GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 3

Volcanic Rocks:

Lavas, Landforms, and Products

Jan. 26, 2009

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

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

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)

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

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/

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)

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.

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

CENTRAL VENT VOLCANIC LANDFORMSSCALES

CENTRAL VENT VOLCANIC LANDFORMSCALDERAS AND DOMES

Lava dome structure (from Winter, Fig. 4-7)

Lava dome building in Mt. St. Helens crater

FISSURE ERUPTION LANDFORMS

Laki fissure, Iceland – Erupted 1783 creating the largest lava flow in human history

FISSURE ERUPTION LANDFORMSPLATEAU (FLOOD) BASALTS

FISSURE ERUPTION LANDFORMSDIKE SWARMS

Feeder conduits to eroded plateau basalts

LAVA FLOW FEATURES

Massive Massive BasaltBasalt

AmygdaloidalAmygdaloidalBasaltBasalt

LAVA FLOW FEATURESBASALTIC LAVA FLOW CONTACT

NORTH SHORE

PahoehoePahoehoe

AAAA

HAWAII

PahoehoePahoehoe

AAAA

LAVA FLOW FEATURESBASALTIC LAVA FLOW SURFACES

LAVA FLOW FEATURESTOE LOBES

1.1 Ga North Shore Volcanics

Modern-day Hawaii

LAVA FLOW FEATURESPILLOW STRUCTURES

(SUBMARINE ERUPTIONS)

LAVA FLOW FEATURESCOLUMNAR JOINTING

Shovel Point, MNShovel Point, MN

Gooseberry Falls, MNGooseberry Falls, MNCRB, OR (Winter, 2001)CRB, OR (Winter, 2001)

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

PYROCLASTIC DEPOSITSPRODUCTS OF EXPLOSIVE ERUPTIONS

TUFFTUFF

Winter (2001) Fig. 2-5

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

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

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