محاضره جيو نضري الخامسه

Volcanoes and Other Volcanoes and Other Igneous ActivityIgneous Activity

Chapter 5

NOT all Volcanic Eruptions are the SameNOT all Volcanic Eruptions are the Same

NOT all Volcanic Eruptions NOT all Volcanic Eruptions are the Sameare the Same

• Three factors determine the “violence” or explosiveness of a volcanic eruption:

1. Composition of the magma2. Temperature of the magma3. Dissolved gases in the magma

ViscosityViscosity

• These three factors control the viscosity of a given magma.

• Which in turn controls the nature of an eruption.

• Viscosity is a measure of a material’s resistance to flow. – Higher viscosity materials flow with great

difficulty.

Factors Affecting ViscosityFactors Affecting Viscosity

1. Temperature – Hotter magmas are less viscous.

2. Composition – Silica (SiO2) content– Higher silica content = more SiO4

chains/structures = higher viscosity • felsic lava – rhyolite

– Lower silica content = fewer SiO4

chains/structures = lower viscosity or more fluid like behavior

• mafic lava – basalt

3. Dissolved Gases• The violence of an eruption is related to

how easily gases escape from magma. – Gas content affects magma mobility.– Volatiles migrate upward and accumulate

near the top of the magma chamber. enriching the upper portion of the magma chamber with dissolved gases.

– Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure.

– Escaping gases provide the force to propel molten rock from the volcanic vent.


3. Dissolved Gases– Very fluid basaltic

magmas allow the gases to migrate upwards and escape the vent with relative ease.

– Produces lava fountains extending hundreds of meters in height.


3. Dissolved Gases– Highly siliceous magmas

undergo magmatic differentiation leaving the upper portion of the magma chamber enriched in silica and dissolved gases.

– The volcano summit begins to inflate and bulge months to years prior to eruption.

– As the magma migrates up the vent, the gases collect into tiny bubbles.

– The mixture is transformed into a gas jet containing tiny magma fragments that are explosively ejected.

– Produces plumes of a hot ash-laden gases called eruption columns that extends thousands of meters into the atmosphere.


The Nature of Volcanic The Nature of Volcanic EruptionsEruptions

• In Summary:– Fluid basaltic lavas generally produce quiet

eruptions.• Basaltic flow rates between 30-1000 ft/hour.• Traveling up to 90 miles (150 km) from the vent.

– Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions.

• Rhyolitic lava are much slower. • Seldom travel more than a few kilometers from

their vents.

Materials Extruded Materials Extruded from Volcanoesfrom Volcanoes

1. Lava Flows

2. Volatiles

3. Pyroclastic Material

Lava FlowsLava Flows

Pahoehoe Lava Flow in Pahoehoe Lava Flow in Hawaii Volcanoes Hawaii Volcanoes National ParkNational Park

• Types of Basaltic Flows:

• Pahoehoe lava resembles a twisted or ropey texture.

Pahoehoe Pahoehoe Lava Flow Lava Flow in Hawaii in Hawaii Volcanoes Volcanoes National National

ParkPark

PahoehoePahoehoe lava flows can lava flows can contain contain lava lava tubestubes, cave-like tunnels that were , cave-like tunnels that were

horizontal conduits for lava.horizontal conduits for lava.

Lava Tube in Hawaii Lava Tube in Hawaii Volcanoes National ParkVolcanoes National Park


– Pahoehoe flows are hotter, richer in gases, and travel faster than aa flows.

– Basaltic lavas can begin as pahoehoe flows and become aa flows.

• Types of Basaltic Flows:

• Aa lava has a rough, jagged blocky texture.

– Cool and thick with gases escaping forming numerous voids and sharp spines.

• Types of Basaltic Flows:– Pillow lavas – lava that formed

underwater.


• Occur along oceanic ridges.

• Lava cools quickly forming an outer skin.

• Lava advances by breaking through the outer rind.

• Forms elongated structures resembling pillows.

Pillow Basalts, Olympic National Park, Washington

• Magmas contain varying amounts of dissolved gases held in the molten rock under confining pressure.– Consists mainly of:

• water vapor and carbon dioxide

– Lesser amounts of: • nitrogen, sulfur dioxide, chlorine, hydrogen,

argon

– Contribute significantly the planet’s atmosphere (natural air pollution).

– Rise into the atmosphere and may reside there for years potentially impacting climate.

Dissolved Gases (Volatiles)Dissolved Gases (Volatiles)

Volcanoes and ClimateVolcanoes and Climate• Explosive eruptions emit huge quantities

of gases and fine-grained debris into the atmosphere which

• Filter out and reflect a portion of the incoming solar radiation.

• This can cause cooling on a global scale.• Examples of volcanism affecting climate:

• Mount Tambora, Indonesia – 1815• Krakatau, Indonesia – 1883• Mount Pinatubo, Philippines – 1991

• Ash and Dust – Fine, glassy fragments compose tuffs and welded tuffs.

• Pumice – Porous rock from “frothy” lava or rhyolitic composition.

• Scoria – Vesicular rock of typically basaltic composition.

• Cinders – Glassy vesicular fragments 4 - 32 mm.

Pyroclastic Materials – “Fire Fragments”

• Lapilli – pyroclastic fragments 2-64 mm in size.

• Particles larger than lapilli:

Pyroclastic Materials – “Fire Fragments”

• Blocks – pyroclasts more than 64 mm in diameter ejected in the solid state (rock torn from the vent wall).

• Bombs – ejected blob of hot lava streamlined during flight (more than 64 mm in diameter).

Bomb is approximately 10 cm long

Anatomy of VolcanoesAnatomy of Volcanoes

Anatomy of VolcanoesAnatomy of Volcanoes• General Features

– Magma chamber is connected to the surface by a conduit or pipe that terminates at a surface vent.


– Opening at the summit of a volcano:• Crater – steep-walled, circular depression at the

summit, generally less than 1 km diameter. – Produced by explosive excavation of rock during eruptions.

• Caldera – a summit depression typically greater than 1 km diameter.

– Produced by collapse following a massive eruption.


– Mature volcanoes develop fissures along the flanks and base producing parasitic cones and fumaroles (emit only gases and smoke).

Types of Volcanoes and Types of Volcanoes and Their CharacteristicsTheir Characteristics

Insert Plate Tectonic Boundary Features Animation #66

Plate Tectonics and Igneous ActivityPlate Tectonics and Igneous Activity• Global distribution of igneous activity is not random.

• Most volcanoes are located within or near ocean basins or along continental margins.

• Each type of plate tectonic boundary produces a specific type of igneous activity.

Plate Tectonics and Plate Tectonics and Igneous ActivityIgneous Activity

• Basaltic igneous activity is common in both oceanic and continental settings.

• Granitic igneous activity is rarely found in the oceans.

Not all Volcanoes are the SameNot all Volcanoes are the Same

• Factors determining the size and shape of volcanoes:

1. Volume of lava erupted2. Viscosity of lava:

• Composition of the magma• Temperature of the magma• Dissolved gases in the magma

Shield VolcanoesShield Volcanoes• Broad, slightly domed-shaped.

• Composed primarily of basaltic lava.

• Generally cover large areas.

• Produced by mild eruptions of large volumes of lava.

Mauna LoaMauna Loa

Shield VolcanoesShield Volcanoes• Most grow up from the seafloor to form islands or sea

mounts.

• From ocean floor to summit, Mauna Loa is over 6 miles high (higher than Mt. Everest [~32,000 feet]).

• Large steep-walled calderas occupy the summit.

• Mature volcanoes erupt lava from the summit and rift zones that develop along the slopes (flank).

Cinder Cones -

Haleakala Maui

• During late stages, shield volcanoes produce clusters of cinder cones in summit area.

Shield Volcanoes and Plate TectonicsPlate Tectonics

• Shield volcanoes are a product of intra-plate volcanism in oceanic crust.– Activity within a tectonic plate. – Associated with plumes of heat

upwelling in the mantle – mantle plume.

– Form localized volcanic regions in the overriding plate called a hot spot.

– Produces basaltic magmas creating:

• Shield volcanoes• Volcanic chains and seamounts

Volcanism on a Volcanism on a Tectonic Plate Tectonic Plate Moving over a Moving over a

Hot SpotHot Spot

Produces Shield Produces Shield Volcanoes Volcanoes

(basaltic) and (basaltic) and Volcanic Island Volcanic Island and Seamount and Seamount

ChainsChains

Global distribution of flood basalt provinces (black) and associated hot-spots (red dots). Red dashed lines are hot spot tracks, which appear as lines

of volcanic structures on the ocean floor. The Keweenawan and Siberian Traps formed in failed continental rifts where the crust had been greatly

thinned. Whether there is a connection between the Columbia River basalts and the Yellowstone hot spot is still a matter of ongoing research.

Cinder Cone VolcanoesCinder Cone VolcanoesSunset Crater – a Cinder Cone Sunset Crater – a Cinder Cone

near Flagstaff, Arizonanear Flagstaff, Arizona • Built from ejected lava fragments – pyroclastic cinders or clinkers (glassy vesicular fragments).

• Fragments range in size from ash to bombs, primarily lapilli.

• Product of gas-rich basaltic magma (scoria).

• Steep slope angle.• Rather small size.• Frequently occur in groups.

Parasitic Cinder Cones, Mauna Kea, Hawaii

Late-Stage Cinder Cones, Mauna Kea

Summit, Hawaii

Cinder Cone Volcanoes and Plate TectonicsPlate Tectonics

• Cinder cone volcanoes are the product of late-stage volcanism in various tectonic environments.– Basaltic magmas:

• Associated with hot spot volcanism in oceanic crust:– Shield Volcano Flanks and Calderas

• Associated with hot spot volcanism in continental crust:– Flanks of Calderas

• Associated with subduction zones between oceanic-oceanic crust and oceanic-continental crust:

– Stratovolcano Flanks and Calderas

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

• Composed of interbedded lava flows and layers of pyroclastic debris.

• Primarily andesitic in composition with lesser basaltic and rhyolitic lavas.

Composite or StratovolcanoesComposite or Stratovolcanoes

• Explosive eruptions that eject huge quantities of pyroclastic material.

• Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens).

Composite or StratovolcanoesComposite or Stratovolcanoes

Mt. St. HelensMt. St. Helens

Mt. St. Mt. St. Helens Helens 1980 1980

EruptionEruption

Mt. St. Helens Following Mt. St. Helens Following the 1980 Eruptionthe 1980 Eruption

• Stratovolcanoes erupt violently…– Often produce a nueé

ardente (glowing avalanche):

• Fiery pyroclastic flow made of hot gases infused with ash, pumice, and other debris.

• Felsic and intermediate magmas.

• Material ejected at high velocities.

Pyroclastic FlowsPyroclastic Flows

NueNueé Ardente on Mt. St. Helensé Ardente on Mt. St. Helens

• Nueé ardente (glowing avalanche):– Forms from the collapse

(overcome by gravity) of tall eruption columns.

– Moves down the slopes of a volcano at speeds up to 200 km (125 miles) per hour.

– Traveling up to more than 60 miles from the vent.

– Ground-hugging portion is rich in particular matter suspended by jets of buoyant gases (nearly frictionless).

Pyroclastic FlowsPyroclastic Flows

• Stratovolcanoes may produce lahars, or volcanic mudflows:– Mixture of volcanic debris and water.– Moves rapidly down slope (30 kph or more)

following stream valleys.– Triggered when large volumes of ice and

snow melt during an eruption.– Also generated when heavy rainfall saturates

weathered volcanic deposits.– Highly destructive.

LaharsLahars

Lahar from Mt. St. Helens -- On March 18, 1980, an explosive eruption on Mt. St. Helens generated a 14-kilometer-high eruption plume. Melted snow from the eruption produced the dark-colored lahar seen in this photo. Part of the lahar flowed into Spirit Lake (lower left). However, most of the lahar flowed into the North Fork of the Toutle River valley (right), eventually reaching the Cowlitz River, 80 kilometers downstream. Courtesy of Thomas J. Casadevall, USGS.

The snow-covered peaks of the Cascade volcanoes in Washington, Oregon, and northern California pose a clear threat to surrounding towns and villages. Past events suggest that a catastrophic lahar could lie in the future of Mt. Rainier, the largest of the Cascade volcanoes. The 4,000 m high summit of Mt. Rainier contains the largest system of alpine glaciers in the Cascade Range. The periodic melting of glacier ice from Mt. Rainier has generated at least 50 major lahars over the past 10,000 years. The largest of these mudflow deposits, one of the world's largest, is the ~5700-year-old Osceola lahar, shown in the adjacent map (green) (courtesy of USGS). The Osceola lahar traveled down the White River, over 112 km from its source. It then spread out at its mouth to cover an area of over 300 square kilometers along the shoreline of Puget Sound. The recent geologic history of Mt. Rainier demonstrates that a major mudflow descends down the White River once every 600 years. The younger 500-year-old Electron lahar (see map – yellow) was also generated from Mt. Rainier. It flowed 56 kilometers down the Puyallup River to within 15 kilometers of Tacoma, Washington. More than 300,000 people now live in the area covered by these extensive lahars! Unlike floods, such catastrophic mudflows can occur with little or no warning. Some volcanologists have predicted that Mt. Rainier will be the site of the next Cascade eruption. Therefore, the volcano is monitored closely, with the hope that we can warn the local population before the next lahar strikes.

Lahars at Lahars at Mt. RainerMt. Rainer

Eruptive Frequency of Cascade Range Stratovolcanoes

• Stratovolcanoes are the product of subduction zone igneous activity along oceanic-oceanic and oceanic-continental convergent plate boundaries. – Occur in conjunction with deep oceanic trenches.– Descending plate causes partial melting (wet

melting) of the mantle.• A volcanic island arc if in the ocean – evolves

from early-stage mantle-derived basalts to mature andesites and rhyolites.

• A continental volcanic arc if along a continental margin – primarily andesites and rhyolites lesser basalts.

Stratovolcanoes and Plate TectonicsPlate Tectonics

Igneous Igneous Activity Activity Along Along

Subduction Subduction ZonesZones

• Associated with the Pacific Ocean Basin margin is known as the “Ring of Fire”.

Stratovolcanoes and Plate TectonicsPlate Tectonics

• Most of the world’s explosive volcanoes are found here.

Size Comparison of the Size Comparison of the Three Types of VolcanoesThree Types of Volcanoes

• Large collapse depressions with approximate circular diameter.• Steep-walled depressions at the summit.• Size generally exceeds 1 km in diameter. • Formed by the eruption of large volumes of magma from a shallow

underground magma reservoir.• Results in loss of structural support for the overlying rock. • Leading to collapse of the ground and formation of a large depression.

CalderasCalderas

Form by one of the following processes:1. Collapse of the summit of a large composite volcano

following the eruption of silica-rich pryroclastic eruption (Crater Lake).

CalderasCalderas

Crater Lake Caldera, Oregon (6 miles diameter)Crater Lake Caldera, Oregon (6 miles diameter)

2. Collapse of the top of a shield volcano caused by subterranean drainage from the central magma chamber to rift zone – flank eruptions (Mauna Loa).

CalderasCalderas

3. Collapse of large area, independent of any preexisting volcanic structures, caused by discharge of colossal volumes of silica-rich pyroclastics along ring fractures (Yellowstone).

CalderasCalderas

Image courtesy of the United States Geological Survey.

Yellowstone Yellowstone CalderaCaldera

Calderas and Plate TectonicsPlate Tectonics


– Produces granitic magmas creating:

• Calderas

• Calderas are a product of intra-plate volcanism in continental crust.– Activity within a tectonic plate. – Associated with plumes of heat upwelling in the

mantle – mantle plume.

Yellowstone

Valles

Long Valley

Large Calderas in the U.S.Large Calderas in the U.S.

• Long Valley, CaliforniaLong Valley, California

• Yellowstone, WyomingYellowstone, Wyoming

• Valles, New MexicoValles, New Mexico

Long Valley CalderaLong Valley Caldera• Located in eastern

California. • Caldera is elliptical in

shape.• Approximately 10 X 20

miles across.• Last volcanic activity

~550-600 years ago at Inyo Craters.

• Huge eruption 730,000 years ago ejected about 600 km3 of material.

Long Valley Long Valley Caldera, Caldera,

Looking from Looking from SW to NESW to NE

Inside the Long Inside the Long Valley CalderaValley Caldera

http://quake.wr.usgs.gov/VOLCANOES/LongValley/gallery/images/32022545-048_large.jpg

Ash Deposits from Long Ash Deposits from Long Valley Eruption, 730,000 Valley Eruption, 730,000

years ago.years ago.

Potential Thickness of Potential Thickness of Tephra from Eruption of less Tephra from Eruption of less

than 1 kmthan 1 km33 of Magma. of Magma.

Fissure Eruptions and Lava Plateaus• Huge volumes of volcanic

material are extruded from fractures in the crust called fissures.

• Fluid basaltic lava extrude via fissure eruptions (flood basalts) building up a thick lava plateau.

• e.g., Columbia River Plateau (nearly 1 mile thick of flood basalts).

Flood Basalts and Plate TectonicsPlate Tectonics• Flood basalts are a product of

intra-plate volcanism in continental crust.– Activity within a tectonic plate. – Associated with plumes of heat

upwelling in the mantle – mantle plume.


– Produces basaltic magmas creating:

• Flood basalts in continental environments (Columbia Plateau).

Global distribution of flood basalt provinces (black) and associated hot-spots (red dots). Red dashed lines are hot spot tracks, which appear as lines

of volcanic structures on the ocean floor. The Keweenawan and Siberian Traps formed in failed continental rifts where the crust had been greatly

thinned. Whether there is a connection between the Columbia River basalts and the Yellowstone hot spot is still a matter of ongoing research.

• Spreading Centers– The greatest volume

of volcanic rock is produced along the oceanic ridge system.

– Continental or oceanic rifting.

– Results in partial melting of mantle (decompression melting).

– Large quantities of basaltic magma are produced.

Lava DomesLava Domes

• Most are associated with composite cones that produce explosive eruptions of silica- and gas-rich lavas.

• Typify late stages if mature, chiefly andesitic cones.

Lava Dome on Mt. St. HelensLava Dome on Mt. St. Helens

• Bulbous mass of congealed lava.

• Form in the summit crater.

Climbing Mount St. Helens



Volcanic PipesVolcanic Pipes• Most volcanic pipes are conduits that connect a magma chamber

to the surface. • Pipes may extend in a tube-like manner to depth exceeding 200

km (rare).• Enables ultramafic rocks from the mantle to reach the surface

that have undergone very little alteration.• Diamond-bearing kimberlite dikes.

Volcanic NecksVolcanic Necks• Erosion of the outer flanks of the

volcanic cone leave behind the more-resistant neck.

Shiprock, NM Shiprock, NM – A Volcanic – A Volcanic

NeckNeck

• Form as igneous rocks cool at or near the surface.

• Shrinkage fractures formed by tensional forces that cracks the rock as it contracts during cooling.

• Produces elongated, pillar-like columns.

• Common is lava flows and sills.

Columnar JointsColumnar Joints

Plutonic Igneous ActivityPlutonic Igneous Activity

Plutonic Igneous Bodieslutonic Igneous Bodies

• Most magma is emplaced at depth in the Earth.

• An underground igneous body, once cooled and solidified, is called a pluton.

Classification of Plutons

2. Orientation with respect to the host (surrounding) rock:– Discordant – cuts

across existing structures or rock units.

– Concordant – parallel to existing structures of rock units.

• Classification of Plutons:

1. Shape• Tabular (sheetlike)

• Massive

3. Size

Types of Intrusive Igneous Features• Dike – a tabular, discordant pluton where

magma has injected into fractures.– Pathways that fed ancient lava flows.

– Occur in clusters – radiating around neck.

Vertical Vertical Dike Near Dike Near Granby, Granby, ColoradoColorado

• Sill – a tabular, concordant pluton where magma has injected along sedimentary bedding surfaces.– Occur in shallow environments less force required to move overlying rock

layers.– Frequently cut across layers and resume concordant nature at a higher

level.

Types of Intrusive Igneous Features

A Sill in the Salt River Canyon, A Sill in the Salt River Canyon, ArizonaArizona

• Laccolith– Similar to a sill (near-surface environment).– Lens or mushroom-shaped mass.– Generated by accumulation of more viscous

magma.


– Arches overlying strata upward.

• Two Types of Plutons (based on size)– Batholiths

• Largest intrusive body.• Surface exposure of 100+ square

kilometers.


• Frequently form the cores of mountains.

• Two Types of Plutons (based on size)– Stocks

• Smaller• Surface exposure less than 100 square

kilometers.


• Batholiths consist of large numbers of distinct plutons (including stocks) that were intruded over millions of years.

Batholiths of Western Batholiths of Western North AmericaNorth America

The Sierra Nevada Batholith exposed in The Sierra Nevada Batholith exposed in Yosemite Natl. ParkYosemite Natl. Park

Documents

محاضره جيو نضري الخامسه