basalt

Basalt is a hard gray or black, mafic igneous volcanic rock that is usually fine-grained due to rapid cooling of lava, though it contain larger crystals in a fine matrix (porphyritic), be vesicular, or be a frothy scoria.

Basalt magmas form by decompression melting of peridotite in the mantle. The crustal portions of oceanic tectonic plates comprised predominantly basalt, derived from upwelling peridotite in the mantle below ocean ridges. The basalt shield volcanoes of the Hawaiian island chain sit above a mantle plume, or 'hot spot'. (left - click to enlarge - aa flows over ropey pahoehoe in Hawaii - image courtesy of USGS.)

Basalt is TAS classified according to the relationships between the combined alkali content and the silica content. Basalt typically containts a preponderance of calcic plagioclase feldspar and pyroxene; olivine can also be a significant constituent. Accessory minerals include iron oxides and iron-titanium oxides, providing basalt with a paleomagnetic signature.

Phaneritic, shallow intrusive igneous rocks with a basaltic composition are generally referred to as dolerite (also called diabase) or gabbro.

(image left - click to enlarge - courtesy USGS - top-down: basaltic lava; lava field; flow-lines in basalt formation; close-up of vesicular basalt with olivine crystals; surface of basalt hand specimen; basalt columns.)

[images - roll-over link for preview (where available); large images (well worth a visit) show only as a corner on preview : water-sculpted basalt at Fossil Falls in Yosemite : Basalt Fall unterhalb des Hengifoss, basalt columns, Dverghamrar basaltic columns in Iceland, 2 : cliff of basalt columns : Columbia River basalts, Catherine Creek arch in Miocene columnar basalts : flowing curves of basalt entablature in Yellowstone : basalt columns Armenia : basalt field : basalt and sandstone : 3.7 Ga moon-rock basalt : hand-specimen : hand-specimen vesicular basalt, vesicular basalt with olivine phenocrysts, 2 : hand-specimen diabase : hand-specimen diabase porphyry : hand-specimen diorite : hand-specimen gabbro : hand-specimen scoria : thin-section basalt, 2, 3; thin-sections moon basalts ]

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fusibles and refractories

Fusibles are rocks or minerals that melt easily, and are the opposite of melting-resistant refractories.

Sedimentary rocks, which are stable at the Earth's cool surface, tend to be fusible because they consist mostly of stable minerals that have resisted weathering. Crystalline rocks tend to be refractory and to resist melting because they consist mostly of minerals that crystallized out of melts. The most refractory rocks, such as gabbro and peridotite, are stable in the lower crust and the mantle.

▪ Bowen's Reaction Series

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peridotite

Peridotite is an ultramafic, ultrabasic (less than 45% silica), dense, plutonic igneous rock comprising mostly olivine and pyroxene. Most of the Earth's upper mantle (asthenosphere) is composed of peridotite that originated during the accretion and differentiation of the Earth, or that has differentiated, by precipitation of olivine ± pyroxenes, from basaltic or ultramafic magmas in turn derived from partial melting of the upper mantle peridotites. Deeper in the crust, olivine is replaced by a high pressure polymorphs, so peridotites do not occur at depths greater than 400 km.

Peridotite emplaced in the continental crust is typically found in obducted ophiolite complexes, as xenoliths in basalts and kimberlite pipes, and as orogenic peridotite massifs and alpine peridotites. Olivine is unstable at shallow depths and reacts rapidly with water, so that much surface peridotite has been altered to serpentinite by a process in which the pyroxenes and olivines are converted to green serpentine.

xenoliths

Xenoliths, meaning 'foreign stones', arise when fragments of metamorphically altered country rock fall into magma or lava and become enveloped within igneous rock. Xenoliths can be engulfed along margins of magma chambers, torn loose from the walls of a conduit for erupting lava or an explosive diatreme, or picked up along the base of flowing lava. Xenoliths can be quite large and xenoliths can occur in clusters within the plutonic rock or demonstrate dikelet intrusions by the magma.

Xenoliths are distinguished from paleosomes, which are older bodies comprising refractory minerals that failed to melt within migmatites (in which more fusible neosomal minerals melted during Barrovian regional metamorphism).

[images : xenolith of peridotite embedded in vesicular basalt?; olivine in peridotite xenolith in alkali basalt; xenolith of partly melted metamorphic rock embedded in solidied lava; xenolith embedded in granite, Garnet Canyon, Tetons, WY; xenolith in pink granite, 2; xenolith in granodiorite, and close-up of boundary; xenolith of biotite-rich schist enclosed in Petersburg Granite; xenolith; xenolith in a basaltic sill; spinel lherzolite xenoliths, San Carlos AZ; xenolith of spinifex-textured basalt within vesicular dacite glass; altered xenolith of spinifex rock in altered and veined dacite; xenoliths in boulder; Shiprock xenolith; formations: xenolith of a quartz-biotite gneiss into which granite was intruded; xenolith in granite quarry, Elberton, GA, 2; excellent, though large bandwidth, illustrations of a xenolith in the Halifax Pluton]