igneous structures

Magma is molten (igneous) rock formed when Earth's radioactivity heats rocks – because of Earth's geothermal gradient, the deeper in the Earth, the greater the temperature.

◙◙ Rock Index: Igneous Rocks ◙◙

Intrusion:
Magma that is emplaced below the surface when molten rock intrudes into or across strata of country rock cools and crystallizes in a variety of intrusive structures, including large plutonic and small hypabassal structures:

intrusive emplacement / structures: ▪ aplite, aplite dike (◙ aplite) ▪ batholith ▪ boudin ▪ bysmalith ▪ concordant ▪ conformable ▪ diapir ▪ diatreme ▪ dike ▪ discordant ▪ enclaves ▪ hypabassal ▪ laccoliths ▪ lopoliths ▪ pluton ▪ sills ▪ strata ▪ volcano ▪ vein ▪

structures reflecting flow/crystallization within magma chambers: ▪ cumulates ▪ enclaves ▪ schlieren ▪ xenoliths ▪

magmatic processes: ▪ anatexis ▪ assimiliation ▪ exchange of volatiles ▪ fractional crystallization ◘ igneous rocks ◘ igneous structures ◘ magma ▪ magmatic differentiation ▪ magmatic mixing ◘ ophiolite complexes ◘ tectonics

Extrusion:
When melted rock flows extrusively or erupts explosively at the surface is called lava – Hawaii's and Iceland's basalt lavas fountain or flow freely, while other lavas are sticky and explosive, producing deadly pyroclastic flows (Mount St. Helens, Vesuvius, Pinatubo).

extrusive: ▪ aphanitic texture ◘ lava ◙ tectonism ◘ volcanoes ▪ vulcanism

Links: Maps of North American rock types : rock types - metamorphic, plutonic, sedimentary, volcanic; tapestry of time and terrain, terrain.

phenocryst

A phenocryst is a conspicuous, large crystal embedded in a finer-grained matrix of smaller crystals in a porphyritic igneous rock.

(left - click to enlarge image - a plagioclase feldspar phenocryst, Lambert Dome, Yosemite, courtesy Daniel Mayer)

Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more). Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass.

[images : phenocrysts of feldspar in matrix of quartz, feldspar and mica; large feldspathic phenocrysts in granite; large zoned plagioclase phenocryst; andesites with phenocrysts; formation: Death Valley vitrophyre, a phenocryst-bearing obsidian; thin-section feldspar and clinopyroxene phenocrysts in Mt. Fuji basalt; thin-section clinopyroxene phenocrysts, ppl; thin-section augite phenocryst, same augite phenocryst in cross-polarized light; resorbed quartz phenocryst, in PPL; thin-section anhedral amphibole phenocryst; igneous rocks in thin section]

sills

Sills are tabular slabs or concordant intrusive (plutonic) sheets of igneous rock that intruded laterally and horizontally, or nearly horizontally, as magma. Sills lie between and parallel to layers of older sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock.

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]