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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