Revised 8 / 06 (Monroe 6th ed.)
Including...
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
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
Generally based upon the composition of the magma
DIGRESS TO: magma vs. lava
REVIEW: Magma compositions
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
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
Mineralogy
Olivine - Mg2SiO4 to Fe2SiO4
Pyroxene - Ca(Mg,Fe,Al)(Al,Si)2O6
Plagioclase - CaAlSi3O8 to NaAlSi3O8
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
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
Mineralogy
Plagioclase - CaAlSi3O8 to NaAlSi3O8
Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
Muscovite/Biotite - KAl2(Si3Al)O10(OH)2
Quartz - SiO2
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
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
Mineralogy
Potash Feldspar - KAlSi3O8
Quartz - SiO2
Muscovite/Biotite - KAl2(Si3Al)O10(OH)2
Amphibole - NaCa2(Mg,Fe,Al)5(Si,Al)8O22(OH)2
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
(chorus)
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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