Revised 8 / 06 (Monroe 6th ed.)

Volcanology - Chapter 5




Global Distribution

Classification of Volcanoes

Mafic eruptions

Intermediate eruptions

Felsic eruptions

Volcano lyrics (Jimmy Buffett)



Very impressive events

Potential for great destruction

And construction!

What a deal - both earth processes in one event

Have certainly occurred throughout geologic time

Fantasia "Rite of Spring" scene

DIGRESS TO: Early formation and heating of earth

No sign of dying out soon

"The book" discusses the intermittent activity of individual volcanoes

Be careful here!

Human time scales are not up to the task of understanding the timing of geological events

DIGRESS TO: Active, Dormant, Extinct

Named after Vulcan - Roman God of Fire and Metalworking

Also Mr. Spock's home planet

More than 500 "active" volcanoes identified (Monroe; Table 5-1, pg. 135) (Monroe; fig. 5-20, pg. 151)

This number is also VERY questionable

DIGRESS TO: active vs. dormant vs. extinct

Many cultures associated volcanoes with gods

Hawaiian - Pele

Greek - Hephaestus

The ugly son of Hera

Europe in the middle ages - lots of crazy ideas (no surprise here)

Gateway to hell

Prisons of the damned

Noises associated with volcanoes were the "screams of tormented souls"

Proposed by the R.C. church, which incinerated thousands for worshiping "false" deities


Global Distribution

Very specific pattern (Monroe; fig. 5-20, pg. 151)

Usually associated with plate boundaries

DIGRESS TO: Pacific Ring of Fire

"The book" claims >80% are associated with the Ring of Fire and the Med (refer to Ch. summary #13)

I have trouble with this

Spreading centers are extremely active

Relative to the earth's time frame

We just don't see them

Both in time and space (covered with water)

Commonly associated with "active seismic areas"

No surprise here

Lots more on the plate boundary associations later


Classification of Volcanoes

Generally based upon the composition of the magma

DIGRESS TO: magma vs. lava

REVIEW: Magma compositions

Mafic vs. intermediate vs. felsic

Composition definitely related to regional tectonic setting

Plate boundaries

REVIEW: Plate margins

These compositional differences lead to different types of volcanoes

Each quite different from the others

Eruptive characteristics (Monroe; fig. 5-22, pg. 158)

Morphology of the resulting pile of debris

To summarize

Mafic: Shield, flood basalt, cinder cones

Intermediate: Composite, stratovolcanoes

Felsic: Calderas, domes, obsidian, pumice


Mafic eruptions

Regional tectonic setting

Tensional environments

Earth splitting through the crust

Expose upper mantle

Newly differentiated basaltic magma (blood of the earth)

Spreading centers

Global network, like stitches on a baseball (Monroe; fig. 5-20, pg. 151)

Continental rifting

Columbia River Basalt

Deccan Traps - India

Great Rift Valley - Africa

Also occur at Hot Spots - mantle plumes

Also referred to as "blowtorches"

EXAMPLE: Hawaiian Islands

Characteristics of mafic lavas and rocks

General description

Dark colored

Silica/Oxygen: 45% to 52%

High in iron, magnesium, calcium

Low in (or don't have) potassium, aluminum, sodium

Low volatile content

High temperature (>1000°C.)

Low viscosity - flow easily


Olivine - Mg2SiO4 to Fe2SiO4

Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6

Plagioclase - CaAlSi3O8 to NaAlSi3O8

Igneous rock formation

Basalt: extrusive, aphanitic

Diabase: medium texture

Commonly associated with dikes (EXPLAIN)

Gabbro: intrusive, phaneritic

Scoria: frothy

Various volcanic breccias

Wide range of clast sizes

Glassy rims commonly associated with submarine eruptions

These often shatter and break off

Eruptive characteristics

Relatively non-explosive

Due to high temperature, low silica, low volatiles

Flow like water in many cases

Better put on your running shoes

DIGRESS TO: Pahoehoe vs. aa lavas (Monroe; fig. 5-4, pg. 137)

Very dependent on temperature and gas content

Flows cool and lose gas as they move away from the vent

Eruptions relatively quiet, but spectacular

Fissure eruptions (Monroe; fig. 5-19, pg. 150)

Extensional environment indeed!

Can be miles long, and pour out floods of very fluid basaltic magma

Also called flood basalt

No respect for anything in their path

Can cover immense areas

Columbia River Basalt (Monroe; fig. 5-19, pg. 150)

Spatter cones.

Bubble, bubble, toil and trouble

Fountains - of all sizes

Some incredible displays on Hawaii (therefore generally mafic)

Lava falls

Lava tubes (Monroe; fig. 5-3, pg. 137)

Lava lakes: Halemaumau, Mauna Loa

Pillow basalt (Monroe; fig. 5-7, pg. 140)

Indicator of submarine eruption

Columnar jointing (Monroe; fig. 5-5, pg. 138)

Indicator of subaerial eruption

Vesicular basalt (church rocks)

Cinder cones (Monroe; fig. 5-11, pg. 144) (Monroe; fig. 5-12, pg. 145)

Basalt flows commonly erupt from vents at the base of the cone

Morphology of mafic volcanoes

Shield volcanos - fluid lava (Monroe; fig. 5-10, pg. 143)

Examples of mafic volcanoes - describe each

Iceland: spreading center

Island of Hawaii: mantle plume

Part of Hawaiian/Emperor seamount chain

Very active throughout its history (at least 60 million years)

An incredible amount of rock has been produced!

Lots of video and film on Hawaii

Also mythology

Columbia River / Modoc Plateau basalt

Continental fissure eruptions

Parícutin: cinder cone

Lava Butte: cinder cone (Lava Lands National Park; Bend, Oregon)

Lava Beds National Park: lava tubes


Intermediate eruptions

Regional tectonic setting

Compressional environments

Subduction zones

Re-melting of mafic crust, continental debris, and organic sediments

Characteristics of intermediate lavas and rocks

General description

Medium colored

Silica/Oxygen: 53% to 65%

Varying amounts of all the major rock-forming elements

Medium volatile content

Medium temperature


Plagioclase - CaAlSi3O8 to NaAlSi3O8

Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

Muscovite/Biotite - KAl2(Si3Al)O10(OH)2

Quartz - SiO2

Igneous rock formation

Andesite: extrusive, aphanitic

Diorite: intrusive, phaneritic

Various volcanic breccias

Eruptive characteristics: (KaBoom)

Eruptions relatively explosive, VERY impressive

Mt. St. Helens is a SMALL example

Can be MUCH larger

Explosiveness due to high volatile content

Like shaking up a can of carbonated soda

Also due to "stickiness" of magma (high silica content)

Tends to plug up the works until the pressure builds to the breaking point

Basically like zits on the earth

Results in the formation of "pyroclastic" material (Monroe; fig. 5-8, pg. 140) (Monroe; fig. 5-16, pg. 147)

Pyro = fire; clast = small piece of broken rock

In this case "small" is a VERY relative term!

All different sizes can be formed

From powder to blocks the size of a church

Get blown out by the force of the blast

Pyroclastic flows - nuée ardente

Can be very dangerous

Very hot and very fast

You CANNOT outrun one of these (Monroe; fig. 5-1c, pg. 134)

Associated features

Volcanic gasses (Monroe; fig. 5-2, pg. 136)

Carbon dioxide

Dense and will smother life when concentrated on surface

Sulfur dioxide

Combines with water to make sulfuric acid

Pyroclastics: Tephra and ash deposits

Ashfalls: Just like snow, but don't melt in the spring

Will develop into good soil, with time (the earth's time frame)

Tephra deposits: similar to ash but bigger pieces (also don't melt!)

Both tephra and ash are pyroclastic in origin

More or less sort themselves by clast size and distance from the vent

Lahars (Monroe; fig. 5-14, pg. 147)

These types of volcanoes often form high mountain peaks

Collect snow and ice

Melt when an eruption occurs

Major flooding downhill

Can pick up massive amounts of pyroclastic (and other) debris

Houses, mobile homes, cows, slow cats

Whatever is loose and in the way

Extensive damage and loss of life

Morphology of intermediate volcanoes

Composite cones (stratovolcanoes) (Monroe; fig. 5-13, pg. 146)

Layered pyroclastics and flows

KaBoom then ooze; KaBoom then ooze; KaBoom then ooze

Examples of intermediate volcanoes

Dante's Peak

Cascade Range (Monroe; pg. 154)

Mt. Pinatubo

Potential risk

DIGRESS TO: Relationship to seismic events

Can expect great quakes in the same areas

Very great risk

From initial blast, pyroclastic flows, lahars, ashfalls

Several major urban areas in potential risk zones

The western flank of the Cascade Range (Monroe; pg. 154)

Mt. Shasta and the I-5 corridor

the Puget Sound in general

Especially the southern end immediately west of Mt. Rainier

Portland, Oregon

Lucked out big time when St. Helens blew to the northeast!

DESCRIBE: difference in lahar distance related to a southern blast

What about Mt. Hood?

A west-directed blast would be a real problem

Japanese islands in general

Mt. Fiji

...and modern Tokyo is worried about Godzilla


Felsic eruptions

Regional tectonic setting

Intra-plate hot spots

Tough to get granitic magma close enough to the surface to erupt

Eruptions are (fortunately) very infrequent (by the human time scale)

Origins VERY uncertain

Characteristics of felsic lavas and rocks

General description

Light colored

Silica/Oxygen: >65%

High in potassium, aluminum, sodium

Low in (or don't have) iron, magnesium, calcium

High volatile content

Low temperature (600°C. - 900°C.)

High viscosity


Potash Feldspar - KAlSi3O8

Quartz - SiO2

Muscovite/Biotite - KAl2(Si3Al)O10(OH)2

Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2

Igneous rock formation

Rhyolite: extrusive, aphanitic

Granite: intrusive, phaneritic

Pumice: frothy

Obsidian: glassy

Various volcanic breccias

Eruptive characteristics: (KaBoom)2

Often called "Supervolcanoes" (Monroe; Geology at Work, pg. 167)

Eruptions very explosive, MOST impressive

Explosiveness again due to very high volatile and silica content

DIGRESS TO: Most felsic magmas cool as intrusive igneous rocks

The cooler temperatures lead to rapid crystallization of selected silicate minerals (amphibole, feldspar)

And this high silica content really plugs up the pipes

Long chains common in the felsic minerals

Like a log jam in a river - restricts the flow

Porphyritic textures common in the felsic rocks

Begins to cool just below the surface, plugs itself up, and then erupts

Obsidian and pumice: volcanic glass

Very quick cooling

No time for crystallization

Non-crystalline solids (amorphous)

Morphology of felsic volcanoes

Lava domes (Monroe; fig. 5-15, pg. 147)

Often associated with obsidian flows

Caldera (Monroe; fig. 5-9, pg. 142)

Examples of felsic volcanoes

Yellowstone National Park (Monroe; Geology at Work, pg. 167)

0.6 mya: 45 mi.

1.2 mya: 60 mi.

1.9 mya: 85 mi.

Long Valley Caldera, California

Newberry Crater obsidian flows

Potential Risk

Ask the folks in Bishop, Lone Pine, or Wyoming how safe they feel


Volcano (Jimmy Buffett)


Now, I don't know

I don't know

I don't know where I'm a-gonna go

When the volcano blows

Let me say now


I don't know

I don't know

I don't know where I'm a-gonna go

When the volcano blows


Ground she's movin' under me

Tidal waves out on the sea

Sulfur smoke up in the sky

Pretty soon we learn to fly

Let me hear ya now


chorus (1)


Now my girl quickly said to me

Mon, you better watch your feet

Lava come down soft and hot

You better lava me now or lava me not

Let me say now


chorus (1)


No time to count what I'm worth

Cuz I just left the planet Earth

Where I go I hope there's rum

Not to worry, mon soon come


chorus (2)


But I don't want to land in New York City

Don't want to land in Mexico

Don't want to land on no Three Mile Island

Don't want to see my skin aglow


Don't want to land in Comanche Sky Park

Or in Nashville Tennessee

Don't want to land in no San Juan Airport

Or the Yukon Territory


Don't want to land no San Diego

Don't want to land in no Buzzard's Bay

Don't want to land on no Ayatollah

I got nothing more to say


chorus (2)


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