Extrusive rock
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Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out onto the surface as lava or explodes violently into the atmosphere to fall back
A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope
Basalt
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Basalt; US: buh-SALT is an aphanitic extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the surface of a rocky planet or moon
Extrusive rock
…components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in pumice, scoria, or vesicular basalt. Other examples of extrusive rocks are rhyolite and andesite. The texture of extrusive rocks is characterized by fine-grained crystals indistinguishable…
Basalt 12
QAPF diagram with basalt/andesite field highlighted in yellow. Basalt is distinguished from andesite by SiO2 < 52%.
Vesicular basalt at Sunset Crater, Arizona. US quarter (24mm) for scale.
Columnar basalt flows in Yellowstone National Park, US