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see basaltbasalt
, fine-grained rock of volcanic origin, dark gray, dark green, brown, reddish, or black in color. Basalt is an igneous rock, i.e., one that has congealed from a molten state.
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magmatic rock of basal composition, containing approximately equal parts of basal plagioclase (labradorite, bytownite) and colored minerals (pyroxenes, less frequently amphibole and olivine) and mineral ores in small quantities. Strictly, gabbro consists of plagioclase and monoclinic pyroxene. Varieties of gabbro include norites of plagioclase and rhombic pyroxenes and troctolites of plagioclase and olivine. If the gabbro contains olivine together with pyroxene, the rock is called olivine gabbro. Gabbros rich in plagioclase (85-90 percent) are called plagioclasites. Among these are labradorites, whose plagioclases often possess a beautiful bluish or greenish play of colors. The chemical composition of gabbro is approximately 45-50 percent silica and up to 20 percent oxides of iron and magnesium. The color is black or dark green, sometimes mottled. Typical gabbros that have hardened at great depths are characterized by approximately identically-sized crystals of the light and dark minerals that constitute it; gabbro in small masses usually has an ophitic structure. (The plagioclase is in the form of well-formed crystals, and the dark minerals fill the interstices between the plagioclases.) The forms and spatial distributions of the gabbro are large laccoliths, lopoliths, dikes, and stocks. Dimensions of the formations reach enormous magnitudes. (For example, the Duluth massif in Canada covers an area up to 5,000 sq km.) In the USSR there are large massifs of gabbro in the Urals, the Ukraine, the Kola Peninsula, the Transcaucasus, and other regions; outside the USSR there are gabbro masses in the Republic of South Africa, France, Scotland (Great Britain), and other countries. When gabbro disintegrates, ferrous clays, ochers, and, less frequently, low-quality kaolins are formed. Gabbros sometimes contain masses of ore minerals and in such cases may be used as ores of copper, nickel, and titanium. Gabbros are often used as building or facing stone of great strength, for exterior and interior facing, predominantly in the form of polished slabs (for example, the columns of the Lenin Library in Moscow), and for manufacturing gravel and road-paving stone.


A group of dark-colored, intrusive igneous rocks with granular texture, composed largely of basic plagioclase and clinopyroxene.


Igneous rock similar to diorite, predominantly composed of ferromagnetic minerals with crystals visible to the eye; has the same mineral composition as basalt.
References in periodicals archive ?
The amount of magma generated and the proportion of it that erupts varies along mid-ocean ridges, leading to significant variations in total crustal thickness and in the relative proportions of gabbroic rocks, sheeted dikes, and pillow lavas.
The aim of this paper is to present the petrologic and structural characterization of the units exposed in the Sierra San Miguel region, corresponding to the southernmost border of the Laguna Merin Basin (East Uruguay), with particular emphasis in the gabbroic intrusion.
Compilations of dredge results and gravity lows centered over dredge segments also suggest that a continuous gabbroic layer simply does not exist at slow-spreading ridges, and that their internal stratigraphy is governed by ongoing faulting and deformation accompanied by hydrothermal alteration as cold seawater sinks into the crust, heats up, and reacts with the rock as much as by intrusion of new lava at depth and its eruption to the seafloor.
Gabbroic rocks occur in the northwestern part of the unit, intruded by small crosscutting dykes of alkali-feldspar granite.
1991, Syn- and post-kinematic intrusions of troctolite, gabbro, and peridotite into layered gabbroic cumulates in the Bay of Islands ophiolite: genesis of anorthosite by partial assimilation, and troctolite by hybridization: Geological Survey of Canada, Current Research, Paper 91-1, part D, p.
It is suggested that local gabbroic pottery and stoneware continued in use alongside these imports.
They show that low-grade gold mineralization does extend south of the Kiaka Central Area, where it occurs on both sides of a large unmineralised gabbroic intrusion.
However, gabbroic bodies in the same area are interpreted to be part of the Landry Brook pluton, based on petrological features described below.
The drilling has intersected packages of both cumulate and intrusive style magnetite-ilmenite mineralisation intercalated with gabbroic and anorthositic country rock.
hosted by monotonous sequences of clastic rocks intruded by gabbroic to granitic plutons, and metamorphosed to at least the greenschist fades.
There was also a sandhill find from Hardelot in the Pas-de-Calais, France, which is of gabbroic clay.