Transcript CHAPTER – 3
Chapter Three Igneous Activity and Plate Tectonics 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 – Representation of how rocks are formed, broken down, and processed in the geosphere – Arrows indicate possible process paths within the cycle CHAPTER – 3 IGNEOUS ROCKS *IGNEOUS ROCKS: ROCKS THAT COOLED AND FIRE CRYSTALLIZED DIRECTLY FROM MOLTEN ROCK, EITHER AT THE SURFACE OR DEEP UNDERGROUND *MAGMA: MOLTEN ROCK WITHIN THE EARTH : *LAVA: WHEN MAGMA REACHES EARTH’S SURFACE *MOST IGNEOUS PROCESSES ARE HIDDEN FROM VIEW *REGIONAL EROSION EXPOSES ANCIENT IGNEOUS EVENTS DISTRIBUTION OF MAJOR CONTINENTAL IGENOUS ROCKS MOLTING ROCKS AND CRYSTALLIZING MAGMA • MAGMA FORMATION: HEATED UNGERGROUND MINERALS – BONDS BROKEN- BECOMES MAGMA – CHEMICAL COMPOSITION OF MAGMA CHANGES • MAGMA COOLING AND CRYSTALLIZATION: AS COOLING PROGRESSES, DIFFERENT MINERALS CRYSTALLIZE EFFECT ON THERMAL ENERGY The Rock Cycle and Plate Tectonics • Magma is created by melting of rock above a subduction zone • Less dense magma rises and cools to form igneous rock • Igneous rock exposed at surface gets weathered into sediment Convergent plate boundary • Sediments transported to low areas, buried and hardened into sedimentary rock • Sedimentary rock heated and squeezed at depth to form metamorphic rock • Metamorphic rock may heat up and melt to form magma CLASSIFICATION OF IGNEOUS ROCKS: • TEXTURE: SIZE AND SHAPE OF MINERAL CRYSTALS CRYSTAL GROWTH DURING COOLING • MINERAL CONTENT:CHEMICAL COMPOSITION COOLING HISTORY • TEXTURE: RATE AT WHICH MAGMA OR LAVA COOL WHEN 100 – 1000 YRS FOR COOLING TIME TO GROW LARGER CRYSTALS CRYSTALS CAN BE VISIBLY SEEN PHANERTIC TEXTURE • INTRUSIVE ROCKS (OR PLUTONIC ROCKS) SLOW COOLING OCCURS WHEN MAGMAS INTRUDE PREEXISTING SOLID ROCKS Igneous Rocks • Magma is molten rock • Igneous rocks form when magma cools and solidifies – Intrusive igneous rocks form when magma solidifies underground Granite • Granite is a common example – Extrusive igneous rocks form when magma solidifies at the Earth’s surface (lava) • Basalt is a common example Basalt •PEGMATITES: IGNEOUS ROCKS WITH EXCEPTIONALLY LARGE CRYSTALS (QUARTZ, MICA, FELDSPAR ARE COMMON) •EXTRUSIVE OR VOLCANIC ROCKS: WHEN ROCKS SOLIDIFY QUICKLY, CRYSTALS ARE SMALL APHANITIC TEXTURE – ROCKS WITH AHANITIC STRUCTURE ARE CALLED EXTRUSIVE ROCKS •PORPHYRITIC STRUCTURE: LARGER AND SMALLER GRAINS – SLOW COOLING FOLLOWED ABRUPTLY BY RAPID COOLING *VOLCANIC GLASS: WHEN LAVA SUDDENLY COOLS, NO TIME TO FORM CRYSTALS. TEXTURE IS GLASSY. *PUMICE: FORMS WHEN HIGHLY GASEOUS, SILICA – RICH LAVA COOLS VERY RAPIDLY *OBSIDIAN: VERY Si-RICH LAVAS CONTAINING LESS GAS, COOL VERY QUICKLY Igneous Rock Textures • Texture refers to the size, shape and arrangement of grains or other constituents within a rock • Texture of igneous rocks is primarily controlled by cooling rate • Extrusive igneous rocks cool quickly at or near Earth’s surface and are typically finegrained (most crystals <1 mm) • Intrusive igneous rocks cool slowly deep beneath Earth’s surface and are typically coarse-grained (most crystals >1 mm) Coarse-grained igneous rock Fine-grained igneous rock Special Igneous Textures • A pegmatite is an extremely coarse-grained igneous rock (most crystals >5 cm) formed when magma cools very slowly at depth • A glassy texture contains no crystals at all, and is formed by extremely rapid cooling Pegmatitic igneous rock • A porphyritic texture includes two distinct crystal sizes, with the larger having formed first during slow cooling underground and the small forming during more rapid cooling at the Earth’s surface Porphyritic igneous rock IGNEOUS COMPOSITION • MAGMA O2, Si, Al, Fe, Ca, Mg, Na, K, S. • DISSOLVED GASSES WATER VAPOR, CO2, SO2. • SILICATES ARE THE MAJOR CONSTITUENTS OF IGNEOUS ROCKS Igneous Rock Identification • Igneous rock names are based on texture (grain size) and mineralogic composition Textural classification • – – • Plutonic rocks (gabbro-diorite-granite) are coarse-grained and cooled slowly at depth Volcanic rocks (basalt-andesite-rhyolite) are typically fine-grained and cooled rapidly at the Earth’s surface Compositional classification – – – Mafic rocks (gabbro-basalt) contain abundant dark-colored ferromagnesian minerals Intermediate rocks (diorite-andesite) contain roughly equal amounts of dark- and light-colored minerals Felsic rocks (granite-rhyolite) contain abundant light-colored minerals CLASSIFICATION OF IGNEOUS ROCKS AND MAGMAS COMPOSITION Si TYPE (%) OTHER MAJOR ELEMENTS VISC. OF IGNEOUS ROCKS PRODUCED MAGMA FELSIC >65 Al, K, Na HIGH ~ 600 – 800 0C INTERMEDIATE 55-65 Al, Ca, Na, Fe, Mg MEDIUM ~ 800 – 1000 0C MAFIC (BASALT) 45 - 55 Al, Ca, Fe, Mg LOW ~ 1000 – 1200 0C ULTRAMARIC (PERIDOTITE) < 40 Al, Ca, Fe, Mg VERY LOW > 1200 0C IGNEOUS ROCK - CHART ULTRA MAFIC IGNEOUS ROCKS: * < 40% Si. * EX: PERIDOTITE & KOMATITE * OCCURRENCE: RARE AT EARTH’S SURFACE MAFIC IGNEOUS ROCKS: * 45 – 55 % Si. * EX: BASALT & GABBRO * OCCURRENCE: COMMON ON OCEAN FLOORS AND CONTINENTS INTERMEDIATE IGNEOUS ROCKS: * 55 – 65 % Si. *EX: ANDESITE & DIORITE * OCCURRENCE: ABUNDANT VOLCANIC ROCK. FELSIC IGNEOUS ROCKS: * > 65% Si. * EX: GRANITE & RHYOLITE * OCCURRENCE: COMMON ON CONTINENTS CREATION OF MAGMA: * PARTIAL MELTING WHEN ROCKS MELT TO PRODUCE MAGMA PARTIAL MELTING DIFF. MELTING POINT EX: ALBITE = 1118 0C ANORTHITE = 1553 0C * MELTING OF ROCKS DEPENDS ON HEAT PRESSURE AMOUNT OF H2O IN THE ROCKS. • THERMAL ENERGY HEAT SOURCES *HEAT: SOURCE OF HEAT IN THE INTERIOR * DECAY OF RADIOACTIVE ISOTOPES *RESIDUAL FROM EARTH’S FORMATION *FRICTIONAL HEAT FROM PLATE MOTION *HIGH PRESSURE: THE IONS AND ATOMS IN A CRYSTALLINE SOLID CLOSER TOGETHER – HIGH TEMP IS REQUIRED TO VIBRATE, WEAKEN, AND BREAK THEIR BONDS. *AS PRESSURE INCREASES, THE TEMPERATURE AT WHICH ROCKS MELT INCREASES EX: Na – FELDSPAR ALBITE MELTS AT 1118 0C AT 100 KM PRESSURE IS 35, 000 TIMES HIGHER–MP 1440 0C GEOTHERMAL GRADIENT MELTING TEMPERT.-DRY MELTING TEMPERT.-WET FLUIDITY AND VISCOSITY OF MAGMA: MAGMA RISES BECAUSE IF IT IS LESS DENSE THAN SURROUNDING ROCK EXPANDING GASES DRIVE IT UPWARD IT IS SQUEEZED UPWARD BY SURROUNDING ROCKS VISCOSITY: FLUID RESISTANCE TO FLOW A) INCREASES WITH DECREASING TEMPERATURE B) MINERAL (SILICA) CONTENT INCREASES VISCOSITY VALUE. CRYSTALLIZATION OF MAGMA: *MINERALS MELT AT THE SAME TEMPERATURE AT WHICH THEY CRYSTALLIZE FIRST TO MELT LAST TO CRYSTALLIZE * AT EACH STAGE OF COOLING, CRYSTAL/LIQUID RATIO CHANGES OCEANIC PLATE SUBDUCTS MAGMA MIXING BOWEN’S REACTION SERIES: A) BOTH MAFIC AND FELSIC ROCKS CAN CRYSTALLIZE FROM AN ORIGINALLY MAFIC MAGMA B) EARLY – FORMING CRYSTALS REMAINING IN CONTACT WITH THE STILL – LIQUID MAGMA REACT WITH IT TO EVOLVE INTO DIFFERENT MINERALS BOWEN’S REACTION SERIES Bowen’s Reaction Series • Minerals crystallize in a predictable order, over a large temperature range Discontinuous branch • – – • Ferromagnesian minerals (olivine, pyroxene, amphibole, biotite) crystallize in sequence with decreasing temperature As one mineral becomes chemically unstable in the remaining magma, another begins to form Continuous branch – Plagioclase feldspar forms with a chemical composition that evolves (from Ca-rich to Na-rich) with decreasing temperature EARLY-FORMING CRYSTALS MAGMA & EARLY FORMING CRYSTALS SILICATE MINERALS CAN CRYSTALLIZE FROM MAFIC MAGMAS TWO WAYS: DISCONTINUOUS SERIES CONTINUOUS SERIES • • • • • • CALCIUM PLAGIOCLASE • SODIUM PLAGIOCLASE OLIVINE PYROXINE AMPHIBOLE BIOTITE MICA MINERALS WITHOUT Fe, Mg. CONTD*AFTER BOTH SERIES COMPLETE, HIGH – SILICA MINERALS FORM EX: K – FELDSPAR MUSCOVITE MICA QUARTZ HOW MAGMA CHANGES AS IT COOLS: CRYSTALS CAN * REMAIN SUSPENDED AND REACT WITH MAGMA * SINK * BE PLASTERED TO THE WALLS OR CEILING OF THE MAGMA *BE FILTERED OUT AS MAGMA FLOWS ELSEWHERE OTHER MAGMA CRYSTALLIZATION PROCESSES: * OTHERS – ASSIMILATION OF ROCK BODIES * MAGMA MIXING INTRUSIVE ROCK FORMATION: • RISING MAGMA MAY FORCE OVERLYING ROCKS TO BULGE UPWARD RESULTING ROCK APPEARS AS A DOOMED INTRUSION WITHIN OTHER ROCKS THIS STRUCTURE IS KNOWN AS DIAPIR • XENOLITHS: WHEN PREEXISTING ROCK IS ASSIMILATED IN A MAGMA, THEY APPEAR IN THE SOLIDIFIED ROCK AS DISTINCT BODIES – XENOLITHS PLUTONS: MAGMA THAT COOL UNDERGROUND FORM PLUTONS CONCORDANT PLUTONS: DISCORDANT PLUTONS: TABULAR PLUTONS PARALLEL TO THE PREEXISTING ROCK LAYERS – CALLED “SILLS” CUT ACROSS THE PREEXISTING LAYERS – CALLED “DIKES” MASSIVE SIZE BATHOLITHS RELATIVELY THIN TABULAR PLUTONS 1. DIKES 2. SILLS – DISTINGUISHED FROM EXTRUSIVE FLOWS BY A)EVIDENCE OF HEATING OF ADJACENT ROCK SURFACES B)EVIDENCE OF INCLUSIONS OF COUNTRY ROCK IN BOTH UPPER & LOWER SILL SURFACE C)LACK OF VESICLES ( HOLES FROM GAS BUBBLES ) ON UPPER SURFACE D)LACK OF WEATHERING OF LARGE SURFACE. PLUTONIC IGNEOUS FEATURES BATHOLITHS AND LARGE PLUTONS: 1. LACCOLITHS 2. LAPOLITHS 3. BATHOLITHS A)DEFINITION B)EXAMPLES C)TEXTURE SEMINARY RIDGE TOPOGRAPHIC RIDGE SILLS AND LAVA FLOWS LACCOLITH PLATE TECTONICS AND IGNEOUS ROCKS: A) THE ORIGIN OF BASALT & GABBROS 1) INTRODUCTION * UPPER MANTLE LACKS LIGHT ELEMENTS *DEEPER MANTLE POSSESSES SOME LIGHT ELEMENTS *PRESENCE OR ABSENCE OF LIGHT ELEMENTS IN GABBRO & BASALT IDENTIFIES SOURCE OF PARENT MAGMA Igneous Activity and Plate Tectonics • Igneous activity occurs primarily at or near tectonic plate boundaries • Mafic igneous rocks are commonly formed at divergent boundaries – • Increased heat flow and decreased overburden pressure produce mafic magmas from partial melting of the asthenosphere Intermediate igneous rocks are commonly formed at convergent boundaries – Partial melting of basaltic oceanic crust produces intermediate magmas Igneous Activity and Plate Tectonics • Felsic igneous rocks are commonly formed adjacent to convergent boundaries – • Hot rising magma causes partial melting of the granitic continental crust Intraplate volcanism – – Rising mantle plumes can produce localized hotspots and volcanoes when they produce magmas that rise through oceanic or continental crust Hawaii is an example PLATE SETTINGS & BASALTS BASALTS-OCEAN & LAND 2) OCEANIC BASALTS a) MORBS FROM UPPER MANTLE b)OIBS ( OCEAN ISLAND BASALTS) FROM DEEPER MANTLE 3) CONTINENTAL BASALTS a) COMPOSITION VARIES WIDELY b) BASALTS NEAR CONTINENTAL RIFTS FROM DEEP MANTLE c)BASALTS NEAR SUBDUCTION ZONES FROM UPPER MANTLE ANDESITE & DIORITE ORIGIN B) ORIGIN OF ANDESITES & DIORITE 1. PROXIMITY TO SUBDUCTION ZONES 2. FACTORS IN FORMATION a) WATER CONTENT b) ASSIMILATION OF COUNTRY ROCKS c) OCEANIC SEDIMENTS C) ORIGIN OF RHYOLITES & GRANITES 1. NEARLY ALL FOUND ON CONTINENTS 2. DERIVE FROM PARTIAL MELTING OF LOWER CONTINENTAL CRUST 3. EXIST NEAR MODERN OR ANCIENT SUBDUCTION ZONES GEOLOGY AT A GLANCE CHAPTER SUMMARY 1. MAGMA, LAVA, DIFFERENCE BETWEEN THEM 2. PROPERTIES TO IDENTIFY IGNEOUS ROCKS 3. FACTOR(S) THAT GOVERN ROCK TEXTURE 4. APHANITIC, PHANERTIC, PLUTONIC, PORPHYRITIC 5. MAJOR ELEMENTS IN IGNEOUS ROCKS 6. CLASSIFICATION OF IGNEOUS ROCKS – BASIS 7. FASTEST COOLING RATE RESULTS 8. EXAMPLES OF APHANITIC ROCK, PHANERTIC ROCK 9. WHEN ROCKS MELT UNDER LOWER TEMPERATURE? 10.FACTORS THAT CONTROL MELTING POINT OF A MINERAL. 11. WHAT IS BOWEN’S REACTION SERIES?IT EXPLAINS WHAT?WHAT IS DISCONTINUOUS SERIES? 12. DIKE, SILL, BATHOLITHS, XENOLITHS 13. OCEANIC CRUST – BASALTS & GABBROS 14. WHAT TYPES OF MAGMA ASSOCIATED WITH WHAT BOUNDARIES