intrusive rocks harden slowly beneath the surface of the earth, and often form large mineral crystals within the rock. Granite is a good example of an intrusive rock.Porphyritic intrusive rocks have large crystals embedded in a matrix of smaller crystals. Pegmatite is the only porphyritic rock on the Science Olympiad list.Extrusive rocks harden quickly during a volcanic eruption and are usually smooth-grained. Basalt is the most common form of extrusive rock. |IgneousName |Classification|Description | |Andesite |Extrusive |Usually blackish-brown, sometimes greenish. Papier-mache look.Contains less than 5% quartz. | |Basalt |Extrusive |Very dark, often black. Often contains phenocrysts of feldspars, olivine, and other dark minerals | |Diorite |Intrusive |Dark gray to blackish gray, mottled. Evenly speckled with dark and light minerals, salt-and-peppery look.| |Gabbro |Intrusive |Gray or light green, very coarse-grained. | |Granite |Intrusive |Crystals of feldspar (pink or red), mica (dark brown or black), and quartz (clear pink, white, or | | | |black).Coarse-grained. | |Obsidian |Extrusive |Shiny black.Volcanic glass, has a conchoidal fracture (see explanation of cleavage and fracture below) Be| | | |careful | |Pegmatite |Intrusive |Same composition as granite but has very large, usually light crystals. | |Pumice |Extrusive |Very light gray. Also volcanic glass, but very light and bubbly. Only rock that floats. | |Rhyolite |Extrusive |Usually light grayish-pink. Made of the same minerals as obsidian and pumice, but did not cool as | | | |quickly. | |Scoria |Extrusive |Dark gray, red, or black. Composed of basalt that cooled very quic |
Clastic rocks, like sandstone, form from other rocks and minerals.Organic rocks, like limestone and coal, form from the bodies or shells of organisms. |Sedimentary Name |Classification |Description | |Anthracite Coal |Organic |93-98% pure carbon. Shiny, scaly black. Conchoidal fracture. Can be used like black | | | |chalk. | |Arkose |Clastic |Formed mostly from feldspar. Gray or pink. Coarse grained, looks like sandstone with | | | |redder tint (mostly quartz). | |Bituminous Coal |Organic |50-65% carbon Black. Not very shiny. Well-jointed, splinters under pressure. Hardness: | | | |2. | |Breccia |Clastic |Conglomerate of sharp, angular fragments. Often forms after rock slides. | |Conglomerate |Clastic |Conglomerate of smooth, rounded fragments. Has the largest grain sizes. Often forms in | |...
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