Basalt rock face on the shores of Loch Na Keal, Isle of Mull. Argyll. Scotland. SCO 9238.
Basalt is a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet or moon. By definition, basalt is an aphanitic igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and where at least 65% of the feldspar is in the form of plagioclase. (In comparison, granite has more than 20% quartz by volume.) Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into rust. It almost always has a fine-grained mineral texture due to the molten rock cooling too quickly for large mineral crystals to grow, although it can sometimes be porphyritic, containing the larger crystals formed prior to the extrusion that brought the lava to the surface, embedded in a finer-grained matrix. Basalt with a vesicular or frothy texture is called scoria, and forms when dissolved gases are forced out of solution and form vesicles as the lava decompresses as it reaches the surface. #During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns. The topology of the lateral shapes of these columns can broadly be classed as a random cellular network. These structures are predominantly hexagonal in cross-section, but polygons with three to twelve or more sides can be observed. The size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (
Size: 6063px × 4035px
Location: Loch Na Keal, Isle of Mull. Argyll. Scotland
Photo credit: © David Gowans / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: actinolite, air, assemblages, basalt, basalts, belt., belts, brown, chlorite, colour, compared, conditions, content, due, earth, elements, epidote, event., facies, fast., flows, formed, important, information, iron, iron-rich, lanthanide, lava, metamorphic, metamorphism, mineral, minerals, moon, named, oxide, oxidise, pressures, produces, provide, rapidly, rare, red, rock, rocks, silica, staining, subjecting, surface, temperatures, types, typically, vital, water, weather
