ROCK PICTIONARY
ROCK TYPES IGNEOUS ROCKS Rocks formed when magma (molten material beneath the earth s surface) or lava (molten material on the earth s surface) cools and becomes solid. SEDIMENTARY ROCKS Rocks composed of pieces of other rocks, remains of animals or plants (i.e. fossils), or minerals formed by chemical precipitation or evaporation. METAMORPHIC ROCKS Rocks formed when existing rocks (parent rock) are changed or altered due to pressure, heat, or chemically active fluids. IGNEOUS ROCK TEXTURES TEXTURE DEFINITION APHANITIC PHOTOGRAPH Crystalline texture with individual crystals too small to be visible to the unaided eye. Caused by rapid extrusive cooling of lava. PHANERITIC Crystalline texture in which the individual components are distinguishable with the unaided eye. Caused by slow intrusive cooling of magma. PORPHRYTIC Crystalline texture in which larger crystals (phenocrysts) are set in an aphanitic groundmass. Caused by initial slow intrusive cooling of magma then subsequently fast extrusive cooling of lava.
PEGMATITIC Crystalline texture with exceptionally large crystals. Caused by extremely slow intrusive cooling of magma. VESICULAR Texture of a rock, esp. a lava, characterized by abundant vesicles (holes) formed as a result of the expansion of gases during the fluid stage of the lava. GLASSY Having the appearance of glass. Caused by extremely fast extrusive cooling of lava. So quick the atoms are all in a chaotic distribution and not crystallized. FELSIC Is a light colored igneous rock rich in potassium and sodium silicates.
INTERMEDIATE Is a dark and light igneous rock with a composition in between felsic and mafic. MAFIC Is a dark colored igneous rock rich in iron and magnesium silicates. ULTRA-MAFIC Is made almost entirely of dark-colored iron and magnesium silicates.
ROUNDED CLASTS SEDIMENTARY ROCK TEXTURES Sharp edges of the clasts have been rounded off due to transportation within a stream. ANGULAR CLASTS Clasts are angular and have sharp edges since the material has not been transported in a river. WELL SORTED Composed of grains or clasts of approximately the same size. POORLY SORTED Composed of grains or clasts of a variety of different sizes. FOSSILIFEROUS Contains fossils (evidence of ancient life) embedded in a fine grained matrix.
EFFERVESENCE (i.e. the acid test) Reaction to acid of some sedimentary rocks (limestone, dolostone) which contain calcite or dolomite. Bubbles are generated when dilute HCl acid is applied. Video: http://www.youtube.com/watch?v=lw3bwr0ws7m CROSS-BEDDING Is a primary sedimentary structure in which one set of inclined sedimentary layers is beveled off by an erosional process and a new set of sedimentary layers is deposited on top of the truncated original layers; this type of primary sedimentary structure is common in stream sediments and sand dunes. METAMORPHIC ROCK TEXTURES FOLIATED: SLATY CLEAVAGE Fine-grained texture caused by the parallel orientation of microscopic grains, causing the rock to separate along parallel planes. This feature is a property known as slaty cleavage. (Slaty cleavage or rock cleavage is not to be confused with cleavage in a mineral, which is related to the internal atomic structure of the mineral.) FOLIATED: PHYLLITIC TEXTURE Fine-grained texture formed by the parallel arrangement of platy minerals, usually micas, that are barely macroscopic (visible to the naked eye). The parallelism is often silky, wavy or crenulated. The predominance of micaceous minerals imparts a sheen to the hand specimens. FOLIATED: SCHISTOSE TEXTURE Coarse-grained texture resulting from the suhparallel to parallel orientation of large platy minerals such as chlorite or micas, which cause the rock to sparkle. Other common minerals present are quartz and amphiholes. The average grain size of the minerals is generally smaller than in a gneissic texture.
FOLIATED: GNEISSIC TEXTURE Coarse-grained texture in which the minerals have been segregated into bands, each of which is dominated by one or two minerals. These bands range in thickness from 1 mm to several centimeters. The individual mineral grains are macroscopic and impart a striped appearance to a hand specimen. Light-colored bands commonly contain quartz and feldspar. And the dark hands are commonly composed of hornblende and ochise. Accessory minerals are common and are useful in applying specific names to these rocks. NON-FOLIATED Contains no linear structure, is massive in appearance. Named on the basis of its composition (e.g. quartz, calcite). EFFERVESENCE (i.e. the acid test) Reaction to acid of some metamorphic rocks (marble) which contain calcite or dolomite. Bubbles are generated when dilute HCl acid is applied. Video: http://www.youtube.com/watch?v=lw3bwr0ws7m
PHOTOGRAPH CREDIT angular clasts aphanitic cross-bedding effervescence acid test video felsic fine felsic large fossiliferous glassy gneissic texture intermediate fine Intermediate large mafic fine mafic large non-foliated pegmatitic phanaritic phyllitic texture poorly sorted porphrytic rounded clasts schistose texture slaty cleavage ultra-mafic vesicular well sorted http://skywalker.cochise.edu/wellerr/rocks/sdrx/breccia1.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/rhyolite2.htm http://skywalker.cochise.edu/wellerr/rocks/sdrx/sedstruct7.htm http://www.geol.umd.edu/~jmerck/geol100/lectures/09.html http://www.youtube.com/watch?v=lw3bwr0ws7m http://skywalker.cochise.edu/wellerr/rocks/igrx/rhyolite007.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/granite-red2.htm http://skywalker.cochise.edu/wellerr/rocks/sdrx/limestone3.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/obsidian2.htm http://skywalker.cochise.edu/wellerr/rocks/mtrx/gneiss10.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/andesite-misc7.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/diorite-misc5.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/basalt-fresh6.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/gabbro-misc1.htm http://skywalker.cochise.edu/wellerr/rocks/mtrx/quartzite1.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/pegmatite4.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/granite-red3.htm http://skywalker.cochise.edu/wellerr/rocks/mtrx/phyllite5.htm http://skywalker.cochise.edu/wellerr/rocks/sdrx/conglomerate2.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/andesite2657.htm http://skywalker.cochise.edu/wellerr/rocks/sdrx/conglomerate6.htm http://skywalker.cochise.edu/wellerr/rocks/mtrx/schist1.htm http://skywalker.cochise.edu/wellerr/rocks/mtrx/slate1.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/dunite1.htm http://skywalker.cochise.edu/wellerr/rocks/igrx/basalt-vesic3.htm http://skywalker.cochise.edu/wellerr/rocks/sdrx/sandstone-orthoquartzite2.htm