chapter iv volcanism

7/31/2019 Chapter IV Volcanism

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Volcanism

Chapter 4

Course notes are available on the web at:

http://www.colorado.edu/GeolSci/courses/GEOL1010-3/

First Midterm is tomorrow, covers first four chapters in the

Course notes. Mostly multiple choices, true false, etc.


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The Nature of Volcanic EruptionsFactors determining the violence orexplosiveness of a volcanic eruption

Composition of the magma

Temperature of the magma

Dissolved gases in the magma

These three factors control the viscosityof magma (which controls the nature ofan eruption)


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The Nature of Volcanic Eruptions

Viscosityis a measure of a materials resistanceto flow (higher viscosity materials flow withgreat difficulty; water has very low viscosity)

Factors affecting viscosity Higher temperature - magma is less viscous (or

more fluid)

Composition - higher silica (SiO2) content -magma is more viscous (rhyolite)

Lower silica content = lower viscosity or morefluid-like behavior (e.g., mafic lava such as basalt)


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Factors affecting viscosity continued

Dissolved Gases

Gas content affects magma mobility Gases expand in a magma as it nears the

Earths surface due to decreasing pressure

The violence of an eruption is related to how

easily gases escape from magma


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Summary

Fluid basaltic lavas generally produce

quiet eruptions (Hawaiian volcanos gurgle)

Highly viscous lavas (rhyolite or andesite)

produce more explosive eruptions

(Yellowstone or St Helens go boom)


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Materials extruded from a 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 water vapor and carbon dioxide


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A Pahoehoe lava flow


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A typical aa flow


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QuickTime an d a Cinepak decompressor are needed to see this picture.


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Materials extruded from a volcanoPyroclastic materialsFire fragments

Types of pyroclastic debris

Ash and dust - fine, glassy fragments

Pumice - porous rock from frothy lava

Lapilli - walnut-sized material

Cinders - pea-sized material

Particles larger than lapilli

Blocks - hardened or cooled lava

Bombs - ejected as hot lava


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A volcanic bomb

Bomb is approximately 10 cm long


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Volcanoes

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

Ventopening connected to the magma

chamber via a pipe


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Volcanoes

Types of Volcanoes

Shield volcano

Broad, slightly domed-shaped

Composed primarily of basaltic lava

Generally cover large areas

Produced by mild eruptions of large volumesof lava

Mauna Loa on Hawaii is a good example


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A size comparison of the three

types of volcanoes


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Olympus Mons

Caldera


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VolcanoesTypes of Volcanoes continued

Cinder cone

Built from ejected lava (mainly cinder-sized)

fragments

Steep slope angle

Rather small size Frequently occur in groups


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Sunset Cratera cinder cone near

Flagstaff, Arizona


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VolcanoesTypes of volcanoes continued

Composite cone (Stratovolcano)

Most are located adjacent to the Pacific

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

Large, classic-shaped volcano (1000s of ft.

high & several miles wide at base)

Composed of interbedded lava flows and

layers of pyroclastic debris


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A composite volcano


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. . composite volcano (prior to

eruption)

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Mt. St. Helens following the

1980 eruption

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St Helens

Eruption

Sequence

How would

ScientistsMonitor this

Process?

Seismometers

Tilt Meters

INSAR


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St Helens Base Surge (on the scale of an atomic bomb)


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Volcanoes

Composite cones continuedMost violent type of activity (e.g., Mt.

Vesuvius)

Often produce a nue ardente

Fiery pyroclastic flow made of hotgases infused with ash and otherdebris

Move down the slopes of a volcanoat speeds up to 200 km per hour

May produce a lahar, which is avolcanic mudflow


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A nue ardente on Mt. St. Helens

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Location of Montserrat

- a historical Nuee Ardente

Town of St Pierre on Montserrat after Nuee Ardente


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Town of St Pierre on Montserrat after Nuee Ardente


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Other volcanic landforms

Calderas (form by collapse of evacuated

magma chamber)

Steep-walled depressions at the summit

Size generally exceeds 1 km in diameter

Pyroclastic flows (explosive mix of rock,

gas and heat)

Only with felsic & intermediate magma

Consists of ash, pumice, and other

fragmental debris

Material propelled from vent at high speed

C ld f fill d b C k


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Caldera of Mt Mazama now filled by Crater Lake


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Mt Mazama Eruption and Caldera Collapse


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Other volcanic landforms

Fissure eruptions and lava plateaus

Fluid basaltic lava extruded from crustal

fractures called fissures

e.g., Columbia River Plateau

Flood basalts cover huge areas


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Flood Basalt erupted

from fissures - Snake

River Plain, southern

Idaho


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Volcanic landforms

Lava Domes

Bulbous mass of congealed lava

Most are associated with explosive

eruptions of gas-rich magma

St H l L D


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St Helens Lava Dome


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Volcanic 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


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Formation of a volcanic neck

Shiprock in Northern New Mexico


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Shiprock in Northern New Mexico


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Plutonic igneous activity

Types of intrusive igneous features

Dikea sheetlike injection into a fracture (cutsacross pre-existing rocks)

Silla thick, sheetlike injection into a bedding

plane (lies parallel to bedding)

Laccolith

Similar to a sill

Lens or mushroom-shaped mass

Arches overlying strata upward

S i h P k d R di ti Dik ( th CO)


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Spanish Peaks and Radiating Dikes (southern CO)

Spanish Peaks


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Spa s ea s

Dike Outcrop

Spanish Peaks Digital Elevation


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Spa s ea s g a eva o

Spanish Peaks Dikes Volcanic Necks


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p

Dikes

Spanish Peaks Dikes - Digital 3D Elevation


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Spanish Peaks Dikes Digital 3D Elevation

Dikes

Volcanic Necks


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Some intrusive igneous structures


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Injection of a laccolith

QuickTi me and a Video decompressor are needed to see this picture.


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A sill in the Salt River Canyon, AZ


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Plate tectonics and igneous activity

Global distribution of magmatism is not

random

Most volcanoes are located within or near

ocean basins Basalt common in both oceanic and

continental settings

Granite is rare in oceans, mostly found incontinents

Di ib i f j l


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Distribution of major volcanoes


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Plate tectonics and igneous activity

Igneous activity along plate margins

Spreading centers

Great volumes of volcanic rock produced alongoceanic ridges

Mechanism of spreading

Lithosphere pulls apart and thins

Less pressure results in partial melting in mantle


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Plate tectonics and igneous activityIgneous activity along plate margins

Subduction zones

Descending plate partially melts Magma slowly moves upward

Rising magma can form either

An island arc if in the ocean (Aleutians)

A volcanic arc if on a continentalmargin(Sierra Nevada Mtns - California)

l i i bd i


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Volcanism in subduction zones

QuickTime and a Video decompressor are needed to see thi s picture.


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Plate tectonics and magmatism

Intraplate volcanism

Associated with plumes of heat in mantle

Form localized volcanic regions in theoverriding plate called a hot spot

Produces basaltic magma sources in oceanic

crust (Hawaii and Iceland)

Produces granitic magma sources incontinental crust (Yellowstone Park)

V l i i


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Volcanism on a tectonic

plate moving over a hot spot


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End of Volcanism

(Chapter 4)These notes are available on the web at:

http://www.colorado.edu/GeolSci/courses/GEOL1010-03/


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chapter iv volcanism

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