2004 Physical Geology Lecture 9 Eruptions

8/2/2019 2004 Physical Geology Lecture 9 Eruptions

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The Wrath of Vulcan:Volcanic EruptionsThe Wrath of Vulcan:Volcanic Eruptions

Geology 211, Physical Geology Copyright, 2004, Ron ParkerGeology 211, Physical Geology Copyright, 2004, Ron Parker

February 17, 2004February 17, 2004

RonParker,2003

RonParker,2003

Geo lo gy 2 11 , P hy si ca l Geo lo gy 2 00 3, R on P arke r

Volcanic Eruptions

Vulcan The Roman god of fire.

Volcano

An erupting vent through which moltenrock (magma) reaches the surface

A mountain built from magmatic eruptions

Volcanoes pose hazards to humans.

Many populated areas are located

near volcanic centers

Seattle Portland

Mexico City

Naples, Italy

Vulcan The Roman god of fire.

Volcano

An erupting vent through which moltenrock (magma) reaches the surface

A mountain built from magmatic eruptions

Volcanoes pose hazards to humans.

Many populated areas are located

near volcanic centers

Seattle Portland

Mexico City

Naples, Italy

Geo lo gy 2 11 , P hy si ca l Ge ol og y 2 00 3, Ro n P arke r

Vesuvio

79 A.D. Roman Empire

Mount Vesuvius erupted violently.

Pompeii and Herculaneum were

destroyed by pyroclastic flows.

20,000 + were incinerated

The ignimbrite preserved a complete

record of Roman daily life.

Jocelyn Karlans Vesuvius Webpage

W. W. Norton W. W. Norton

Leo C. CurranLeo C. Curran

W. W. Norton W. W. Norton

Mt. Vesuvius remains a threat to the 100s of thousands

of people living in and around modern Naples, Italy.

Expanding human populations put people at risk.

Volcanoes have killed 100,000 in the past 100 years

Mt. Vesuvius remains a threat to the 100s of thousands

of people living in and around modern Naples, Italy.

Expanding human populations put people at risk.

Volcanoes have killed 100,000 in the past 100 years

, 2002, DIGIT, Prentice-Hall

NASA NASA


Volcanoes are unpredictable and dangerous

Volcanically active areas may erupt without warning

50 to 60 volcanoes erupt annually

Most in sparsely populated areas.

Rarely eruptions occur near cities.

Indonesia

Japan

Philippines

Mexico

Volcanoes are unpredictable and dangerous

Volcanically active areas may erupt without warning

50 to 60 volcanoes erupt annually

Most in sparsely populated areas.

Rarely eruptions occur near cities.

Indonesia

Japan

Philippines

Mexico

Volcanoes


Paracutin (Central Mexico)

A man plowing his cornfield noticed strange smoke

1 week later cinders and

ash piled up 100m

Lava buried the

nearby village

Paracutin (Central Mexico)

A man plowing his cornfield noticed strange smoke

1 week later cinders and

ash piled up 100m

Lava buried the

nearby village

Volcanoes


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Lava flows Sheets or mounds of lava

that flowed on the ground or sea floor

and then solidified.

Pyroclastic debris Fragmented

igneous material deposited on the

ground or the sea floor.

Volcanic gases Gases

and aerosols.

Volcanic Materials


The type of lava flow depends on the magma composition

Magma compositions vary in silica (SiO2) content

Rhyolitic > 70% Explosive eruptions

Andesitic > 55% Intermediate

Basaltic < 50% Mellow eruptions

Lava Flows

The 3 Main Magma TypesBasaltic Andesitic Rhyolitic

Silica-poor Silica-rich Silica-rich


Low viscosity basaltic magmas produce quiet eruptions.

95% ofbasaltic magmas reach the surface.

High viscosity rhyolitic magmas produce violent eruptions.

Most of these magmas do not reach the surface, cooling at depth

Those that do reach the surface explode to form calderas.

Intermediate andesitic magmas are in between

Composition and Extrusive Style

DIGIT, 2002DIGIT, 2002

PahoehoePahoehoe

Hot, fluid basalt flows rapidly

Creates a ropy texture

Develops a glassy surface

Hot, fluid basalt flows rapidly

Creates a ropy texture

Develops a glassy surface


Aa Far from the vent,

basaltic lava cools, loses

gases and thickens.

This type of lava:

Flows slowly for

short distances

Is rough and jagged

Is known as Aa (ahh

ahh) because thats

what you say if you

walk on it with bare

feet!

Far from the vent,

basaltic lava cools, loses

gases and thickens.

This type of lava:

Flows slowly for

short distances

Is rough and jagged

Is known as Aa (ahh

ahh) because thats

what you say if you

walk on it with bare

feet!


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Pahoehoe can form lava tubes preventing cooling

Lava can flow for many miles underground in tubes.

Lava tubes act to expand the size of the island

Pahoehoe can form lava tubes preventing cooling

Lava can flow for many miles underground in tubes.

Lava tubes act to expand the size of the island

Lava TubesLava Tubes


Columnar JointingColumnar Jointing When lava flows cool uniformly, they sometimes

form vertical fractures called columnar jointing.

Columns have a hexagonal cross-section.

When lava flows cool uniformly, they sometimes

form vertical fractures called columnar jointing.

Columns have a hexagonal cross-section.

http://www.ivanweb.net/Nelmondo/Irlanda/irlanda.htmhttp://www.ivanweb.net/Nelmondo/Irlanda/irlanda.htm


Pyroclastic Fire fragments

Types of pyroclastic debris

Ash and dust - fine, powdery, glassy fragments

Cinders - pea-sized material

Lapilli - walnut-sized material

Pumice - porous rock from frothy lava

Particles larger than lapilli

Blocks - hardened or cooled lava basketball to TV sized Bombs - streamlined blocks ejected as hot lava

Pyroclastic Fire fragments

Types of pyroclastic debris

Ash and dust - fine, powdery, glassy fragments

Cinders - pea-sized material

Lapilli - walnut-sized material

Pumice - porous rock from frothy lava

Particles larger than lapilli

Blocks - hardened or cooled lava basketball to TV sized Bombs - streamlined blocks ejected as hot lava

Pyroclastic DebrisPyroclastic Debris

TephraTephra

Tephra is particulate material ejected by volcanoes.

Tephra ranges in size from car size blocks to fine

clay sized ash.

Bombs ejected by volcanoes can kill.

Tephra is particulate material ejected by volcanoes.

Tephra ranges in size from car size blocks to fine

clay sized ash.

Bombs ejected by volcanoes can kill.

Airfall Volcanic AshAirfall Volcanic Ash

Volcanic Ash FragmentVolcanic Ash Fragment


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Pyroclastic Flow (Nue Ardente)Pyroclastic Flow (Nue Ardente)

Pyroclastic flows:

Are 100s of degrees

Race downslope at 30 kph

Incinerate everything

Famous examples:

Pompeii (Italy) 79 A.D.

St. Pierre (Martinique) - 1902

Pyroclastic flows:

Are 100s of degrees

Race downslope at 30 kph

Incinerate everything

Famous examples:

Pompeii (Italy) 79 A.D.

St. Pierre (Martinique) - 1902

Mount PeleeNue Ardente destroyed the city of St. Pierre, Martinique in 1902

Mount PeleeNue Ardente destroyed the city of St. Pierre, Martinique in 1902

Pyroclastic HazardsPyroclastic Hazards

Ash fall Buries landscapes

Lateral blasts Destroy large land areas

Pyroclastic flows Incinerate and bury land

Ash fall Buries landscapes

Lateral blasts Destroy large land areas

Pyroclastic flows Incinerate and bury land


One to six percent of a magma by weight

Mainly water vapor and carbon dioxide

Gas content affects magma mobility.

Gases expand within 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. Low viscosity (basalt) easy escape, mellow eruption

High viscosity (rhyolite) difficult escape, violent eruption

Volatiles (Dissolved Gases)


Magma composition controls most volcanic features.

Magma chamber

Vent

Crater

Caldera

Size and shape

Volcanic Architecture


Volcanic ventsVolcanic vents

Flank of Mauna Kea shield volcanoFlank of Mauna Kea shield volcano

Volcanic vents are smaller scale openingsVolcanic vents are smaller scale openings


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Crater Conical depression at the top of a volcano.

Craters are several km across.

Crater Conical depression at the top of a volcano.

Craters are several km across.


CalderaCaldera A giant depression formed by collapse of a volcano

into emptied magma chamber.

Much larger than a crater (10s of km across).

Crater Lake is a Caldera formed by the collapse of

Mount Mazama ~8000 years ago.

A giant depression formed by collapse of a volcano

into emptied magma chamber.

Much larger than a crater (10s of km across).

Crater Lake is a Caldera formed by the collapse of

Mount Mazama ~8000 years ago.

USGS Fact Sheet FS092-02USGS Fact Sheet FS092-02


Volcano Types Shield volcano

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


Cinder cone

Built from ejected lava (mainly cinder-sized) fragments

Steep slope angle

Rather small size

Occur in groups

Volcano Types


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

Volcano Types


A Composite Volcano



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W. W. Norton W. W. NortonGeo lo gy 2 11 , P hy si ca l Geo lo gy 2 00 3, R on P arke r

Composite Volcanoes Most violent type of activity (Mt. Vesuvius)

Often produce a nue ardente

Fiery pyroclastic flow made of hot gases, ash and other debris

Move down the slopes of a volcano at speeds up to 300 km per hour

May produce a lahar (a volcanic mudflow)

Lahars common if volcano was glaciated

Lahars magnify the damage and destruction

Most violent type of activity (Mt. Vesuvius)

Often produce a nue ardente

Fiery pyroclastic flow made of hot gases, ash and other debris

Move down the slopes of a volcano at speeds up to 300 km per hour

May produce a lahar (a volcanic mudflow)

Lahars common if volcano was glaciated

Lahars magnify the damage and destruction


Case History Mount St. Helens

May 18th, 1980, 8:32 A.M.

Initial debris avalanche and lateral blast removed the upper

396 meters of the volcano

This killed 57 people

USGSUSGSGeo lo gy 2 11 , P hy si ca l Geo lo gy 2 00 3, R on P arke r

Eye Witnesses Mount St. Helens Geologists Keith and Dorothy Stoffel, were in a small plane 1000

feet over Mount Saint Helens when i t erupted.

"noticed landsliding of rock and ice debris in-ward into thecrater. Within a matter of seconds, the whole north side ofthe summit crater began to move. The nature of movementwas eerie.... The entire mass began to ripple and churn up,without moving laterally. Then the entire north side of thesummit began sliding to the north along a deep-seated slide

plane. We took pictures of this slide sequence occurring, butbefore we could snap off more than a few pictures, a hugeexplosion blasted out. We neither felt nor heard a thing.

W. W. Norton W. W. Norton DIGIT, 2002DIGIT, 2002

Mount St. Helensbefore May 18th, 1980 8:32 a.m.

Mount St. Helensbefore May 18th, 1980 8:32 a.m.


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Mount St. Helensafter May 18th, 1980 8:32 a.m.

Mount St. Helensafter May 18th, 1980 8:32 a.m.

Melted glaciers triggered debris flows (lahars) that

clotted the Toutle River

Temporarily stopped shipping on the Columbia River

Melted glaciers triggered debris flows (lahars) that

clotted the Toutle River

Temporarily stopped shipping on the Columbia River

Case History Mount St. HelensCase History Mount St. Helens


Case History Mount St. HelensCase History Mount St. Helens

Disrupted highways and rail lines.

The blast devastated 596 square kilometers,

Destroyed timber valued at several 100 million dollars.

Ash fell as far east as North Dakota.

Disrupted highways and rail lines.

The blast devastated 596 square kilometers,

Destroyed timber valued at several 100 million dollars.

Ash fell as far east as North Dakota.

USGSUSGS


Plate Tectonics & Igneous Activity Global distribution of igneous activity is not random

Most volcanoes are located within or near ocean basins

Basaltic rocks are common in both oceanic and

continental settings, whereas granitic rocks are rarely

found in the oceans



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Intraplate volcanism

Associated with plumes of heat in the mantle

Form localized volcanic regions in the overriding plate

called a hot spot

Produces basaltic magma sources in oceanic crust (e.g., Hawaii

and Iceland)

Produces granitic magma sources in continental crust (e.g.,

Yellowstone Park)

Plate Tectonics & Igneous Activity


Volcanoes and Climate

Explosive eruptions emit huge quantities of gases and fine-

debris into the atmosphere. These filter and reflect incoming

solar radiation. Examples of volcanism affecting climate

Mount Tambora, Indonesia 1815

Krakatau, Indonesia 1883

Mount Pinatubo, Philippines 1991

El Chicon, Mexico 1982 Sulfur dioxide


Predicting Eruptions

Usually, signals that precede an eruption permit

advanced warning.

Seismic activity Earthquakes accompany magma movement

Heat Volcanoes heat-up before eruptions, sometimes visible

on Infrared satellites.

Topographic inflation Volcanoes expand when magma

chambers inflate.

Lasers

Tiltmeters

Volcanic gas emanation

Geologic history

2004 Physical Geology Lecture 9 Eruptions

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