρη or Ἀπέννινον ὄρος; la, Appenninus or – a singular with plural meaning;''Apenninus'' (Greek or ) has the form of an adjective, which wou ...
in Italy. It was named after
Gabbro
Gabbro () is a phaneritic (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 che ...
, a hamlet near
Rosignano Marittimo
Rosignano Marittimo is a ''comune'' (municipality) in the Province of Livorno in the Italian region Tuscany, located about southwest of Florence and about southeast of Livorno.
Geography
Rosignano Marittimo borders the following municipalities: ...
in
Tuscany
Tuscany ( ; it, Toscana ) is a Regions of Italy, region in central Italy with an area of about and a population of about 3.8 million inhabitants. The regional capital is Florence (''Firenze'').
Tuscany is known for its landscapes, history, art ...
. Then, in 1809, the German geologist
Christian Leopold von Buch
Christian Leopold von Buch (26 April 1774 – 4 March 1853), usually cited as Leopold von Buch, was a German geologist and paleontologist born in Stolpe an der Oder (now a part of Angermünde, Brandenburg) and is remembered as one of the most im ...
used the term more restrictively in his description of these Italian ophiolitic rocks. He assigned the name "gabbro" to rocks that geologists nowadays would more strictly call "metagabbro" (
metamorphosed
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
gabbro).
Petrology
Gabbro is a coarse-grained (
phaneritic
A phanerite is an igneous rock whose microstructure is made up of crystals large enough to be distinguished with the unaided human eye. In contrast, the crystals in an aphanitic rock are too fine-grained to be identifiable. Phaneritic texture fo ...
)
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
that is relatively low in
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
and rich in iron, magnesium, and calcium. Such rock is described as ''
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
''. Gabbro is composed of
pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
and calcium-rich
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
, with minor amounts of
hornblende
Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rocks ...
,
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
, and
accessory minerals
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...
. When present, hornblende is typically found as a rim around
augite
Augite is a common rock-forming pyroxene mineral with formula . The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.
Characteristics
Augite is a solid solution in the pyroxene group. ...
crystals or as large grains enclosing smaller grains of other minerals (''
poikilitic
Poikilitic texture refers to igneous rocks where large later-formed less perfect crystals ('oikocrysts') surround smaller early-formed idiomorphic crystals ('chadacrysts') of other minerals. A poikilitic texture is most easily observed in petrog ...
'' grains).
Geologists use rigorous quantitative definitions to classify coarse-grained igneous rocks, based on the mineral content of the rock. For igneous rocks composed mostly of silicate minerals, and in which at least 10% of the mineral content consists of
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
,
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
, or
feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are usually not found in rocks containing prim ...
minerals, classification begins with the
QAPF diagram
A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym QAPF stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for ...
. The relative abundances of quartz (Q),
alkali feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldspa ...
(A), plagioclase (P), and feldspathoid (F), are used to plot the position of the rock on the diagram.
The rock will be classified as either a gabbroid or a
dioritoid
Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is Intermediate composition, inter ...
if quartz makes up less than 20% of the QAPF content, feldspathoid makes up less than 10% of the QAPF content, and plagioclase makes up more than 65% of the total feldspar content. Gabbroids are distinguished from dioritoids by an
anorthite
Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon.
Mineralo ...
(calcium plagioclase) fraction of their total plagioclase of greater than 50%.
The composition of the plagioclase cannot easily be determined
in the field, and then a preliminary distinction is made between dioritoid and gabbroid based on the content of mafic minerals. A gabbroid typically has over 35% mafic minerals, mostly pyroxenes or olivine, while a dioritoid typically has less than 35% mafic minerals, which typically includes hornblende.
Gabbroids form a family of rock types similar to gabbro, such as
monzogabbro Monzogabbro is an intrusive rock with a composition intermediate between gabbro and monzonite. It is defined in the QAPF classification as coarse-grained igneous rock in which quartz makes up 0% to 5% of the QAPF mineral fraction, plagioclase
...
,
quartz gabbro Quartz gabbro is an intrusive rock with a composition intermediate between gabbro and tonalite. It is defined in the QAPF classification as coarse-grained igneous rock in which quartz makes up 5% to 20% of the QAPF mineral fraction, plagioclase
...
, or
nepheline-bearing gabbro. Gabbro itself is more narrowly defined, as a gabbroid in which quartz makes up less than 5% of the QAPF content, feldspathoids are not present, and plagioclase makes up more than 90% of the feldspar content. Gabbro is distinct from
anorthosite
Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most c ...
, which contains less than 10% mafic minerals.
Coarse-grained gabbroids are produced by slow crystallization of
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
having the same composition as the
lava
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
that solidifies rapidly to form fine-grained (
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geologic ...
)
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
.
Subtypes
There are a number of subtypes of gabbro recognized by geologists. Gabbros can be broadly divided into leucogabbros, with less than 35% mafic mineral content; mesogabbros, with 35% to 65% mafic mineral content; and melagabbros with more than 65% mafic mineral content. A rock with over 90% mafic mineral content will be classified instead as an
ultramafic rock
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
. A gabbroic rock with less than 10% mafic mineral content will be classified as an anorthosite.
A more detailed classification is based on the relative percentages of plagioclase, pyroxene, hornblende, and olivine. The end members are:
* Normal gabbro (gabbro ''sensu stricto''
) is composed almost entirely of plagioclase and
clinopyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
(typically augite), with less than 5% each of hornblende, olivine, or
orthopyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
.
*
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite, orthopyroxene, and olivine. The name ''norite'' is derived from ''Norge'', the Norwegian name for Norway.
Norite also known as orthopyroxen ...
is composed almost entirely of plagioclase and
orthopyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
, with less than 5% each of hornblende, clinopyroxene, or olivine.
*
Troctolite
Troctolite (from Greek τρώκτης 'trout' and λίθος 'stone') is a mafic intrusive rock type. It consists essentially of major but variable amounts of olivine and calcic plagioclase along with minor pyroxene. It is an olivine-rich anorth ...
is composed almost entirely of plagioclase and olivine, with less than 5% each of pyroxene or hornblende.
*
Hornblende gabbro Hornblende gabbro is an intrusive rock similar to normal gabbro (gabbro ''sensu stricto''). In the QAPF classification it falls within the gabbro field, in which quartz makes up 0% to 5% of the QAPF mineral fraction, plagioclase makes up 90% or more ...
is composed almost entirely of plagioclase and hornblende, with less than 5% each of pyroxene or olivine.
Gabbros intermediate between these compositions are given names such as
gabbronorite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite, orthopyroxene, and olivine. The name ''norite'' is derived from ''Norge'', the Norwegian name for Norway.
Norite also known as orthopyroxen ...
(for a gabbro intermediate between normal gabbro and norite, with almost equal amounts of clinopyroxene and orthopyroxene) or olivine gabbro (for a gabbro containing significant olivine, but almost no clinopyroxene or hornblende). A rock similar to normal gabbro but containing more orthopyroxene is called an orthopyroxene gabbro, while a rock similar to norite but containing more clinopyroxene is called a clinopyroxene norite.
Gabbros are also sometimes classified as alkali or tholleiitic gabbros, by analogy with
alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
or
tholeiitic
The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma i ...
basalts, of which they are considered the intrusive equivalents. Alkali gabbro usually contains olivine, nepheline, or
analcime
Analcime (; ) or analcite is a white, gray, or colorless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is Na Al Si2 O6· H2O. Minor amounts of potassium and calciu ...
, up to 10% of the mineral content, while tholeiitic gabbro contains both clinopyroxene and orthopyroxene, making it a gabbronorite.
Gabbroids
Gabbroids (also known as gabbroic-rocks
) are a family of coarse-grained igneous rocks similar to gabbro:
* Quartz gabbro contains 5% to 20% quartz in its QAPF fraction. One example is the ''cizlakite'' at
Pohorje
Pohorje (), also known as the Pohorje Massif or the Pohorje Mountains (german: Bachergebirge, ''Bacherngebirge'' or often simply ''Bachern''), is a mostly wooded, medium-high mountain range south of the Drava River in northeastern Slovenia. Acco ...
in northeastern Slovenia,
* Monzogabbro contains 65% to 90% plagioclase out of its total feldspar content.
*
Quartz monzogabbro combines the features of quartz gabbro and monzogabbro. It contains 5% to 20% quartz in its QAPF fraction, and 65% to 90% of its feldspar is plagioclase.
* Foid-bearing gabbro contains up to 10% feldspathoids rather than quartz. "Foid" in the name is usually replaced by the specific feldspathoid that is most abundant in the rock. For example, a
nepheline
Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites ...
-bearing gabbro is a foid-bearing gabbro in which the most abundant feldspathoid is nepheline.
* Foid-bearing monzogabbro resembles monzogabbro, but containing up to 10% feldspathoids in place of quartz. The same naming conventions apply as for foid-bearing gabbro, so that a gabbroid might be classified as a
leucite
Leucite is a rock-forming mineral of the feldspathoid group, silica-undersaturated and composed of potassium and aluminium tectosilicate KAlSi2O6. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 18 ...
-bearing monzogabbro.
Gabbroids contain minor amounts, typically a few percent, of iron-titanium oxides such as
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
,
ilmenite
Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...
, and
ulvospinel.
Apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...
,
zircon
Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...
, and
biotite
Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...
may also be present as accessory minerals.
[
Gabbro is generally coarse-grained, with crystals in the size range of 1 mm or larger. Finer-grained equivalents of gabbro are called ]diabase
Diabase (), also called dolerite () or microgabbro,
is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-graine ...
(also known as dolerite
Diabase (), also called dolerite () or microgabbro,
is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grained ...
), although the term ''microgabbro'' is often used when extra descriptiveness is desired. Gabbro may be extremely coarse-grained to pegmatitic
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...
. Some pyroxene-plagioclase cumulates
Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group o ...
are essentially coarse-grained gabbro, and may exhibit acicular crystal habits.
Gabbro is usually equigranular
An equigranular material is composed chiefly of crystals of similar orders of magnitude to one another. Basalt and gabbro commonly exhibit an equigranular texture
Texture may refer to:
Science and technology
* Surface texture, the texture mea ...
in texture, although it may also show ophitic texture
Poikilitic texture refers to igneous rocks where large later-formed less perfect crystals ('oikocrysts') surround smaller early-formed idiomorphic crystals ('chadacrysts') of other minerals. A poikilitic texture is most easily observed in petrog ...
[ (with laths of plagioclase enclosed in pyroxene).
]
Distribution
Nearly all gabbros are found in plutonic bodies, and the term (as the International Union of Geological Sciences
The International Union of Geological Sciences (IUGS) is an international non-governmental organization devoted to international cooperation in the field of geology.
About
The IUGS was founded in 1961 and is a Scientific Union member of the Inte ...
recommends) is normally restricted just to plutonic rocks, although gabbro may be found as a coarse-grained interior facies
In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
of certain thick lavas. Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
and plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
, or as part of a layered intrusion
A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around to over and several hundred metres to ...
as a cumulate
Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group o ...
formed by settling of pyroxene and plagioclase.
An alternative name for gabbros formed by crystal settling is ''pyroxene-plagioclase adcumulate''.
Gabbro is much less common than more silica-rich intrusive rocks in the continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called ''sial'' be ...
of the Earth. Gabbro and gabbroids occur in some batholith
A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such ...
s but these rocks are relatively minor components of these very large intrusions because their iron and calcium content usually makes gabbro and gabbroid magmas too dense to have the necessary buoyancy. However, gabbro is an essential part of the oceanic crust, and can be found in many ophiolite
An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ''ophis'' (''snake'') is found i ...
complexes as layered gabbro underling sheeted dike complex
A sheeted dyke complex, or sheeted dike complex, is a series of sub-parallel intrusions of igneous rock, forming a layer within the oceanic crust. At mid-ocean ridges, dykes are formed when magma beneath areas of tectonic plate divergence travels ...
es and overlying ultramafic
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
rock derived from the Earth's mantle
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up about 84% of Earth's volume. It is predominantly sol ...
. These layered gabbros may have formed from relatively small but long-lived magma chamber
A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it upw ...
s underlying mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
s.
Layered gabbros are also characteristic of lopolith
A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body. The term was first defined ...
s, which are large, saucer-shaped intrusions that are primarily Precambrian
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
in age. Prominent examples of lopoliths include the Bushveld Complex
The Bushveld Igneous Complex (BIC) is the largest layered igneous intrusion within the Earth's crust. It has been tilted and eroded forming the outcrops around what appears to be the edge of a great geological basin: the Transvaal Basin. It i ...
of South Africa, the Muskox intrusion of the Northwest Territories
The Northwest Territories (abbreviated ''NT'' or ''NWT''; french: Territoires du Nord-Ouest, formerly ''North-Western Territory'' and ''North-West Territories'' and namely shortened as ''Northwest Territory'') is a federal territory of Canada. ...
of Canada, the Rum layered intrusion
The Rum layered intrusion is located in Scotland, on the island of Rùm ( Inner Hebrides). It is a mass of intrusive rock, of mafic-ultramafic composition, the remains of the eroded, solidified magma chamber of an extinct volcano that was active ...
of Scotland, the Stillwater complex of Montana, and the layered gabbros near Stavanger
Stavanger (, , American English, US usually , ) is a city and municipalities of Norway, municipality in Norway. It is the fourth largest city and third largest metropolitan area in Norway (through conurbation with neighboring Sandnes) and the a ...
, Norway. Gabbros are also present in stock
In finance, stock (also capital stock) consists of all the shares by which ownership of a corporation or company is divided.Longman Business English Dictionary: "stock - ''especially AmE'' one of the shares into which ownership of a company ...
s associated with alkaline volcanism of continental rifting
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.
Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben wi ...
.
Uses
Gabbro often contains valuable amounts of chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
, nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
, cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
, gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
, silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
, platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
, and copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
sulfide
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
s. For example, the Merensky Reef
The Merensky Reef is a layer of igneous rock in the Bushveld Igneous Complex (BIC) in the North West, Limpopo, Gauteng and Mpumalanga provinces of South Africa which together with an underlying layer, the Upper Group 2 Reef (UG2), contains most of ...
is the world's most important source of platinum.
Gabbro is known in the construction industry by the trade name of ''black granite''. However, gabbro is hard and difficult to work, which limits its use.
See also
*
*
*
References
External links
Ocean drilling program gabbro petrology
{{Authority control
Mafic rocks
Plutonic rocks
Phaneritic rocks