0 TRAINING ON SURFACE EXPLORATION STUDIES FOR GEOTHERMAL RESOURCES AND DEVELOPMENT OF CONCEPTUAL MODELS UNDER THE AUSPICES OF INTERIM PROJECT COORDINATION UNIT OF THE AFRICA GEOTHERMAL CENTER OF EXCELLENCE Rock types and minerology (Igneous/Sedimentary /Metamorphic) with emphasis Igneous petrology/volcanology Dr. Tobias Björn Weisenberger, Iceland GeoSurvey Petrography - overview 1. Rock Types 2. Igneous Rocks 3. Minerals (primary & secondary) 4. Volcanology Earth Materials • All Earth materials are composed of atoms bound together. • Minerals are composed of atoms bonded together and are the building blocks of rocks. • Rocks are composed of minerals and they record various geologic processes. Rock Types Rocks are naturally occurring solid aggregates of minerals, or in some cases, non-mineral solid matter. Identity is determined by: texture composition Sedimentary Rocks Metamorphic rocks Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Metamorphic mineral assembalges (for mafic rocks) Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies Facies Definitive Mineral Assemblage in Mafic Rocks Zeolite zeolites: especially laumontite, wairakite, analcime Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite) Greenschist chlorite + albite + epidote (or zoisite) + quartz ± actinolite Amphibolite hornblende + plagioclase (oligoclase-andesine) ± garnet Granulite orthopyroxene (+ clinopyrixene + plagioclase ± garnet ± hornblende) Blueschist glaucophane + lawsonite or epidote (+albite ± chlorite) Eclogite pyrope garnet + omphacitic pyroxene (± kyanite) Contact Facies Mineral assemblages in mafic rocks of the facies of contact meta- morphism do not differ substantially from that of the corresponding regional facies at higher pressure. After Spear (1993) Low Temperature Metamorphic rocks Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Igneous rocks Igneous Rocks - classification Textures: Aphanitic: crystals too small to see by eye Phaneritic: can see the constituent minerals Fine grained- < 1 mm diameter Medium grained- 1-5 mm diameter Coarse grained- 5-50 mm diameter Very coarse grained- > 50 mm diameter Porphyritic: bimodal grain size distribution Glassy: no crystals formed Idealized rates of crystal nucleation and growth as a function of temperature below the melting point. Slow cooling results in only minor undercooling (Ta), so that rapid growth and slow nucleation produce fewer coarse-grained crystals. Rapid cooling permits more undercooling (Tb), so that slower growth and rapid nucleation Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. produce many fine-grained crystals. Very rapid cooling involves little if any nucleation or growth (Tc) producing a glass. Igneous Textures Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine- grained. Porphyry Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Intrusive rocks Some phenocrysts grow large, but the remaining melt cools faster, forming smaller crystals Porphyry during an eruption. Igneous Rocks Igneous Rocks – Classification Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Igneous Rocks – Geochemcial Discrimination Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Igneous Rocks Igneous Rocks – Bowen´s reaction serious Magma composition Temperature Orthoclase feldspar Muscovite mica Felsic, ~600°C Rhyolitic Quartz (high silica) Sodium- Biotite rich mica Intermediate, andesitic Amphibole Mafic, Pyroxene basaltic Simultaneous Ultramafic Olivine Calcium- (low silica) ~1200°C crystallization rich Igneous Rocks – Pyroclastic Rocks Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Common Primary Minerals 4– Silicate ion (SiO4 ) The silicate ion forms tetrahedra. Oxygen ions (O2–) Silicon ion (Si4+) Common Primary Minerals Cleavage planes and number of Silicate Mineral Chemical formula cleavage directions structure Specimen 1 plane Isolated tetrahedra Olivine (Mg, Fe)2SiO4 2 planes at 90° Single chains Pyroxene (Mg, Fe)SiO3 2 planes at 60° and 120° Double chains Amphibole Ca2(Mg, Fe)5Si8O22(OH)2 1 plane Sheets Muscovite: KAl (AlSi O )(OH) Micas 2 3 10 2 Biotite: K(Mg, Fe)3AlSi3O10(OH)2 2 planes at 90° Three-dimensional framework Orthoclase feldspar: KAlSi O Feldspars 3 8 Plagioclase feldspar: (Ca, Na) AlSi3O8 Hydrothermal Alteration Empty vesicles in rock ORIGINAL ROCK DEVITRIFICATION RECRYSTALLIZATION DEPOSITION Fresh rock Vesicles filled by REACTION WITH THERMAL deposition FLUID AND/OR STEAM HYDROTHERMALLY ALTERED ROCK Altered rock Hydrothermal Alteration Hydrothermal Alteration Epidote (>250°C) ca. 1 mm Weisenberger & Selbekk, 2009 Physical Properties of Minerals Mica and its cleavage Calcite and its cleavage Hematite and its streak Volcanology Volcanology Magmas form today in four distinct tectonic environments: • Mid-ocean ridges • Continental rift systems (e.g. East African rift) • Subduction zones (e.g. circum- Pacific belt) • Intraplate settings Oceanic (e.g. Hawaii and other ocean islands) isolated continental volcanoes (e.g Mt. Cameroon) Volcanology Viscosity Calculated viscosities of anhydrous silicate liquids at one atmosphere pressure, calculated by the method of Bottinga and Weill (1972) by Hess (1989), Origin of Igneous Rocks. Harvard University Press. b. Variation in the viscosity of basalt as it crystallizes (after Murase and McBirney, 1973), Geol. Soc. Amer. Bull., 84, 3563- o 3592. c. Variation in the viscosity of rhyolite at 1000 C with increasing H2O content (after Shaw, 1965, Amer. J. Sci., 263, 120-153). Eruption Types Thordarsson & Höskuldsson 2014 − There are two types of eruptions in terms of activity: Effusive eruptions – outpouring of lava Explosive eruptions – gas-driven explosions Types of Basaltic Lava Pahoehoe lava Pillow lava Aa lava Magmatic Eruptions Eruptive Styles & Landforms Caldera – Hollahraun/Bárdarbunga Magmatic Eruptions Phreatomagmatic eruption – Surtseyan Eruption Eruptive Styles & Landforms Schematic cross section through a lava dome. Cross sectional structure and morphology of small explosive volcanic landforms with approximate scales Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Types of pyroclastic flow deposits. a. Collapse of a vertical explosive or plinian column that falls back to earth, and continues to travel along the ground surface. b. Lateral blast, such as occurred at Mt. St. Helens in 1980. c. “Boiling-over” of a highly gas-charged magma from a vent. d. Gravitational collapse of a hot dome. e. Retrogressive collapse of an earlier, unstably perched ignimbrite. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Subglacial Eruptions after Werner & Schmincke (1999) Flood Basalt (Fissure eruption) Explosive Eruptions Pyroclastic deposits − Fallout deposits Pyroclastic deposits − Flow deposits −surges −lahars Pyroclastic deposits Laacher See, Germany Volcaniclastic fragments Ignimbrite - consolidated Fall deposit (pumice) unconsolidated Pyroclastic fragments Ash Bombs Lapilli.