Volcanoes & TectonicsVolcanoes & Tectonics

Seamounts and Guyots

Lava Viscosity (silica, Temp.) & explosivenessLava Viscosity (silica, Temp.) & explosiveness

• Basaltic lava - less silica and higher temperature, low viscosity, gasses escape at surface low pressures, not explosive

e.g. Shield volcanoes, Rifts, MOR, cinder cones• Intermediate silica, explosive, composite =

stratovolcanoes, Andes, Cascades, Aleutians Japan, Philippines, often pyroclastic clouds = nuée ardente

• Rhyolitic lava - more silica and lower temperatures, high viscosity, gasses cannot escape, explosive.

Plate Tectonics and VolcanismPlate Tectonics and Volcanism

Volcanism on a tectonic Volcanism on a tectonic plate moving over a hot plate moving over a hot

spotspot

Pacific Basin 90%Volcanic Output Pacific Basin 90%Volcanic Output Seamounts and MOR => ocean Seamounts and MOR => ocean

floorfloor

Plumes under Continents yield more silica rich, explosive eruptionsPlumes under Continents yield more silica rich, explosive eruptions

Great Basalt PlateausGreat Basalt Plateaus

Volcanic geomorphic provinces

A size comparison of the three A size comparison of the three main types of volcanoesmain types of volcanoes

Seamounts are these largestSeamounts are these largest

Plume below ocean

Above subduction zone

Divergent Zone Cinder Cones

Volcanic Activities and their productsVolcanic Activities and their products

• Exhalative (gas: H2O, CO2, SO2)

• Effusive (lava)

• Explosive (tephra)

Hawaii, low silica, low viscosity, notice no tephra in view

Lava

Flo

w

Upper surface pitted Scoria

Pahoehoe Near vent

Jagged A'a'away from vent

http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html

Basaltic Pillow LavasBasaltic Pillow Lavas

The reason there is so much water carried down into a subduction zone

Volcanic Activities and their productsVolcanic Activities and their products

• Explosive (tephra)

Intermediate to high silica - Andesitic to Rhyolitic

nuée ardente = pyroclastic cloud

TephraTephra• Coarse = close

• Thick = close

Lahars - volcanic mudflowsLahars - volcanic mudflows

Copyright © Stephane Veyrat-Charvillon 2002

Melting collapse of ice embedded cone

Characteristic Eruptive TypesCharacteristic Eruptive Types

Special case: Phreatic eruptions occur when magma hits water. And the water flashes to steam explosively

Copyright © Richard Kesel 2002

Basaltic lava Basaltic lava flowsflows

• Layered– Lava/sediment

• Jointed– Columnar

Rift Valley FlowsRift Valley Flows

Volcanic Landforms 1: BasalticVolcanic Landforms 1: Basaltic

• Columbia Plateau

• Deccan Traps

• Very large in scale, worldwide effects

Basalt Plateaus - Flood BasaltsBasalt Plateaus - Flood Basalts

Basaltic Cinder and Basaltic Cinder and Spatter Cones Spatter Cones

• Projectiles– Ballistic– Wind-borne

• Divergent Margin +

Shield VolcanoesShield Volcanoes

Mauna Kea from Mauna Loa

•Form above Plumes, hugeForm above Plumes, huge• also Basaltic, gentle eruptionsalso Basaltic, gentle eruptions•As plate moves old volcanoes As plate moves old volcanoes become inactive, new volcano become inactive, new volcano forms over the plumeforms over the plume

Volcanic Landforms, Basaltic continued

Andesitic - Composite conesAndesitic - Composite cones

Mt. Fuji in summer

Volcanic Landforms 2: Andesitic Volcanoes

Form above Subduction zones

"Stratovolcano"

The Cascades, Washington State

Composite cones - sectionComposite cones - section

Lava and tephra alternate

Andesitic EruptionsAndesitic Eruptions

Extensive Welded Tuff, Pumice Lavas, and Lahars

Cleveland Volcano, Aleutians

Plug DomesPlug Domes

Rhyolite Rhyolite EruptionsEruptions

• Rare, extremely explosive

• Size of ash cover = violence

• Toba and our near-extinction

Volcanism on a tectonic Volcanism on a tectonic plate moving over a hot plate moving over a hot

spotspot

CalderasCalderas

Caldera – Crater LakeCaldera – Crater Lake

Copyright © Ann Dittmer 2002

Caldera topography – Crater LakeCaldera topography – Crater Lake

MaarsMaars

Tuff Ring

Ukinrek Maars

A maar is a broad, low-relief volcanic crater that is caused by a phreatomagmatic eruption, an explosion caused by groundwater coming into contact with hot lava or magma

Panum Crater is a good example of both a tuff ring and a Rhyolite dome

Rising magma hit groundwater, explosion left crater, then fountain of hot cinders made ring, then remaining magma rose as dome

Mono Co, CA Rhyolite, very high silica, very viscous, can’t get rid of gas bubbles, explodes

IcelandIceland• Is on the mid-Atlantic Ridge, • 13 volcanoes have erupted since AD 874.• most active & volatile is Grímsvötn.• Over the past 500 years, Iceland's

volcanoes have erupted a third of the total global lava output.

• • One quarter of Iceland ‘s population died

due to eruption of the craters of Laki in 1783-84. The craters are a part of a larger volcanic system with the subglacial Grímsvötn as a central volcano. Because most of Grímsvötn’s eruptions have been subglacial, the interaction of magma and meltwater from the ice causes phreatomagmatic explosions

Tephra forms TuffTephra forms Tuff

Source: Gerald & Buff Corsi/Visuals Unlimited, Inc.

Ignimbrite or Welded Tuff

Erosional Erosional RemnantsRemnants• Plugs

• Dikes

Ship Rock, NM

Intrusive terrains - LaccolithsIntrusive terrains - Laccoliths

• Laccoliths - Henry Mts.

• Colorado Plateau

First studied in 1875-1876 by Grove Karl Gilbert. The term laccolith) is used to describe the characteristic shapes of some of the igneous intrusions that core the mountains.

Intrusive terrains - Sierra Nevada BatholithIntrusive terrains - Sierra Nevada Batholith