Volcanoes Volcanoes Lecture 4Lecture 4

http://www.soest.hawaii.edu/GG/HCV/kilauea.html

Igneous Rock Summary.doc will be helpful for this chapter’s homework.

The Nature of Volcanic EruptionsThe Nature of Volcanic Eruptions• Factors determining the “violence” or

explosiveness of a volcanic eruption• Composition of the magma –Silica Content• Temperature of the magma• Dissolved gases in the magma

• Composition and Temperature control the viscosity (resistance to flow) of magma. Viscous magmas cannot release gasses coming out of solution, and explode lava as it freezes.

http://www.soest.hawaii.edu/GG/HCV/kilauea.html

The Nature of Volcanic EruptionsThe Nature of Volcanic Eruptions• Water has very low viscosity, cold molasses high viscosity

• Factors affecting Viscosity

• Temperature - hotter magma is less viscous (more fluid). Basaltic (mafic) magmas (Olivine, Pyroxene, Ca-Feldspars) are hotter than Granitic (felsics) (Quartz, K- feldspars)

• Composition

- Felsic lava (e.g. rhyolite) is most viscous due to high silica content

- intermediate lavas (e.g. andesite) viscous.

- mafic lava (basalt) has lower viscosity - more fluid-like due to lower silica content

The Nature of Volcanic The Nature of Volcanic EruptionsEruptions

• Factors affecting explosiveness

• Dissolved Gases– Gases come out of solution and expand in

a magma as it nears the Earth’s surface due to decreasing pressure

– The violence of an eruption is related to how easily gases escape from magma – trapped gasses expand and shatter solidifying lavas, causing explosions

http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/framework.html

The Nature of Volcanic The Nature of Volcanic EruptionsEruptions

• Summary• Fluid basaltic lavas generally produce

quiet eruptions (Hawaiian lava flows)• Viscous lavas (rhyolite or andesite)

produce more explosive eruptions (Yellowstone & Mt. St. Helens hot ash explosions)

Low-Viscosity Basaltic LavaLow-Viscosity Basaltic Lava

Source: Phil Degginger/Earth Scenes

Viscous Andesitic Lava over crater floorViscous Andesitic Lava over crater floor

Source: Eugene Iwatsubo/Cascade Volcano Observatory, USGS

Very Viscous Rhyolite FlowVery Viscous Rhyolite Flow

Source: Martin Miller

Viscous, short path

Materials extruded from a Materials extruded from a Basaltic VolcanoBasaltic Volcano

• Lava Flows• Basaltic lavas are much more fluid

• Types of basaltic flows

– Pahoehoe lava (- twisted or ropey texture)

– Aa lava (rough, jagged blocky texture)

• Dissolved Gases• 1-6% of a magma by weight

• Mainly H2O vapor and CO2 and SO2

A Pahoehoe lava flowA Pahoehoe lava flow

TTypicalypical a’a’ flow a’a’ flow

Fluid basalt forms lava tubes

Checking Bowens Reaction Series

Materials extruded from a volcanoMaterials extruded from a volcano

• Pyroclastic materials – “Tephra”

Propelled through the AirTypes of pyroclastic debris

• Dust 0.001 mm and Ash < rice sized

• Cinders or Lapilli - pea to walnut-sized material

Particles larger than lapilli

• Bombs - > 64 mm ejected as hot lava

-Surtsey Is. Bombs the size of busses

A volcanic bombA volcanic bomb

Bomb is approximately 10 cm long

Tephra forms TuffTephra forms Tuff

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

Tephra layers fine away from sourceTephra layers fine away from source

PumicePumice

• Felsic magmas with high water content may bubble out of a vent as a froth of lava.

• Quickly solidifies into the glassy volcanic rock known as Pumice.

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

VolcanoesVolcanoes

• General Features• Opening at the summit of a volcano

– Crater - steep-walled depression at the summit, generally less than 1 km diameter

– Caldera - a summit depression typically greater than 1 km diameter, produced by collapse following a massive eruption.

• Vent – opening connected to the magma chamber via a pipe

Types of Volcanoes 1Types of Volcanoes 1

• Shield volcano - Largest–Broad, slightly domed-shaped

–Composed primarily of basaltic lava

–Generally cover large areas

–Produced by mild eruptions of large volumes of lava

–Mauna Loa on Hawaii is a good example

Shield Volcano Shield Volcano (Hawaii's K’ilaueau Volcano)(Hawaii's K’ilaueau Volcano)

Source: Jeff Greenberg/Visuals Unlimited, Inc.

Shield Volcanoes are often the largest in a chain of islands. They have basaltic lava, which is NOT very viscous, so it easily releases it’s gasses. Hence explosive pyroclastic eruptions are rare.

A size comparison of the three A size comparison of the three types of volcanoestypes of volcanoes

A Shield VolcanoA Shield Volcano

Olympus MonsOlympus MonsCalderaCaldera23 km high

Mars

Types of Volcanoes - 2Types of Volcanoes - 2

•Cinder cone - Smallest–Built from ejected lava fragments (mainly cinder-sized)

–Steep slope angle

–Rather small size

–Frequently occur in groups

Cinder ConeCinder Cone

A Cinder Cone FountainA Cinder Cone Fountain

Sunset Crater – a cinder cone Sunset Crater – a cinder cone near Flagstaff, Arizonanear Flagstaff, Arizona

Types of Volcanoes - 3Types of Volcanoes - 3

• Composite cone (Stratovolcano)–Most are located adjacent to the

Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)

–Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)

–Composed of interbedded lava flows and layers of pyroclastic debris

A composite volcanoA composite volcano

Mt. St. Helens – a typical Mt. St. Helens – a typical composite composite volcano (prior to eruption)volcano (prior to eruption)

Composite volcanoes typically have intermediate silica, andesitic magma. Gasses are trapped in the magma. When it erupts out onto the surface, low pressure causes dissolved gasses to come out of solution just as the lava is freezing. The lava explodes,Resulting in a nuee ardente.

Mt. St. Helens after 1980 eruptionMt. St. Helens after 1980 eruption

•explosive mix of rock, gas and heatexplosive mix of rock, gas and heat•only with felsic & intermediate magmaonly with felsic & intermediate magma•consists of ash, pumice, other fragmentsconsists of ash, pumice, other fragments•material propelled from vent at high speedmaterial propelled from vent at high speed

Pyroclastic Flows

St HelensEruptionSequence

How wouldScientistsMonitor thisProcess?

SeismometersTilt Meters

Composite Volcanoes –continuedComposite Volcanoes –continued

–Most violent type (e.g., Mt. Vesuvius, Mt. St. Helens, Mt. Pinatubo)

–Often produce a nuée ardente• Fiery pyroclastic flow made of

hot gases infused with ash and other debris

• Move down the slopes of a volcano at speeds up to 200 km per hour

• Forms Welded Tuffhttp://volcanology.geol.ucsb.edu/pfs.htm

A nueA nueé ardente on Mt. St. Helensé ardente on Mt. St. Helens

Location of Montserrat- 1997 Nuée ArdentePlus massive ashfall. Leaves of trees preserved.

North American plate’s ocean crustSubducted under Caribbean Plate

Montserrat Eruption with Montserrat Eruption with NuNuéée Ardente, 1997e Ardente, 1997

Source: Kevin West/Liaison Agency

LaharsLaharsPyroclastics on upper slopes may produce a lahar, which is a volcanic mudflow. Heat of volcanics melts ice.

CalderasCalderas

• Calderas form by collapse of evacuated Calderas form by collapse of evacuated magma chambermagma chamber

• Steep-walled depressions at the summitSteep-walled depressions at the summit• Size generally exceeds 1 km in diameterSize generally exceeds 1 km in diameter

Caldera of Mt. Mazama now filled by Crater Lake

Mt Mazama Eruption and Caldera Collapse4700 BC S Oregon

Ngorongoro Crater in Tanzania similar 2 mya

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

spotspot

Hey, the plate changed direction !

Flood Basalts

Hot Spot currently forming Hawaii

Flood BasaltsFlood Basalts

• Fluid basaltic lava extruded from crustal fractures called fissures

• e.g., Columbia River Plateau, • Deccan Traps in India

• Cover huge areas• Plumes from Mantle

Lava Plateau FormationLava Plateau Formation

Flood Basalt erupted from fissures - Snake River Plain, southernIdaho

Plume Activity

Volcanic landformsVolcanic landforms

• Lava Domes•Bulbous mass of congealed lava•Most are associated with explosive eruptions of

silica-rich magma

http://vulcan.wr.usgs.gov/Volcanoes/MSH/Images/MSH04/framework.html

St Helens Lava DomeViscous magmas

Volcanic landformsVolcanic landforms

• Volcanic Pipes and Necks• Pipes are short conduits that connect a magma chamber to the surface

• Volcanic necks (e.g., Devils Tower in Wyoming and Ship Rock in New Mexico) are resistant vents left standing after erosion has removed volcanic cone

Formation of a volcanic neckFormation of a volcanic neck

Spanish Peaks and Radiating Dikes (southern CO)

Plutonic igneous activityPlutonic igneous activity

• Types of intrusive igneous features

• Dike – a sheetlike injection into a fracture Discordant - cuts across pre-existing

• Sill – a sheetlike injection into a bedding

plane Concordant - lies parallel to bedding

• Laccolith – A mushroom-shaped concordant

Some intrusive igneous Some intrusive igneous structuresstructures

A sill in the Salt River Canyon, AZA sill in the Salt River Canyon, AZ

Sill: Sediments above and below sill are baked. Lava Flow, just baked below.

• Global distribution of magmatism is not randomGlobal distribution of magmatism is not random

• Most volcanoes are located within or near Most volcanoes are located within or near ocean basinsocean basins

• Basalt common in both oceanic and Basalt common in both oceanic and continental settingscontinental settings

• Granite is rare in oceans, mostly found in Granite is rare in oceans, mostly found in continentscontinents

• This result is predicted by a crust melt model This result is predicted by a crust melt model of Granite formationof Granite formation

WhyNoC-Ccollisions

Plate tectonics and igneous Plate tectonics and igneous activityactivity

• Igneous activity along plate margins• Mid-Ocean Ridges – Basaltic Pillow Lavas• Great volumes of volcanic rock produced along

oceanic ridges – New ocean floor– Mechanism of spreading or “rifting”

• Lithosphere pulls apart and thins• Less pressure results in partial melting in mantle

http://www.archipelago.nu/SKARGARD/ENGELSKA/ICELAND/surtsey.htm

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

Basaltic Pillow LavasBasaltic Pillow Lavas

Plate tectonics and igneous activityPlate tectonics and igneous activity• Igneous activity along Subduction zones

– Descending plate partially melts– Magma slowly moves upward – Rising magma can form either

• A Volcanic Island Arc if ocean-ocean plate collision (Aleutians, Japan, etc.)

• A Continental Volcanic Arc if ocean-continent plate collision (Sierra Nevada)

The Cascades, Washington State

Plate tectonics and magmatismPlate tectonics and magmatism

• Intraplate volcanism• Associated with plumes of heat in mantle• Form localized volcanic regions in the

overriding plate called a hot spot–Produces basaltic magma sources in

oceanic crust (Hawaii)–Produces granitic magma sources in

continental crust (Yellowstone Park)–These differences are predicted by a

Crust-Melting model of Granite generation

End of VolcanoesEnd of Volcanoeshttp://www.mt-fuji.co.jp/Photo/Photo.html