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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:
texturecomposition
Sedimentary Rocks
Metamorphic rocks
Winter (2010) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
Metamorphic mineral assembalges (for mafic rocks)
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
After Spear (1993)
Table 25-1. Definitive Mineral Assemblages of Metamorphic 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.
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 produce many fine-grained crystals. Very rapid cooling involves little if any nucleation or growth (Tc) producing a glass.
Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Intrusive rocks
Porphyry
Pyroclasts
Extrusive rocks
Volcanic ash Bomb PumiceExtrusive pyroclasts form in violent eruptions from lavain the air.
Mafic Felsic
Basalt RhyoliteExtrusive igneous rocks cool rapidly and are fine-grained.
Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form.
Phenocrysts
Porphyry
Some phenocrysts grow large, but theremaining melt cools faster, forming smaller crystals during an eruption.
Igneous Textures
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
Temperature
~600°C
~1200°C
Orthoclase feldspar
Muscovite mica
Quartz
Biotitemica
Amphibole
Pyroxene
Olivine
Sodium-rich
Calcium-rich
Magmacomposition
Felsic,Rhyolitic
(high silica)
Intermediate,andesitic
Mafic,basaltic
Ultramafic(low silica)
Simultaneouscrystallization
Igneous Rocks – Pyroclastic Rocks
Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Common Primary Minerals
Silicate ion (SiO44–)
Oxygen ions(O2–)
The silicate ion forms tetrahedra.
Silicon ion(Si4+)
Common Primary Minerals
Mineral Chemical formula
Cleavage planesand number of
cleavage directionsSilicate
structure Specimen
1 plane Isolatedtetrahedra
2 planes at 90°Single chains
2 planes at 60°and 120° Double chains
1 plane Sheets
Olivine
Pyroxene
Amphibole
Micas
Feldspars
2 planes at 90° Three-dimensionalframework
(Mg, Fe)2SiO4
(Mg, Fe)SiO3
Ca2(Mg, Fe)5Si8O22(OH)2
Muscovite:KAl2(AlSi3O10)(OH)2
Biotite:K(Mg, Fe)3AlSi3O10(OH)2
Orthoclase feldspar:KAlSi3O8
Plagioclase feldspar: (Ca, Na) AlSi3O8
ORIGINAL ROCK
REACTION WITH THERMAL
FLUID AND/OR STEAM
DEVITRIFICATION
RECRYSTALLIZATION
DEPOSITION
HYDROTHERMALLY
ALTERED ROCK
Hydrothermal Alteration
Vesicles filled by
deposition
Altered rock
Empty vesicles in
rock
Fresh 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-
3592. c. Variation in the viscosity of rhyolite at 1000oC 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
Pillow lava
Aa lava
Pahoehoe lava
Magmatic Eruptions
Eruptive Styles & Landforms
Caldera – Hollahraun/Bárdarbunga
Magmatic Eruptions
Phreatomagmatic eruption – Surtseyan Eruption
Eruptive Styles & Landforms
Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Cross sectional structure and morphology of small explosive volcanic
landforms with approximate scales
Schematic cross section through a lava dome.
Types of pyroclastic flow deposits.
Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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.
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
