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 Properties chemistry 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 Landforms flood (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 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 Al2O3 3.1 15.9 16.6 14.1 FeO 11.2 10.3 7.2 2.8 MnO 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. Total 99.0 99.3 98.3 97.8 Trace Elements (ppm) Cr 3000 200 10 2 Ni 1000 150 15 Ba 20 40 300 350 Zr 35 170

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 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 Eruption Model Water solubility (carrying capacity) in rhyolite as function of pressure; from Yamashita (1999) http://www.geology.sdsu.edu/how_volcanoes_work/

Central Vent Volcanic Landforms STRATOVOLCANOES 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)

Central Vent Volcanic Landforms SHIELD 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 Landforms SMALL 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 Landforms SCALES

Central Vent Volcanic Landforms CALDERAS 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 Landforms PLATEAU (FLOOD) BASALTS

Fissure Eruption Landforms DIKE SWARMS Feeder conduits to eroded plateau basalts

Lava Flow Features

BASALTIC LAVA FLOW CONTACT Lava Flow Features BASALTIC LAVA FLOW CONTACT Massive Basalt Amygdaloidal Basalt

BASALTIC LAVA FLOW SURFACES Lava Flow Features BASALTIC LAVA FLOW SURFACES AA AA Pahoehoe HAWAII Pahoehoe NORTH SHORE

Lava Flow Features TOE LOBES Modern-day Hawaii 1.1 Ga North Shore Volcanics

PILLOW STRUCTURES (SUBMARINE ERUPTIONS) Lava Flow Features PILLOW STRUCTURES (SUBMARINE ERUPTIONS)

Lava Flow Features COLUMNAR JOINTING Shovel Point, MN CRB, OR (Winter, 2001) Gooseberry Falls, MN

Lava Flow Features MISCELLANEOUS Convoluted Flow Banding in Rhyolite Amygdule Cylinders in Basalt Plagioclase Porphyritic Texture in Basalt Coarse Ophitic Texture in Basalt

Pyroclastic Deposits PRODUCTS OF EXPLOSIVE ERUPTIONS TUFF Winter (2001) Fig. 2-5

Pyroclastic Deposits FALL DEPOSITS Mt. St. Helens 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. 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.

Pyroclastic Deposits FLOW DEPOSITS 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 COMPLETE ERUPTIVE PACKAGE - IGNIMBRITE 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 Graded Tuff- Episodic Eruptions/Surges Tuff + Heating + Pressure  Welded Tuff