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Introduction to volcanoes, volcanic eruptions, and volcanic landforms

Today:What are the eruptive processes?What are the resulting volcanic landforms?

Previous: Magmatic differentiation, Bowens Reaction series (mineral melting points), mafic vs. felsic (fluid vs. sticky)

Video: “Lava Flows and Tubes” [basalt! A fluid mafic lava]

• Minerals have different melting points• Minerals can vary in Fe, Mg, or SiO2 content• Differentiation (magma evolves as it moves

through the crust via fractional xtl (partial melt)• Result: Volcano types have tectonic context • TODAY: basic volcano behavior and landforms

Why are there different kinds of volcanoes?

What mainly controls eruptive style?

• Gas content of magma• Viscosity of magma• http://dsc.discovery.com/convergence/pomp

eii/interactive/interactive.html

Viscosity in magma

• High viscosity magma = high silica content = more explosive

• Low viscosity magma = low silica content =less explosive (effusive)

How would this affect volcanic hazards?

EX: RHYOLITE

EX: BASALT

2. Eruptive style:Explosive or effusive?

Felsic (high SiO2) = high viscosity and more explosive (e.g. dacite)

Mafic (low SiO2) = low viscosity and effusive (e.g. basalt)

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Explosive eruption: pyroclastic flow Unzen Volcano, Japan, 1991

Where do volcanoes erupt—and how does this relate to plate tectonics?

types of volcanoes and volcanic landformsShield volcano ex: Mauna Loa; Larch Mtn, OR

Composite volcano (stratovolcano) ex: Vesuvius, Mount Rainier, Popo, Mount Fuji

Dome ex. Goat Mountain, Black Buttes & Mt. Shasta (CA)

Cinder cones and spatter cones ex: Paracutin, Puu O (Kilauea), Sunset Crater (AZ)

Maar volcano ex: Battleground Lake, Ubehebe (CA)

Caldera ex: Yellowstone, Toba, Crater Lake, Long Valley

Flood basalt ex: CRBs; Deccan Traps, Siberian Traps

Shield Volcano (typically basaltic!)

Below: aerial view of Hawaii

Kilauea’s summit caldera, southwest and east rift zones that erupt most of the lava flows, and major fault zones

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Kupaianaha lava pond, Kilauea: photo by Pat Pringle, July 1987

Lower viscosity basaltic lava (mafic) is ~45% to 54% silica

Effusive eruptions - chiefly lava flows.

Builds shield volcanoes --gentle relief + cinder cones.

Shield Volcanoes

<= Tumac Mtn shield, NE of White Pass; view to East

Clayton (1983) estimated the age of Tumac Mtn at 30 to 20 ka.

ka = kiloannum

Basaltic volcanism in the Cascade Range

Spray Park and Mount Rainier view to the east; 1903 photo by W.P. Romans

Larch Mtn shield, OR

Underwood Mtnshield, WA

Mount Adams composite volcano is constructed on top of a broad shield

Composite volcanoes(aka stratovolcanoes)

• Related to subduction zones• ~55–65 % silica (intermediate viscosity)• Moderately violent explosive behavior• ~50/50 ratio of lavas/ fragmental deposits• Thick lavas (to 20 or even 100 m)• Steep cones; can have domes (implications?)• Lahars! (volcanic debris flows)

Composite volcano

Mount Hood

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Where are Composite volcanoes?

• Ring of Fire (Circumpacific belt)• Mediterranean belt

May 18, 1980 Plinian eruption column Looking NNWPhoto by Bob Krimmel, USGS

Composite Volcano

aka stratovolcano

Mount St. Helens => changed the way we think about composite volcanoes!

At composite volcanoes, glacial erosion can increase relief and supply ground-water that contributes to hydrothermal alteration and weakening of the cone.

Mount Rainier from Glacier View Wilderness

Photo by Pat Pringle

Mount Baker

Fig. 7-15, p.157 Fig. 7-16, p.158

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Fig. 7-18, p.160

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

Mount St. Helens hot dome avalanche of May 9, 1986

Lava domes Photo by Pat Pringle

Note person for scale

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.)

basaltic dike

Basalt of Castle Creek age (~1900 yr B.P.)

Summit dome dacite of Kalama age (post AD 1479)

Domes

Black Buttes

Mount Shasta, a composite volcano, and ShastinaMount Shasta’s summit consists of 4 domes

Lidar images of crater floor, Sept. 2003 and Sept. 2004

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MSH04_dome_from_sugarbowl_10-10_to_11-21-04

Aerial view of Spiral Butte dacite dome at White Pass

Clayton ~ 30 – 20 ka; Tom Sisson of USGS estimates its age at ~100 ka

Cinder cones

Paricutin, Mexico, 1941

Typically basaltic

1.2 Ma basalt lava flows from the Uinkaret Plateau in Grand Canyon

Calderas

Aniakchak caldera, AKMount Katmai caldera lake, below

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Rhyolitic volcanism and calderas

• ~65 – 76 % Silica (viscous!!!)

Can be very violent and highly explosive

Produce mostly pyroclasts/ extensive ash flows

Significant % of magma erupts producing calderas

Rhyolitic volcanism and calderas• 100 to 1000+ km3 deposit volumes!!!• >138 active in historic times• Examples: Toba, Yellowstone, Campi Flegrei,

Long Valley (CA), Crater Lake, Aniakchak• Largest known (Miocene) La Garita in the San

Juans (CO) => >3000 km3 !!!!!

Volcanoes and human history: Santorini volcano on the island of Thera

Fig. 7-23, p.163 Fig. 7-32, p.168

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Fig. 7-33, p.168 Fig. 7-36, p.169

Fig. 7-37, p.170

Lava Creek ash from Yellowstone Caldera, 1,000 km3

Flood Basalts

Flood Basalts (aka Plateau Basalts)Fissure eruptions of basalt—Earth’s largest lava flowsCRB surface exposures ~175,000 km2 & extend 400-600 km!

Exs: Columbia River Basalt Gp, Deccan Traps, Parana Brazil

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Features of flows (simplified): pillows and pillow palagonite breccia, colonade, entablature, vesicular top.

Grande Ronde Basalt of the Columbia River Basalt Group as exposed at Table Mountain, Columbia River Gorge; photo by Pat Pringle

contact with Miocene Eagle Creek Formation—Table Mtn

Entablature

Colonade

Grand Ronde flow GSOC, 1997, SR 410

Palagonite pillow delta of Pliocene age, Columbia River Gorge

Longview quarry, 2006

Columbia River Basalt flows

Columbia River Basalt flows separated by sedimentary interbeds

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Sill of Columbia River Basalt on

north coast of Oregon; basalt

invaded marine sediments of the Miocene coastal

plain

Maar volcanoes-created by phreatic eruptions

Volcanic necks—Beacon Rock in Columbia River Gorge

Wind Mountain shallow intrusive body in the Columbia Gorge (~6 Ma).

Volcano HazardsVolcano Hazards••Proximal areas (up to 15 mi) Proximal areas (up to 15 mi) subject to multiple hazardssubject to multiple hazards

••Lahars and floods affect Lahars and floods affect valleys for many tens of milesvalleys for many tens of miles

••Tephra fall affects areas tens Tephra fall affects areas tens to hundreds of miles, or to hundreds of miles, or more, downwindmore, downwind

••Excess sediment in Excess sediment in watersheds makes channels watersheds makes channels unstable for years to decadesunstable for years to decades

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Learning objectives

• What is a volcano?• Where do we find volcanoes? (Review)• What is the origin of volcanoes?• Why do volcanoes erupt? • Why do some volcanoes erupt explosively and

some non-explosively? • How does eruptive style affect creation of

volcanic landforms and volcanic hazards?