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Volcanism and Other Igneous Processes
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On Sunday, May 18, 1980, the largest volcanic eruption to occur in
North American historic times transformed a picturesque volcano into
a decapitated remnant. On this date in southwestern Washington
State, Mount St. Helens erupted with tremendous force.
Before
After
What happened??
Mt. St. Helens
•Approximately 1 km3 of ash erupted.
•Summit decreased by 1,350 feet.
•Claimed 59 lives
•Ash propelled 11 miles into the atmosphere.
•Ash covered surrounding areas of Yakima, Tri-cities, and
northern Oregon for 3 days – Noon felt like night.
• Feb. 1981- highest birth rate in Portland and surrounding
areas –TRUE FACT
Advice from the authorities:
If there is another major eruption, put your head between your
legs and kiss your ash goodbye!
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Goals for mastering this chapter:
•Describe factors that affect the nature of volcanic
eruptions.
•What factors affect viscosity of magma?
•Describe the various types of materials associated
with volcanic eruptions.
•Describe the major types of volcanoes recognized by
volcanologists and their eruptive styles – the
geomorphology.
•Describe magmatic differentiation and how it is
related to Bowen’s reaction series.
•Describe various intrusive bodies associated with
plutonic rocks.
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•Understand the general relationship between volcanic
activity and plate tectonics.
The “buzzword” is VISCOSITY
What is viscosity?
Viscosity = how well a material flows
more viscous – flows very slowly
(high viscosity)
less viscous – flows quickly
(low viscosity)
Does glass have viscosity?
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Why do volcanoes have different eruptive
•high viscosity styles???
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•high SiO2
Factors influencing eruptions
•felsic
• dependant on the magma’s viscosity
•“pasty”
•explosive
• high viscosity –”pasty” explosive
• low viscosity –”fluid” flows easily
A
•low viscosity
•low SiO2
•mafic
•“fluid”
•non-explosive
B
Factors influencing viscosity
• Temperature of magma
• T
• T
viscosity
viscosity
= fluid flow
= pasty flow
• Chemical composition
• SiO2 content (high or low)
mafic composition: (50% SiO2)=“fluid” flow
intermediate comp.: (60% SiO2)
felsic composition: (70% SiO2) =“pasty” flow
Dissolved gasses – influencing the movement of magma
(volatiles – water, CO2, SO2….)
Silica content and volatiles erupt two types of materials:
Gas charged lava
expands 100 times its volume
lava fountains
Gas charged
expands 100 times its
volume
very explosive
lava flows fluidly
volatiles easily migrate upward
magma low in SiO2
Volatiles migrate upwards
with difficulty
magma high in SiO2
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sulfur dioxide
> 1%
SO2
5%
Carbon dioxide
water vapor
70%
15%
5%
volatiles
Dissolved gasses (volatiles)
• 1-6% of total magma wt.
Magma chamber
• contributes to atmosphere
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I
VOLCANOES.
Discuss with a friend:
1. What factors affect viscosity?
2. How does silica content (SiO2) influence
the consistency of magma?
3. Name various volatiles that are typically
emitted from a volcanic eruption.
I will get an A on my exams and quizzes.
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Types of Basaltic Lava Flows
(low silica (SiO2) content)
Pahoehoe
Aa
• very fluid, thin,
broad sheets
• very “pasty,” sticky,
thick, cool flows
• flows 10-300 km/hr
(30-900 ft/hr)
• flows 5-50 m/hr
(15-150 ft/hr)
•high volatile gas content •low volatile gas content
• smooth “skin,” ropey
type flow
• rough, blocky, sharp,
angular type flow
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Pyroclastic materials
Ash
Volcanic
Bombs
Bombs
Nuee-Ardente Lahars
Lahars mud flows
Ash
Ryholitic magmas
• high silica
• very explosive
Nuee-Ardente
• thick, pasty
• high viscosity
• pyroclastic ejections
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Why are there various types of volcanoes???
What factors govern the structure of volcanoes??
Features of a typical volcano:
crater
flank
flank
conduit
Magma Chamber
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VOLCANOES.
Discuss with a friend:
1. Describe the differences between aa and
pahoehoe type basaltic flows.
2. Describe a pyroclastic type flow. Describe
3 types of pyroclastic flows.
3. Draw a typical volcano and label the
common features.
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Three types of Volcanoes
•Volcano type is dependant on
SiO2 content.
Shield
Composite
(stratovolcano)
Explain the differences.
Cinder cone
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Shield Volcano - Hawaii
Broad, low angle flanks
Shield Volcanoes
•Hawaiian Islands, Iceland, Galapagos Islands
•commonly rise from the deep ocean floor
•formed by the accumulation of fluid basaltic flows
•low silica content (basaltic composition)
•low viscosity
•less than 1% pyroclastic debris
•non-explosive eruptions
•pahoehoe flows
•aa flows
Stratovolcano
Composite Cones (stratovolcanoes, stratacomposite)
•Western U.S. coast, Western South American coast, Japan
• typically form in the ocean along continent convergent boundaries
• found along the ring of fire
Steep high angle flanks
Pyroclastics
•formed from layering deposits of
ash, lava, and pyroclastic flows
•high silica content (70%)- (Rhyolitic
composition)
•high viscosity flows
•abundant pyroclastic activity
•deadly airborne debris
•explosive eruptions – very hazardous
The Ring of Fire
Cascade Mt. Range
Stratovolcanoes
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Old lahars
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Cinder Cones
•Exist all over the earth’s surface (by the 1000’s)
•Located in volcanic fields (Flagstaff, AZ– about 600)
Very high, steep angled flanks
30-40 degrees
Averages 100 ft – 1000 ft high
•Formed by gas rich basaltic flows (low viscosity, low silica)
producing small sized material. Common
rock scoria and volcanic glass
•Single eruptive episode lasting
a short time
•Composed of scoria and loose
pyroclastic material
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Cinder Cones
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I see extensive lava flows, but where are the volcanoes?
Fissure type eruptions and lava plateaus
• very fluid basaltic lava erupted from fractures in the earth’s
crust
• lava fountains along “linear” fractures spreading out over
wide areas
• extrudes voluminous amounts of low silica basaltic lava
• single flows can travel 100’s of kilometers
Columbia River basalts
My study Area
Linear cracks (fissures)
•
11 m.y. flows
•
very extensive –single flows from
Idaho to Portland, Ore.
•
1 mile thick in southwest
Washington
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VOLCANOES.
1. Fill in the following blanks given the chart
below:
Pyro- Volcano
SiO2
type
Composition content Viscosity clastics
Mafic
basaltic
Intermediate
Felsic
rhyolitic
Low
<50%
Low
None
Approx Intermediate some
60%
High
70%
high
high
Shield type
Flood basalts
Composite
Cider-cone
pyroclastic
Composite type
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Divergent plate volcanism
Plates separate resulting from basaltic magma
ascending into fractures. Shield type volcanoes
form ridges and mountains below the ocean.
•
•
•
•
Very fluid eruptions
Less than 50% SiO2 content
Shield type volcanoes
Basalt rocks
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Ocean – Ocean plate convergence
•Very fluid eruptions
• Less than 50% SiO2
• Shield type volcanoes
• Basalt rocks
Oceanic plate subducts beneath oceanic plate. Melting
subducted plate ascends upward forming shield type
volcanoes in the form of island arc systems “mountainous
arcs” that rise above the ocean floor. – Japan, Aleutian
Islands
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Baker
Pacific Plate
Rainier
St. Helens
Adams
Hood
Jefferson
North
American
Plate
Three Sisters
Newberry Volcano
Crater Lake
McLaughlin
Medicine Lake Volcano
Shasta
Lassen Peak
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Ocean to Continent
convergence
Oceanic plate is subducted
beneath continental plate.
Melting plate ascends
upward mixing with
continental material.
• High SiO2 – High viscosity
• explosive volcanoes
• “pasty” lava flows
• composite type volcanoes
• andesite/rhyolite rocks
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VOLCANOES.
Discuss with a friend:
1. Draw a diagram that shows how the
shield and composite type volcanoes are
related to their respective plate boundary
or “plate tectonic setting.”
I will get an A on my exams and quizzes.
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Why and How Rocks Melt
Granite/Shale (common
rocks) typically begin to
melt at 8000C and turn to
liquid at 12000C.
Why a temperature range?
The range of temperature for complete melting of a rock
is due to various individual mineral melting point
characteristics.
Amphibole (hornblende) melts
at around 9000C.
Feldspar (orthoclase) melts
at around 7000C.
Quartz melts at around 6000C.
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What factors influence the melting points
of rocks?
• Temperature:
• geothermal gradient
• Pressure:
• influence of pressure at depth
• pressure/temperature relationship
• Presence of water in the rocks:
• water in the subduction zone
• how water influences the melting point
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Temperature inside the earth
0 500
1000
Geothermal
gradient
1500 2000
• the rate at which
temperature increases
with depth
Continent gradient
5,000
200
10,000
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• In thicker crust,
gradient increases.
• average 7oC/km rate
• temperature increases
gently
Oceanic gradient
300
400
Pressure (mpa)
Depth (km)
100
15,000
• Below the ocean floor,
temperature increases
rapidly.
• average 130C/km
How does pressure influence the melting point
of rock?
Pressure
inside the earth with depth.
• Pressure/Temperature increase with depth.
• Mantle (mesosphere) reaches temperatures
beyond rock melting point, but the mantle
remains “solid!”
• Increased pressures raise melting points.
• At a depth of 100 km, pressure is 35,000
times greater than at sea level.
• Many minerals will begin melting at
temperatures above melting points at
sea level.
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How does the presence of water influence the
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rock’s melting point?
The presence of water or water vapor
the rock’s melting point.
• water present in subduction zones
The introduction of water
decreases melting points
and creates melted plate
material that moves upward.
Magma stays liquid due
to the decrease in pressure
as magma rises.
(decompression melting)
Introduction of
water
Cooling and Crystallization of Magma
• What is magmatic differentiation?
• What is the significance of Bowen’s reaction series?
• How do you get a rhyolitic composition from a
basaltic magma?
Put the following igneous rocks in the order of
their compositional equivalence.
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granite
basalt
diorite
rhyolite
gabbro
andesite
Is it possible to
create a granite
composition from
a basaltic magma?
Which minerals form first, second, third…….etc.?
Bowen’s reaction series
Gabbro
Diorite
Granite
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Magmatic differentiation:
• the formation of many kinds of igneous rocks from
a single magma
Simple example
SiO2
Fe
SiO2
Fe
SiO2
Mg
SiO2
Mg
SiO2
Fe
Fe
SiO2
Mg
Mg SiO2
SiO2
SiO2
Liquid magma
FeSiO2
Cooling
liquid
SiO2
SiO2
SiO2
FeSiO2
MgSiO2
MgSiO2
FeSiO2
FeSiO2
MgSiO2 solid
Part liquid/solid
How has the liquid magma changed composition?
As the liquid magma begins to cool, various minerals precipitate as
solids and become separated from the liquid melt. This separation
of various chemistries changes the composition of the original magma.
Three ways crystals separate from a melt:
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Filter pressing:
• Remaining melt is pushed
through a fracture and
separated from xlized melt.
Crystal settling:
• The first minerals to
xlize are denser and
sink to the bottom.
Crystal floatation
• The first crystals are
less dense and rise
to the top.
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crystallizing.
Discuss with a friend:
1. Why do rocks typically have a
temperature range to become completely
melted?
2. Describe magmatic differentiation.
3. Describe the three ways crystals are
separated.
4. What is Bowen’s reaction series?
I will get an A on my exams and quizzes.
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What types of features are
formed
when magma
cools below the surface?
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Dikes
Tabular
Tabular intrusive bodies
forming below the earth’s
surface
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Intrusive Bodies:
Batholith: intrusive body GREATER than 40 mi2
Stock: intrusive body LESS than 40 mi2
Dike: intrusive body cutting across strata
(disconcordant)
Sill: intrusive/extrusive body parallel to strata
(concordant)
Laccolith: “mushroom-shaped” intrusive body
forming a dome-like structure
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Intrusive Bodies
Sill
Loccolith
Stock
Dike
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Melting magma rises and
mixes with continental
material (high SiO2) and
solidifies beneath the
surface.
Sierra Nevada Batholith
Granite/Diorite
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intrusive
rocks.
1. Given the block diagram below, describe
the following plutonic (intrusive) type
features:
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