Igneous Review 2 Mostly Extrusives Igneous Rock Summary.doc will be helpful.

Igneous Review 2Igneous Review 2Mostly ExtrusivesMostly Extrusives

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

Igneous Rock Summary.doc will be helpful

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• Amount of 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 (Silica content)

- 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)

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 FlowsLava Flows• Basaltic lavas are much more fluid Basaltic lavas are much more fluid

• Types of basaltic flowsTypes of basaltic flows

– PahoehoePahoehoe lava (- twisted or ropey texture) lava (- twisted or ropey texture)

– AaAa lava (rough, jagged blocky texture) lava (rough, jagged blocky texture)

• Dissolved GasesDissolved Gases• 1-6% of a magma by weight1-6% of a magma by weight

• Mainly HMainly H22O vapor and COO vapor and CO22 and SO and SO22

TTypicalypical a’a’ flow a’a’ flow

Fluid basalt forms lava tubes

Checking Bowens Reaction Series

Products of ExplosionsProducts of Explosions

• 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.

St. Lucia Anecdote

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

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 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

Typical of divergent margins

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

–Above subduction zones

A composite volcanoA composite volcano

Mt. St. Helens – a typical composite Mt. St. Helens – a typical 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 FlowsPyroclastic Flows AKA nuée ardenteAKA nuée ardente

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

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

Volcano FeaturesVolcano Features

• 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

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

Mt Mazama Eruption and Caldera Collapse4700 BC S Oregon

Ngorongoro Crater in Tanzania similar 2 mya

Caldera of Mt. Mazama now filled by Crater Lake

PlumesPlumes

Hey, the plate changed direction !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

Flood Basalt erupted from fissures - Snake River Plain, southernIdaho

Plume Activity

Volcanic landformsVolcanic landforms

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

silica-rich magmasilica-rich magma

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

St Helens Lava DomeViscous magmas

Volcanic landformsVolcanic landforms

• Volcanic PipesVolcanic Pipes and and NecksNecks• Pipes are short conduits that connect Pipes are short conduits that connect

a magma chamber to the surfacea magma chamber to the surface• Volcanic necks (e.g., Devils Tower in Volcanic necks (e.g., Devils Tower in

Wyoming and Ship Rock in New Wyoming and Ship Rock in New Mexico) are resistant vents left Mexico) are resistant vents left standing after erosion has removed standing after erosion has removed volcanic conevolcanic 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

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.

WhyNoC-Ccollisions

Plate tectonics and igneous activityPlate tectonics and igneous activity• Igneous activity above 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)

Batholiths

Plate tectonics and igneous Plate tectonics and igneous activityactivity

• Igneous activity along plate margins• Ophiolites started at Mid-Ocean Ridges• 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

Ophiolite SuiteOphiolite SuiteSome Serpentine is formeddue to hot water (called Hydrothermal) circulation

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

Basaltic Pillow LavasBasaltic Pillow Lavas

Black SmokersBlack Smokers

Circulation of hot water in cracks at mid-ocean ridge dissolves metals (Copper, Iron, Zinc, Lead, Barium) which are re-precipitated as (mostly) sulfide ores. Hydrothermal waters are capable of metamorphism.

http://collections.ic.gc.ca/geoscience/images/detail/F92S0220.jpg

Mostly Sulfide DepositsField Trip Schedule

Cyprus