1. Introduction to volcanoes, volcanic eruptions, and volcanic landforms. What
mainly controls eruptive style? • Gas content of magma. • Viscosity of magma.
Introduction to volcanoes, volcanic eruptions, and volcanic landforms What mainly controls eruptive style? • Gas content of magma • Viscosity of magma
Viscosity in magma
2. Eruptive style: Explosive or effusive?
• High viscosity magma= high silica content=more explosive • Low viscosity magma= low silica content=less explosive (effusive) Why are viscosity and gas important? How would they affect volcanic hazards?
Effusive eruption: advancing lava flow
Felsic (high SiO2) = high viscosity and more explosive (e.g. dacite)
Mafic (low SiO2) = low viscosity and effusive (e.g. basalt)
Explosive eruption: pyroclastic flow Unzen Volcano, Japan, 1991
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Where do volcanoes erupt—and how does this relate to plate tectonics?
Types of volcanoes and volcanic landforms Shield volcano
ex: Mauna Loa; Larch Mtn, OR
Composite volcano (stratovolcano)
ex: Vesuvius,
Mount Rainier, Popo, Mount Fuji
Domes
ex. Goat Mountain, Black Buttes & Mt. Shasta (CA)
Cinder cones and spatter cones
ex: Paracutin, Puu O
(Kilauea), Sunset Crater (AZ)
Maar volcano Caldera
ex: Battleground Lake, Ubehebe (CA)
ex: Yellowstone, Toba, Crater Lake
Flood basalt
ex: CRBs 17–15 Ma; Deccan Traps--India 65 Ma; Siberian Traps ~250 Ma)
Lower viscosity basaltic lava (mafic) is ~45% to 54% silica
Shield Volcano (typically basaltic!)
Shield Volcanoes
Effusive eruptions (less explosive) Gentle relief + cinder cones. Volcano = chiefly lava flows.
Below: aerial view of Hawaii
Kupaianaha lava pond, Kilauea: photo by Pat Pringle, July 1987
Humongous landslides from Hawaiian volcanoes (dark gray = islands). The landslide deposits (stippled area) are found on the sea floor (note scale in km!) 2007 gps velocities ~ 7 cm/yr
So what? There is more to see below the surface of the ocean!
Pacific Plate moves over the Hawaiian hot spot; ages of the Hawaiian Islands + geologic factors reflect plate movement— older to the NW
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Basaltic volcanism in the Cascade Range
Larch Mtn shield, OR
hydrothermal alteration and weakening
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Composite volcano Below: isopach map of Vesuvius’ AD 79 tephra
Mount Hood Left: tectonic setting of Italian volcanoes; right above: thickness of deposits from AD 79 Vesuvius eruption
Composite Volcano
Mount Baker
aka stratovolcano
May 18, 1980 Plinian eruption column Looking NNW Photo by Bob Krimmel, USGS
Mount St. Helens => changed the way we think about composite volcanoes!
Vesuvius, 1944 Monte Somma – remnant of pre79 AD edifice
Crater walls of Mount St. Helens: note abundant fragmental debris!
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Martha Sabel, USGS, 1982
Incised canyon on the north flank of Mount St. Helens near north end of 1980 Crater floor; note great percentage of fragmental debris! View looks NNE; photo by Pat Pringle, September, 1982
Summit dome dacite of Kalama age (post AD 1479) Basalt of Castle Creek age (~1900 yr B.P.)
basaltic dike
Crater walls of Mount St. Helens; photos by Pat Pringle; above from south crater rim; right, from top of Lava Dome.)
Dacite dome rock of Pine Creek age (~2500 yr B.P.)
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