A fine grained igneous rock composed essentially of orthoclase feldspar and quartz, or of rock glass having the same composition, with or without biotite, hornblende, or pyroxene. The fine grained equivalent of granite.
A fine-grained, extrusive igneous rock of the same composition as granite. Rhyolite is formed when the parent magma solidifies rapidly and large mineral crystals do not have time to grow. Granite, a coarse grained or large crystal rock, is formed when the parent magma solidifies underground after a long cooling period which promotes the larger mineral crystal growth. Back
a group of extrusive, felsic light-colored) igneous rocks which are typically porphyritic, with phenocrysts of quartz and/or orthoclase feldspar in a cryptocrystalline groundmass. The presence of phenocrysts indicates two stages (speeds) of cooling of the original magma.
A group of extrusive (volcanic) igneous rocks with phenocrysts (visible crystals that grew out of the molten lava) of quartz and alkali feldspar in a glassy to cryptocrystalline (too small to be seen through an ordinary microscope) groundmass (matrix).
volcanic rock chemically equivalent to granite Usually light colored, very fine-grained or glassy-looking. May have tiny visible crystals of quartz and/or feldspar dispersed in a glassy white, green, or pink groundmass.
(geology): A dull, pink, streaky, igneous rock formed from minute glassy crystals of volcanic dust. In South Africa it is found only in the Lebombo Mountains, and is used for roads, concrete and insulation.
The fine-grained volcanic or extrusive equivalent of granite, light brown to gray and compact. Fine grained, pink to dark purple felsic volcanic rock often with pink feldspar porphyry; fine grained equivalent to granite;