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Rocks and Minerals Igneous Rocks Igneous Rocks Fig. 5-12d, p. 145 Igneous Rocks Fig. 5-5b, p. 138 Sedimentary Rocks Sedimentary Rocks Conglomerate Sedimentary Rocks Meteetsee Spires – just SE of Red Lodge Metamorphic Rocks Metamorphic rocks exposed at Mt. Everest. Deformation occurs at various scales A mica garnet schist Metamorphic Rocks Earth Materials • The Crust and its Composition • Atoms and Minerals • Igneous Rocks • Sediments and Sedimentary Rocks • Metamorphic Rocks • The Rock Cycle The Crust is the outer 8 to 75 km of the Earth Composition of he Crust oxygen and silicon account for about 75% of the earth's crust metallic elements iron, aluminum and the base elements account for most of the rest The Crust and its Composition the elements of the crust are combined in inorganic chemical compounds called minerals these minerals are mixed together in various proportions to form different rock classes rocks of the Earth's crust are grouped into three major classes: igneous, sedimentary and metamorphic rocks The Nature of Minerals • Mineral – A naturally occurring – inorganic solid – that has an exact (or clearly defined range) chemical composition – with an orderly internal arrangement of atoms – generally formed by inorganic processes. Minerals • Internal structure – Repetitive geometric pattern of atoms – Expressed in physical properties • Interfacial angles • Cleavage • Revealed in X-ray diffraction Ionic Bonding Periodic Table of the Elements 8.3 Polymorphs of Carbon Minerals Physical Properties • • • • • Cleavage/Fracture Optical Properties: Luster, Color, Streak Hardness Density Other: Magnetism, reaction to dilute HCl, salty taste, … Perfect cleavage in 1 plane Perfect cleavage in 2 planes Perfect cleavage in 2 planes @ 90° Perfect cleavage in 2 planes NOT @ 90° Rock-Forming Minerals • About 20 common minerals make up most rocks – Silicates dominate – Quartz, Feldspars, Mica, Amphiboles, Pyroxenes – Carbonates are common; limestones – Evaporite minerals, salt mines, gypsum – Secondary minerals formed during weathering, e.g. Aluminum and Iron oxides Silicate Minerals • Most common minerals on Earth – Comprise 95% of the volume of the crust – Approximately 75% of the Earth’s mass is made up of silicon and oxygen – All silicate minerals are based on the silica tetrahedron • SiO4-4 Silica Tetrahedron Silicate Structures Isolated Single chain Double chain Sheet Solid Clay Minerals • Sheet silicates similar to mica • Products of chemical weathering near the Earth’s surface • Usually microscopic crystals – Kaolinite SEM photograph of clay minerals: authigenic chlorite flake from the Watahomigi Formation in Andrus Canyon, Supai Group, Grand Canyon; x 20,900. Figure 05-D, U.S. Geological Survey Professional Paper 1173. Nonsilicate Minerals • Usually form at low temperatures (reactions that occur at the surface of the E arth ) – Carbonates (biologic) • Calcite - Ca CO3 • Dolomite - CaMg(CO3)2 – Evaporite Minerals (seawater evaporation) • Gypsum - CaSO4-2H2O • Halite - NaCl – Oxides (rust and weathering) • Hematite Igneous Rocks and the Rock Cycle The Rock Cycle • A rock is a naturally formed, consolidated material usually composed of grains of one or more minerals • The rock cycle shows how one type of rocky material gets transformed into another Imagine the first rock and the cycles that it has been through. Igneous Rocks • Form from Magma – Hot, partially molten mixture of solid liquid and gas – Mineral crystals form in the magma making a crystal “slush” – Gases - H2O, CO2, etc. - are dissolved in the magma Igneous Rocks • Magma vs. Lava – Magma is molten rock beneath the surface – Lava is molten rock that has reached the surface – Magma solidifies to form intrusive igneous rocks – Lava solidifies to form extrusive igneous rocks Distribution of igneous rocks in North America Distribution of igneous rocks in Montana - Plutonic Distribution of igneous rocks in Montana - Volcanic Igneous Textures • Texture - the size, shape and relationship of mineral crystals in the rock • Reflects cooling history of the magma or lava • Slow cooling rate • Fast cooling rate • Very fast cooling rate >> Big crystals >> Small crystals >> glass Phaneritic (coarse-grained) texture in granite Aphanitic (fine-grained) texture in rhyolite Glassy texture in obsidian Porphyritic Texture • Well formed crystals (phenocrysts) • Fine grained matrix (groundmass) • Complex cooling history – Initial stage of slow cooling • Large, well formed crystals form – Later stage of rapid cooling • Remaining magma crystallizes more rapidly Porphyritic igneous rock: Big xtals in a fine grain matrix Pyroclastic Texture • Produced by explosive volcanic eruptions • May appear porphyritic with visible crystals – Crystals show breakage or distortion • Matrix may be dominated by glassy fragments – Fragments also show distortion – Hot fragments may “weld” together Pyroclastic texture Extrusive Rock Bodies Volcanic • Form of extrusive bodies influenced by magma properties – Composition • Silica content – Viscosity • Volatile content • Temperature Aa flow Pahoehoe flow Figures 4.6 A & B movies F ountain.mov Stromboli.proj.mpg Lava4.mov MainC rater.mpg Redflow.mpg PyroclasticFlow.mov O ldfaith.av i JuandeFucaSmok er.av i Smoker.mov Tube3.mov Devil’s Tower; a volcanic neck, a feeder pipe Shiprock, New Mexico; a volcanic neck Rhumski, Cameroon; a volcanic neck Sill; parallels layers in the country rock Dike; cuts across layers in the country rock Half Dome; part of the Sierra Nevada batholith Beginnings of a spatter cone Large cinder cone Flood basalts with several thick and thin layers. Each layer represents a separate eruption. pillow lavas http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html Formation of Volcanic Domes Mt Fuji: Stratovolcano Mt. St. Helen's prior to 1980 eruption, a classic stratovolcano http://www.youtube.com/watch?v=bgRnVhbfIKQ Process of formation of ash flow caldera - e.g., Crater Lake, Oregon or the super Caldera of Yellowstone Size comparison of various volcanic features Intrusive Rock Bodies Plutonic • Less dense magmas rise through the crust • Rising magmas slowly cool – Viscosity increases – Density increases • Intrusions form as magma solidifies beneath the surface Figure 4.18. Types of magmatic intrusions Metamorphosis… One of the oldest rocks in the world. A gneiss produced by metamorphosis of an even older shale. Metamorphism • The transformation of rock by temperature and pressure • Metamorphic rocks are produced by transformation of: • Igneous, sedimentary and even other metamorphic rocks What causes metamorphism? • Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals • Pressure (stress) • Increases with depth • Pressure can be applied equally in all directions or differentially, i.e. directed Main factor affecting metamorphism • Parent rock • Metamorphic rocks typically have the same chemical composition as the rock they were formed from • Different minerals, but made of the same stuff. Metamorphism • Three types of metamorphic settings: • Contact metamorphism – from a rise in temperature within host rock • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock Contact metamorphism Produced mostly by local heat source Hydrothermal Metamorphism Circulation of hot fluids through cracks and porous rock Important source of ores Regional Metamorphism: Subduction zones ….. …and/or deep burial Metamorphic textures • Foliation • Foliation can form in various ways: – Rotation of platy or elongated minerals – Recrystallization of minerals in a preferred orientation – Changing the shape of equidimensional grains into elongated and aligned shapes Development of foliation due to directed pressure Foliated vs. Nonfoliated textures under the microscope Flattened Pebble Conglomerate = flattening Progressive metamorphism of a shale Shale Slate Phyllite (left) and Slate (right) lack visible mineral grains Phyllite Schist Schist A mica garnet schist Gneiss Gneiss displays bands of light and dark minerals Gneiss Change in metamorphic grade with depth Metamorphic rocks exposed at Mt. Everest. Deformation occurs at various scales Outcrop of foliated gneiss Common metamorphic rocks • Nonfoliated rocks • Quartzite – Formed from a parent rock of quartz-rich sandstone – Quartz grains are fused together – Forms in intermediate T, P conditions Sample of quartzite Thin section of quartzite Common metamorphic rocks • Nonfoliated rocks • Marble – Coarse, crystalline – Parent rock usually limestone – Composed of calcite crystals – Fabric can be random or oriented Marble (Random fabric = annealing; nonfoliated) Marble Question: Where do we see metamorphic rocks in outcrops? North American Craton Shield Western North American Mobile Belt Platform Eastern Nor American Mobile Be Answer: In continental shields and uplifted basement rocks Sedimentary Rocks The Nature of Sedimentary Rocks • Sedimentary rocks are composed of: – Fragments of other rocks (detrital or clastic) – Chemical precipitates – Organic matter or biochemically produced materials Types of Sedimentary Rocks Detrital Chemical Biologic Clastic Texture Crystalline Texture The Nature of Sedimentary Rocks • Sedimentary rocks are common at the Earth’s surface – Cover ~75% of the continents – Cover nearly all of the ocean floor – Easily eroded – Occur in distinct layers (strata) The Nature of Sedimentary Rocks • Layers are easily identified – Majors layers (formations) easily recognized over large distances – Smaller layers within a formation are separated by bedding planes – Gradation in grain size, composition or physical features may vary Sedimentary layers may extend for many miles Identifying and correlating the layers is Stratigraphy. More on that later. Rock Identification is based on: • Composition What minerals make up the rock? These can easily be confused • Texture What is the shape, size and orientation of the mineral grains that make up the rock? Major Classes: Clastic Crystalline (chemical and/or biochemical) Biologic (coal, fossiliferous limestones, etc.) Clastic Sedimentary Rocks • Made of rock & mineral fragments or clasts • Clasts are broken and worn particles transported by water, wind or ice • Clastic rocks are subdivided by grain size Clastic Sedimentary Rocks • Grain size is controlled by: – Size and mineralogy of grains in source rock – Carrying capacity of transport process – Weathering and erosion that occurs during transportation – Energy of the depositional environment Grain size ranges for classification of common clastic sedimentary rocks Clastic Sedimentary Rocks • Common clastic sedimentary rocks – Conglomerate – Sandstone – Mudstone or Shale Conglomerate Sandstone Shales Shales Shales erode very easily and form slopes Chemical/Biochemical Sedimentary Rocks • Formed by a process that takes ions from solution to form a solid – Chemical Sediments • Precipitates from water by an inorganic process, e.g. evaporites – Biochemical Sediments • Formed during the growth of some organism Chemical/Biochemical Sedimentary Rocks • Subdivided by composition and mode of formation • e.g., Limestone – Biochemical formation by algae, coral, etc. – Direct chemical precipitate from warm sea water oolites – Chemical precipitate from springs and in caves Chemical/Biochemical Sedimentary Rocks • Common Chemical/Biochemical rocks: – Dolostone - composed of dolomite – Chert - microcrystalline quartz • Various modes of formation – Evaporites • Rock salt - halite • Gypsum Limestones Limestones Limestones Oolitic Limestone Chalk (Coccolithophores) Travertine (Limestone) Dolostone Chert (Flint, Jasper, Agate…) Evaporites: Bonneville Salt Flats, Utah Rock Gypsum Rock Salt