g eol 2312 i gneous and m etamorphic p etrology lecture 3 volcanic rocks: lavas, landforms, and...
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GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY
Lecture 3
Volcanic Rocks:
Lavas, Landforms, and Products
Jan. 26, 2009
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LECTURE OUTLINE Properties of Lavas and Styles of Eruption
Physiochemical Propertieschemistry temperature volatile content viscosity
Factors leading to Explosive Eruptions
Volcanic Landforms Central Vent Landforms
shield volcanoes stratovolcanoes volcanic complexes scoria cones maar tuff rings tuff cones domes calderas
Fissure Eruption Landformsflood (or plateau) lavas submarine lavas dike swarms
Lava Flow Features Flow Top Morphology
AA, pahoehoe, toe lobes Flow Interior Structures
pillows, columnar joints, flow banding
Pyroclastic Deposits Fall Deposits Flow Deposits Surge Deposits
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PHYSIOCHEMICAL PROPERTIES OF LAVAS
General Magma Type: Ultramafic Mafic Intermediate Felsic
Temperature: 1550º to 1200º 1250º to 1050º 1150º to 950º 1050º to 800º
Viscosity: Low Low Intermediate High
Gas Content: Very Low (<<1%) Low (<1%) Intermediate (1-3%) High (2-5%)
Typical Composition (wt. %)SiO2 46.5 50.0 57.7 70.5
TiO2 0.3 1.9 1.0 0.3
Al2O3 3.1 15.9 16.6 14.1
FeO 11.2 10.3 7.2 2.8
MnO 0.2 0.2 0.1 < 0.1
MgO 32.9 7.0 3.7 0.7
CaO 4.8 9.7 6.5 1.7
Na2O .1 2.9 3.4 3.6
K2O .01 1.1 1.8 3.9
P2O5 n.a. 0.3 0.3 0.1
Total 99.0 99.3 98.3 97.8
Trace Elements (ppm)
Cr 3000 200 10 2
Ni 1000 150 15 2
Ba 20 40 300 350
Zr 10 35 200 170
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CONTROLS ON VISCOSITY (RESISTANCE TO FLOW)
Viscosity increases with:• SiO2 concentration• decreasing temperature• increasing crystallinity of magma• decreasing volatile content (H2O, CO2, SO2, H2, HCl, Cl2, F2)
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EFFUSIVE ERUPTIONS
Mafic magma Relatively low gas content
(<1%) Fountaining followed by flow
as gas content diminishes Creates vesicular to massive
lava flows
Photos from USGS
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EXPLOSIVE ERUPTIONS
Water solubility (carrying capacity) in rhyolite as function of pressure; from Yamashita (1999)
Eruption Model
Driven by degassing of magma as it rises up the neck of the volcanic vent
The dramatic increase of volume resulting from degassing causes the magma to be violently thrust out the neck and shattered into fine fragments – ash
Creates pyroclastic deposits
http://www.geology.sdsu.edu/how_volcanoes_work/
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CENTRAL VENT VOLCANIC LANDFORMSSTRATOVOLCANOES
Steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows, are called stratovolcanoes. Usually constructed over a period of tens to hundreds of thousands of years, stratovolcanoes may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. All but basalt commonly generate highly explosive eruptions.
Mt St. Helens (pre-1980 eruption)Mt St. Helens (pre-1980 eruption)
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CENTRAL VENT VOLCANIC LANDFORMSSHIELD VOLCANOES
Built almost entirely of fluid mafic lava flows. Flow after flow effusively pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape.
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CENTRAL VENT VOLCANIC LANDFORMSSMALL VOLCANOES
Steep-sided cone formed by accumulation of ash, lapilli, bombs and blocks around a central vent resulting from a low volume, weak explosive eruption; also know as cinder cones
Broad accumulation dome with a large central crater resulting from eruption through water-saturated ground; steam (phreatic) explosions are common
Similar to a maar, but lacking central crater; encounter water at a shallower depth than maar and thus is not a phreatic
Steeper-sided and smaller accumulation of volcanic debris than tuff ring; similar shape to scoria cone, but layers dip inward near neck
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CENTRAL VENT VOLCANIC LANDFORMSSCALES
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CENTRAL VENT VOLCANIC LANDFORMSCALDERAS AND DOMES
Lava dome structure (from Winter, Fig. 4-7)
Lava dome building in Mt. St. Helens crater
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FISSURE ERUPTION LANDFORMS
Laki fissure, Iceland – Erupted 1783 creating the largest lava flow in human history
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FISSURE ERUPTION LANDFORMSPLATEAU (FLOOD) BASALTS
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FISSURE ERUPTION LANDFORMSDIKE SWARMS
Feeder conduits to eroded plateau basalts
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LAVA FLOW FEATURES
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Massive Massive BasaltBasalt
AmygdaloidalAmygdaloidalBasaltBasalt
LAVA FLOW FEATURESBASALTIC LAVA FLOW CONTACT
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NORTH SHORE
PahoehoePahoehoe
AAAA
HAWAII
PahoehoePahoehoe
AAAA
LAVA FLOW FEATURESBASALTIC LAVA FLOW SURFACES
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LAVA FLOW FEATURESTOE LOBES
1.1 Ga North Shore Volcanics
Modern-day Hawaii
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LAVA FLOW FEATURESPILLOW STRUCTURES
(SUBMARINE ERUPTIONS)
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LAVA FLOW FEATURESCOLUMNAR JOINTING
Shovel Point, MNShovel Point, MN
Gooseberry Falls, MNGooseberry Falls, MNCRB, OR (Winter, 2001)CRB, OR (Winter, 2001)
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LAVA FLOW FEATURESMISCELLANEOUS
Convoluted Convoluted Flow Banding in Flow Banding in RhyoliteRhyolite
Amygdule Amygdule Cylinders in Cylinders in BasaltBasalt
Coarse Ophitic Coarse Ophitic Texture in Texture in BasaltBasalt
Plagioclase Plagioclase Porphyritic Porphyritic Texture in Texture in BasaltBasalt
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PYROCLASTIC DEPOSITSPRODUCTS OF EXPLOSIVE ERUPTIONS
TUFFTUFF
Winter (2001) Fig. 2-5
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PYROCLASTIC DEPOSITSFALL DEPOSITS
Figure 4-17. Maximum aerial extent of the Bishop ash fall deposit erupted at Long Valley 700,000 years ago. After Miller et al. (1982) USGS Open-File Report 82-583.
Figure 4-16. Approximate aerial extent and thickness of Mt. Mazama (Crater Lake) ash fall, erupted 6950 years ago. After Young (1990), Unpubl. Ph. D. thesis, University of Lancaster. UK.
Mt. St. Helens
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Figure 4-18. Types of pyroclastic flow deposits. After MacDonald (1972), Volcanoes. Prentice-Hall, Inc., Fisher and Schminke (1984), Pyroclastic Rocks. Springer-Verlag. Berlin. 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 (Fig. 4-18d).
PYROCLASTIC DEPOSITS
FLOW DEPOSITS
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Figure 4-19. Section through a typical ignimbrite, showing basal surge deposit, middle flow, and upper ash fall cover. Tan blocks represent pumice, and purple represents denser lithic fragments. After Sparks et al. (1973) Geology, 1, 115-118. Geol. Soc. America
PYROCLASTIC DEPOSITSCOMPLETE ERUPTIVE PACKAGE - IGNIMBRITE
Tuff + Heating + Pressure Welded Tuff
Graded Tuff- Episodic Eruptions/Surges