ROCKS ... and how to identify them THE ROCK CYCLE A tutorial program offered to you by the Applied Science Rocks are naturally occurring combinations or coherent Department of Glendale Community College aggregates of minerals, fossils or other hard materials. They are classified by the way in which they form. The presented by: three rock types are: igneous, sedimentary and Susan Celestian – Curator of the Arizona Mining and metamorphic. Mineral Museum Stan Celestian – Photographer and Instructor All rocks on Earth are locked into a system of cycling and re-cycling known as the ROCK CYCLE. (*) © copyright 2006 This is your cue to advance to the next slide. (*) THE ROCK CYCLE IGNEOUS ROCKS IGNEOUS ROCKS are “born of IGNEOUS fire”. In other words, they w ea vent th er lava were once molten and upon in li g, th i t f r g ic a land s n a n urface cooling, the magma (molten ti t s l io p e n or m ta e, ti rock) crystallized into solid r on u conduit s s es m on e pr i rock. Igneous rocks may , lt magma t in g ea h form deep inside the Earth or at the Earth’s surface when a METAMORPHIC SEDIMENTARY volcano erupts. (*) weathering, transportation, lithification heat, pressure, ions Anatomy of a Volcano (*) CLUES TO IGNEOUS ROCKS IGNEOUS ROCKS COLOR LIGHT COLORED INTERMEDIATE DARK COLORED VERY DARK Felsic COLORED Mafic COLORED Rapid cooling near or at the TEXTURE Ultramafic Earth’s surface, produces COARSE- GRANITE: DIORITE: GABBRO: PERIDOTITE: Slow cooling deep beneath the GRAINED Can see crystals. Usually Can see crystals with Can see crystals — lots of Composed of 90- many small crystals that are (You can see gray or pink. Can see somewhat more light colored flat shiny cleavage surfaces. 100% olivine quartz - gray, glassy grains. feldspar grains than dark Earth’s surface allows crystals different Can see feldspar - pink, colored minerals. A mix of light Usually black to greenish not readily seen by the minerals) buff, or white. and dark but with no quartz. Salt black. PYROXENITE: Composed of pre- to grow to large size (1/8” or & pepper appearance. unaided eye. This group of dominately pyroxene FINE- more). These crystals are igneous rocks is called RHYOLITE: ANDESITE: BASALT: AMPHIBOLITE: GRAINED Usually gray, pink, Light to dark gray. Normally has Usually black or rust red. Composed of pre- pastel. Might see small small black crystals May have some or lots of (You can NOT dominately amphiboles easily visible and distinguish EXTRUSIVE and are typically clear, rectangular crystals. gas bubble holes, some (such as hornblende) see crystals, for Sometimes banded. holes may be filled. May this group of igneous rocks as volcanic in origin. Cooling the most part) see small green grains. may be so rapid that crystals OBSIDIAN: Black, red, green, GLASS GLASSY INTRUSIVE. PERLITE: Usually pearly gray. May contain Apache Tears. do not have a chance to form PUMICE: LOTS of gas bubble holes, very lightweight, will float on water. Abrasive. and instead a glass is PORPHYRITIC ALL CRYSTALLINE IGNEOUS VARIETIES may exhibit porphyritic texture produced. (*) (2 grain sizes) TUFF: Compacted volcanic fragments generally less than 4mm diameter (ash) FRAGMENTAL VOLCANIC BRECCIA: Mixed tuff and angular large (>32mm diameter) fragments (*) AGGLOMERATE: Mixed ash and rounded/sub-rounded large (>32mm diameter) fragments Granite - intrusive Granite - intrusive Graphic Granite quartz biotite mica GRANITE is a coarse to medium- (*) grained rock that forms from the cooling of magma deep within the Earth (intrusive). It feldspar is made up mainly of varying amounts of the minerals: mica quartz, orthoclase, muscovite, biotite and hornblende. The Porphyritic name is from the Latin granum, for “grains”. feldspar (*) Pegmatite Granite - intrusive Pegmatite a Special Case Diorite - intrusive PEGMATITES are tourmaline classified as intrusive igneous rocks, but there is a DIORITE is very similar to difference. They are VERY feldspar coarse grained and strictly granite, but is distinguished speaking are not crystallizing in the hand specimen by the out of a magma. absence of visible quartz. The coarse grained nature is mica the result of crystal growth in aqueous solutions rather than Generally it has a salt and in the molten liquid state. pepper appearance (about The resulting freedom of ion ½ black and ½ white). quartz motion allows the crystal to grow much larger in a shorter length of time. (*) feldspar biotite (*) Gabbro - intrusive Peridotite - intrusive GABBRO is a coarse-grained PERIDOTITE or DUNITE is rock that is high is iron & composed of 90-100% magnesium-bearing minerals olivine. As a result it is (pyroxenes, amphiboles, characteristically olive-greens plagioclase feldspar, olivene). in color. The rocks will be dark in color, somewhat heavier than granitic rocks and devoid of This material is thought to quartz. have originated in the upper Black minerals are primarily amphibole (like mantle of the Earth. (*) hornblende) and plagioclase feldspar. (*) Rhyolite – extrusive Rhyolite – extrusive RHYOLITE’S name comes from sized crystals develop. The the Greek rhyo, from rhyax, volcanoes that produce “stream of lava”. It is formed rhyolite are very explosive when molten rock with the varieties such as Mt. St. same composition as a high Helens, Krakatoa and silica granite oozes (rhyolite is O’Leary Peak (AZ). This is another sample of rhyolite. VERY viscous and does not Frequently it is banded due This has a crystal of garnet that really flow) to the Earth’s to flow alignment of different grew after the rhyolite was surface; and therefore cools associated minerals (quartz, deposited. (*) This is an example of quickly so only microscopic- feldspar, mica, and banded rhyolite. (*) hornblende). (*) If you look closely, you might see tiny clear phenocrysts of feldspar. (*) Andesite - extrusive Basalt - extrusive ANDESITE is the fine- BASALT occurs as thin to is dark, fine-grained and massive lava. flows, often vesicular (having gas grained equivalent of pockets). The pockets may DIORITE. It tends to be sometimes accumulating to be filled with secondary thicknesses of thousands of a darker gray than minerals, e.g. quartz, zeolite feet and covering thousands minerals, calcite, opal, etc. hornblende rhyolite and is often of square miles. The and then it is called phenocrysts (*) porphyritic, with visible volcanoes that produce amygdaloidal (a-mig-duh-loy- hornblende. basaltic lavas are relatively dal) basalt. quiet, such as the Hawaiian The name may have originated Islands volcanoes. Basalt with Pliny who used the Ethiopian word basal for iron- bearing rocks. (*) Basalt - extrusive Basalt - extrusive Vesicular (Scoria) Volcanic Bomb (*) Peridotite xenolith Basalt (*) Gases released near the surface of a lava flow create bubbles or vesicles that are “frozen” in stone. (*) Amygdaloidal Peridotite (*) (*) Basalt - extrusive Obsidian - extrusive Pahoehoe is a feature that forms on the OBSIDIAN is volcanic glass (an surface of very fluid have cooled so quickly that acrystalline “solid” –actually a basalt flows. Much the minerals could not like the skin on a bowl supercooled liquid). Its develop and crystallize. of tomato soup – the glassy, lustrous and Colors vary from black to red, surface in contact with sometimes banded black & red (mahogany), the air begins to appearance makes it rather gray, green, iridescent, crystallize, while the easy to distinguish from all snowflake. fluid lava below other rocks. It is composed Apache Tears are little nodules continues to flow. This of the elements that make of obsidian. drags the upper, still quartz, feldspar and iron/ plastic, surface into a magnesium minerals that series of smooth wrinkles. (*) Obsidian - extrusive Apache tear Perlite - extrusive Flow banding PERLITE is a light gray volcanic Apache Tear glass, having numerous concentric cracks which give rise to a perlitic or onion skin structure. It is generally of rhyolitic composition. It also exhibits a pearly luster. Apache Tears are some times found embedded in perlite. Snowflake Pumice - extrusive Tuff - extrusive PUMICE is highly vesicular Tuff is the accumulation of ash (i.e. it has lots of gas and small pyroclastic debris bubble holes) and is of (<4mm). Thick beds may rhyolitic composition. form for many miles around a very explosive volcano, Due to the many small such as Mt. St. Helens. holes, it is lightweight Often, when the material is still and will often float in hot upon deposition, it welds water. into a very hard rock. Volcanic Breccia - extrusive SEDIMENTARY ROCKS VOLCANIC BRECCIA is SEDIMENTARY ROCKS are composed of particles pyroclastic (fire-formed derived from pre-existing rocks or by the fragments) and forms in crystallization of minerals that were held in solutions. explosive eruptions. It is A general characteristic of this group is the layering or a mix of large angular stratification, as seen in the outcrop. fragments and small ash. Those sedimentary rocks that are CHEMICAL sedimentary rocks are the Often, the material is hot composed of particles of pre-existing result of either precipitation of solids when it comes to rest rocks are considered FRAGMENTAL from solutions (like salt from water) or or CLASTIC. These fragments show by organic process, like shells from and cools (welds) into a evidence of transport – rounding of the marine organisms. very hard rock. grains and size sorting. CLUES TO Breccia - fragmental SEDIMENTARY ROCKS FRAGMENTAL: Composed of pieces of rocks and minerals A BRECCIA is made of varying sizes of angular fragments BRECCIA: Composed of large angular pieces and clay cemented together. The LARGE PIECES CONGLOMERATE: Composed of large rounded pieces and name is from the Italian word (Boulders,cobbles, clay pebbles) for “broken stones” or “rubble”. SMALL PIECES SANDSTONE: Looks sandy (may “shed” sand grains), feels Many form as the result of fault (sand) rough (like sandpaper); may be tan, white, red, gray. movement; others form as the result of rapid and short SHALE: Has very thin layers; often black.