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Rocks & Minerals (Adapted from: Geology and Geological Features: A Photographic Slide Collection compiled by B.F. Kean, 1993) Minerals are naturally formed crystals that are com posed of one or m ore chem ical elem ents. They are disting uished from other natural solid m ate rials by their crysta lline structure. S om e natu ral solid m ate rials form ed in ro ck s are not m inerals bec aus e they lack a crystalline stru cture , i.e., they are am orph ous , eg., obsidian or volcanic glass. Most minerals are formed by inorganic processes; however, a few are formed by organic proc ess es (living orga nism s), eg ., calcite in cora l. Single-element m inerals include diamond (from carbon) and the native (i.e., elemental) metals gold, copper and silver. Other minerals, such as chalcopyrite, sphalerite and mica, are chemical compounds con sisting of se vera l elem ents . Rock s are m ade up of m inerals, the exce ption being glassy rocks (eg., som e rhyolites). Most rock s are polymineralic (i.e., contain more than one mineral); however, a few are monom ineralic, eg., limestone and m arble. Rock-forming processes, some rapid and others requiring millions of years, are as active today as in the geo logica l past. Rocks are divided into three main classes: IGNEOUS ROCKS are formed as m agma (m olten rock) cools and solidifies. Magma is produced far below the Earth's surface by heat generated mainly from radioactive disintegration of uranium, thorium and potassium. The two main types of igneous rock are extrusive and intrusive. Extrusive rocks, or lava, form when m agm as rea ch the surface of the Earth through volcanic fiss ures or vents. They cool ra pidly and are fine-grained or glassy, eg., basalt and obsidian. Explosive eruptions produce rocks composed of rock dust or fragments (pyroclasts). Intrusive rocks form when m agma cools slowly at depth, eg., granite, allowing the constituent minerals to grow much larger. These m agmas intrude fissures and other zones of weakness in the Earth's crust, crystallize, and are eventually uplifted and exposed by erosion. Some magm as carry fragments of rocks and minerals from deep within the Earth, and this allows us to study samples from regions too deep to be reached by drilling. The table below shows the relationship between different igneous rocks and how they form. Extrusive Igneous Rocks (volcanic) Intrusive Igneous Rocks Main Minerals Basalt Andesite Rhyolite Gabbro stock; diabase dyke Diorite stock; or dyke Granite batholith or stock; pegmatite dyke Feldspar pyroxene olivine Feldspar pyroxene olivine Quartz feldspar mica olivine (dark coloured) MAGMA SiO2 (light coloured) MgO SEDIMENTARY ROCKS are fo rm ed b y the ac cum ulation and cem enting of loose s edim ent (e g., sandstone), the deposition of chemical compounds held in solution in water (eg., limestone), or by the acc um ulation of an imal or plant debris (e g., coal). W ind, rive r an d ocean currents are the m ain m eth ods of tra nsport. T his transported m ate rial settles to form distinct layers, which are compressed by overlying layers and cemented to form solid rock. Rock layers c an b e distinguished from eac h other by difference s in grain size, colou r and com pos ition. Sedimentary rocks may have a coarse-grained, gravel-like appearance or be extremely fine grained, and may be hard or soft. The principal varieties are sandstone, limestone and shale. Many sedimentary rocks conta in fo ssils and som e, such as coral re efs, a re com posed entirely of such organic re m ains. T he table below illustrates how s edim entary rocks form . 1. Surface rock Conglomerate erosion Sandstone Shale 2. Chemicals dissolved in sea water precipitation Limestone, Dolostone accumulation 3. Animal (shell, coral) or plant remains Coal seams 4. Salts dissolved in sea water evaporation Salt, Potash, Gypsum METAMORPHIC ROCKS are formed when rocks of any class are subjected to heat and pressure at depth. This causes new minerals to form and other minerals to recrystallize. During the process, material from the rock may be added or lost. Marked changes in temperature (T) and pressure (P) occur, which may produce completely new types of rocks. In addition to the formation of new minerals, existing minerals may be realigned into parallel bands and new textures may be formed. At sufficiently high temperatures, the rock may undergo partial melting to form m agma, which may then become the source of an igneous roc k. Meta m orphic ro ck s include, gneiss, s chist, s late , quartzite and m arble, and the table below show s how they form . Sandstone Shale Limestone T,P T,P T,P high T, P flaky cleavage high T, P partial melting, mineral bands Quartzite Slate Marble Schist Gneiss Igneous, sedimentary or other metamorphic rocks high T, P extensive melting Migmatite The Geological Cycle Soone r or later m ost ro ck s are exp ose d to the proc ess es o f erosion. The res ulting produ cts a ccu m ulate to form sed imenta ry rock s. Thes e m ay be b uried to great depths and con verted by he at and pre ssu re into m etam orphic rocks . At still greater depths, they m ay be m elted to form m agm a, which if crystallized form s igneous rocks; if uplifted and exposed, these renew the cycle. The cycle may, however, be interrupted and follow any of the paths shown in the diagram below. MA MEL TIN G N IO AT CR YS TA L H RP MO S TA CK ME RO IZ MAG MET AM ORPHISM IGNEOUS RO CKS IC I ROMEN CK TA S RY N O SI RO IO (E AT T ING R ER SPO TH WEA TRAN AND SE D N SE DIM EN TS ND GA CEMENTIN NG I N E D HAR Click here for a “Schematic cross-section depicting geological concepts and processes”. PYROCLASTICS VOLCANIC EJECTA VARIABLE CLAST SIZE VOLCANIC VENT STOCK VOLCANIC ROCKS SILL LAVA DUST CLOUD UNC O CUTS ACROSS LAYERS COOLS QUICKLY SMALL CRYSTALS DYKE (erosio n) COOLS RAPIDLY FINE GRAINED PARALLELS LAYERS COOLS QUICKLY SMALL CRYSTALS GRANITE BATHOLITH COOLS SLOWLY LARGE CRYSTALS N FO RM ITY SEDIMENTS SEDIMENTARY ROCKS METAMORPHIC ROCKS U N C ONFO RMIT Y FA UL T TILL UNCONSOLIDATED SURFACE MATERIAL UNSORTED (erosion) OLDER METAMORPHIC ROCKS