Download Volcanoes and Igneous Activity Earth - Chapter 4

Volcanoes and Volcanic Hazards
Mt. Etna, Italy
Classify Volcanoes by…
Eruptive Style
 Volcanic Activity
 Shape of Volcano/ Ejected Material


But first…need to understand viscosity.
Viscosity
Viscosity: measure of a material’s
resistance to flow.
(How much it doesn’t want to flow.)
 Basically, how thick it is.
 Very thick = very viscous
 Less thick = less viscous

High, medium, or low viscosity?
High, medium, or low
viscosity?
High, medium, or low
viscosity?
High, medium, or low viscosity?
High, medium, or low
viscosity?
High, medium, or low
viscosity?
High, medium, or low viscosity?
High, medium, or low
viscosity?
High, medium, or low
viscosity?
High, medium, or low viscosity?
High, medium, or low
viscosity?
High, medium, or low
viscosity?
Viscosity is affected by:
1. Composition
2. Temperature
Composition
Lava is like cookie dough!
A little flour, runny
dough! A little
silica, runny lava!
A lot of flour, thick
dough! A lot of
silica, thick lava!
Composition





Mafic: describes magma or igneous rock that is rich
in magnesium and iron (Fe) and is generally dark in
color.
Felsic: describes magma or igneous rock that is rich
in feldspars and silica and that is generally light in
color.
Intermediate: in between the two.
It is the amount of silica that affects viscosity!
Which is more viscous, a mafic, felsic, or
intermediate magma?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Mafic, intermediate, or felsic?
Composition
The more silica it has, the more viscous it
is.
 Less silica, less viscous.
 Example: Felsic lava (rhyolite) is more
viscous than mafic lava (basalt)

Viscosity is affected by:
1. Composition
2. Temperature
Temperature
Lava is like syrup!
•Hot syrup is runny,
and so is hot lava!
•Cold syrup is thick,
and so is cold lava!
Temperature
The hotter the magma/lava, the less viscous it
is.
 The cooler the magma/lava, the more viscous
it is.

High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
High, medium, or low
temperature?
Classify Volcanoes by…
Eruptive Style
 Volcanic Activity
 Shape of Volcano/Ejected Material

Eruptive Style

The eruptive style or explosiveness of a
volcanic eruption depends on:
 viscosity of the magma
 Amount of dissolved gases
Eruptive Style (cont.)

Dissolved gases play an important role
 Magma has dissolved gasses in it.
 Gases expand within a magma as it nears
the Earth’s surface due to decreasing
pressure
 The violence of an eruption is related to
how easily gases escape from magma
Example:

Soda, like magma, has dissolved gasses in it.
However, the can puts so much pressure on the
gasses that they are dissolved (no bubbles!)
When the pressure is released by the can being
opened, the bubbles form and try to push
through and leave the soda!
It is the same
with magma!

When magma is deep in the earth, there is a
lot of pressure on it.

As the magma moves closer to the surface,
there is less pressure on it.
Since the pressure on the magma is less
now, the dissolved gasses start to form
bubbles, causing the magma to expand.
 This builds up internal pressure.

•The bubbles try to leave the magma, just like
the bubbles in the soda!
•This also builds up internal pressure.
Eruptive Style
Explosive/Violent Eruptions
 Quiet Eruptions

Remember: magma is more viscous than
soda. So…
 If the magma is very viscous, bubbles don’t
escape easily.
 If the bubbles can’t escape, they build up
pressure in the magma.

•Pressure builds up so much that fracturing in
the overlying rock occurs,
•This lowers the confining pressure (the
pressure keeping it in) more, and even more
gas bubbles form.
The volcano eventually explodes when the rocks
above it can’t keep the magma in anymore!
 Therefore, you have a violent eruption!

Or…
 If the magma has a low viscosity, gas bubbles
are able to escape easier.
 Therefore, not much pressure is built up and the
lava just flows out gently.
 Therefore, you have a quiet eruption!
Example: 1980 Mt. St. Helens, WA
Figure 5.1 (top)
Example: 1980 Mt. St. Helens, WA
Building Up Pressure
Mount St. Helens, Washington

Collapse of the volcano’s flank (side) caused
the confining pressure to drop rapidly.
Violently Releasing Pressure
Figure 5.1 (bottom)
Most explosive eruptions are followed by
“degassed” lava.
 Example: Mt. St. Helens
 For six years, extrusions of lava built a
dome inside the crater.
 Then, in October 2004, it erupted again as
small gas plumes.


USGS Photograph taken on May 19, 1982, by Lyn Topinka.
Mount St. Helens, Washington: Debris Avalanche and
Eruption
Explosive Eruptions

Occurs with viscous lavas
 These lavas have high SiO2 contents
 High gas contents also contributes to
explosiveness
 Gas pressure builds up until an explosion
occurs
 Example
• Mt. St. Helens 1980
Quiet Eruptions

The lava is very fluid


These lavas have a low SiO2 content
Examples
Mauna Loa, Hawaii
 Kilauea, Hawaii


The type of eruption depends on the
chemical composition of the lava and the
gas content
Example: Kilauea, Hawaii
In the beginning of the
eruption, there is an initial
release of some pressure,
resulting in a lava fountain.
Then it just gently flows out!
Classify Volcanoes by…
Shape of Volcano/ Ejected Material
Shield Volcano
 Composite Volcano/ Composite Cone/
Stratovolcano

Classify Volcanoes by…
Shape of Volcano/ Ejected Material

Shield Volcano
Volcano has very gentle slopes
 Lavas are very fluid (low viscosity); high
temperature
 Typically erupt basalt
 Hawaiian volcanoes are shields

Mauna Kea, Hawaii. Twice as tall as Mt. Everest!
Much of Mauna Kea is below sea level; when measured from its oceanic base, its height
is 33,500 ft (10,200 m) -- more than twice Mount Everest’s base-to-peak height of 3,650
to 4,650 m. http://en.wikipedia.org/wiki/Mauna_Kea
Mauna Loa, Hawaii.
Example:
 If the lava contains only a little silica, and
has a high temperature, it will have a low
viscosity.
 This will cause the lava to be runny and
travel very far.
 A gently-sloping shield volcano is formed.
Lava Types
Basalt
Andesite
Rhyolite
Magma
Type
Silica
Content
Temperature Gas
Content
Basalt
Low (50%)
High (950C)
Least
Viscosity
Volcanic
Landform
Least
Shields,
Plateaus,
Cinder
Cones
Andesite Intermediate
(60%)
Intermediate Intermediate Intermediate Composite
Cone
(750-950  C)
(Stratovolc
ano)
Rhyolite
Low (650-750 Most
 C)
High (70%)
Most
Pyroclastic
Flows,
Volcanic
Domes
Therefore…
 Basaltic
lavas generally produce quiet
eruptions
 Highly viscous lavas (andesite or
rhyolite) produce more explosive
eruptions
Classify Volcanoes by…
Shape of Volcano/Ejected Material

Composite Volcano/ Composite Cone/
Stratovolcano
 They are characterized by explosive
eruptions
 Eruptions of lava alternate with
explosions that heap volcanic debris
around the vent
 Typically erupt andesite
 Mt. Rainier, Mt. Shasta are examples
Mt. Fuji, Japan
El Misti, is a symmetrical stratovolcano that lies above the city
of Arequipa, Peru.
Mt. St. Helens – a typical
composite volcano
Figure 5.1
Magma
Type
Silica
Content
Temperature Gas
Content
Basalt
Low (50%)
High (950C)
Least
Viscosity
Volcanic
Landform
Least
Shields,
Plateaus,
Cinder
Cones
Andesite Intermediate
(60%)
Intermediate Intermediate Intermediate Composite
Cone
(750-950  C)
(Stratovolc
ano)
Rhyolite
Low (650-750 Most
 C)
High (70%)
Most
Pyroclastic
Flows,
Volcanic
Domes
Stratovolcano
Composite cones first erupt violently
because pressure built up in the magma.
 After the explosion, a viscous magma runs
out in a more quiet type of eruption.
 Pressure begins to build up after the
eruption, and the cycle begins again.

Types of Erupted Material
Gases
 Lava
 Pyroclastic Materials

Types of Erupted Material

Gases






70% Water vapor (H2Ov)
15% Carbon Dioxide (CO2)
5% Nitrogen (N2)
5% Sulfur dioxide (SO2), (STINKY!)
5% others
Example: The April 4, 1982 eruption of El Chichon in
Mexico may have caused global cooling.
• Released a lot of SO2, which combined with water vapor to
make sulfuric acid droplets. These droplets absorb solar
radiation and scatter it back to space.
Types of Erupted Material

Lava - molten (liquid) rock
Rhyolite – felsic (light-colored), viscous,
SiO2-rich, low temperature lava
 Andesite – intermediate in: color, viscosity,
SiO2, and temperature
 Basalt – mafic (dark-colored), fluid, SiO2poor, high temperature lava

Types of Erupted Material

Pyroclastic material/ Tephra

material ejected into the air during an eruption
• Ash: < 2 mm
• Cinders/lapilli: 2-64 mm
• Blocks and bombs: > 64 mm
• Blocks: solid when ejected into the air
• Bombs: still molten when ejected into the air

Ash erupted from Tambora, Indonesia in 1815 may
have blocked sunlight and caused global cooling
• Extremely cold spring and summer in 1816: “Year without a
summer”
• Snowfall and frost in June, July and August
• Crops failed
• Sea ice across Atlantic shipping lanes
Classify Volcanoes by…
Volcanic Activity
Active - eruptions have occurred in
historic times
 Dormant - no historic eruptions have
occurred



Seismographs reveal that they are still
capable of eruption
Extinct - eroded down and essentially
gone
The Cascade Volcanoes





They stretch from Mt. Lassen to the Canadian
border north-south through Calif., Oregon, and
Washington
They are located above a subduction zone at a
convergent plate boundary
They are mostly composite volcanoes made of
andesite
They formed within the last 1 million years.
They are dormant and all are capable of future
violent eruptions
Volcanic Hazards
Lava Flows
 Pyroclastic Material / Tephra
 Pyroclastic Flows
 Lahars
 Caldera collapse

Lava Flows – BASALT!!!

Most lava (90%) is basalt



Lava Flows are streams of molten rock.
You don’t die from lava flows!




Because most eruptions are at MORs
World record: 19 miles per hour
Fast: 6 mph
Typical: less than 1 mile per hour
Just move out of the way! Buildings etc. can’t…
Basalt Hawaii
Hazards - Hawaii

Over the Thanksgiving holidays, eight residences were overrun in the community of
Kapa`ahu, 3 km west of Kalapana. Lava approaches a house in Kapa`ahu as people wait to
see which way the lava will move. Nov. 26, 1986
Lava Flow

Feb. 1991
Types of Lava Flows STILL BASALT

AA


Jagged, blocky surface
Pahoehoe
Smooth or ropey surface
 Means “on which one can walk”


Lava flows can start as Pahoehoe and then can
change into Aa.
Pahoehoe has a higher T
 Cooling and degassing pahoehoe often changes
into Aa

Kalapana, Hawaii 1990
Pahoehoe covered village of Kalapana
 Villagers watched for weeks as their town was
buried.
 Lava moved few meters per hour.

April, 1990
Welcome to Kalapana
1990 eruption of Kilauea:
Kalapana

180 homes, visitor center in a national park,
highways, archaeological sites.
February 1991
1986
1990?
1973: Heimaey, Iceland
Lava flow going to harbor.
 Sprayed water on the lava from ships in the
harbor, “stopping” the lava flow.

The 1973 eruption on the island of Heimaey. Photograph by the
late Sveinn Eirikksen, fire marshal of the town of Vestmannaeyjar
and courtesy of the U.S. Geological Survey.
Pyroclastic Materials
Material ejected into the air from a volcano.
 Pyro = fire
 Clast = fragment
 AKA: Tephra

Tephra
Tephra: material ejected into the air from a
volcano.
 Tephra is classified by size.

Sizes of Tephra
Blocks and Bombs: > 64mm
 Lapilli or cinders: 2-64mm
 Ash: < 2mm


What’s the difference between blocks and
bombs?

Blocks are solid rock blasted out by the
explosion. Bombs are when lava is ejected
into the air and becomes smooth in flight.
Blocks
A volcanic bomb
Bombs
Ash
Ash: < 2mm
 From viscous magma
 As magma moves up the vent, the gases
quickly expand, generating a froth of melt.
 As the gases below it expand explosively, the
froth is blown into very fine fragments.


Mt. Etna, 2002

Mount Pinatubo, Philippines, on June 12, 1991, just days before the volcano's climactic explosion on June 15.

Pinatubo,
Philippines, 1991
Lapilli or Cinders

2-64 mm in size

A cinder cone on Mauna Kea, Hawaii
A size comparison of the three
types of volcanoes
Eruption Column

Plume of hot
ash and gas that
can extend
thousands of
meters into the
atmosphere
Mt. Augustine, Alaska
Pyroclastic Flow
AKA: Nuee Ardante
 A hot mixture of ash, pumice and gas,
traveling down the flanks of a volcano.
 FAST!!! Often 125 mph (some say over
200mph)

Air that is trapped by the pyroclastic flow
may be heated enough to provide buoyancy to
the flow.
 Like air hockey


Montserrat in 1996 The dense part of the flow stopped at the beach, but the lighter-than-water, denserthan-air, ash cloud separated and flowed across the water for about 100 meters.
Pyroclastic Flows (cont.)
Mainly from collapse of the eruption column.
 Gravity overcomes the upward thrust, and the
ejecta falls, then flows downslope.

Figure 5.22B
Mt. St.
Helens
60 mph
Vesuvius, Italy
Ruins of Pompeii, destroyed 79 AD. Vesuvius on
horizon.
Mount Pelee, Island of Martinique
in Caribbean
Destroyed town of St. Pierre in 1902
 Killed all 28,000 people on the land in less
than 5 minutes…except for one.

Lahar
Lahar: a wet, cement-like mixture of water,
mud and volcanic rock fragments that flows
down the slopes of a volcano and its river
valleys. Volcanic mudflow.
 Form when

Heat from the eruption melts snow and ice,
 erupted material runs into a river or lake,
 rain mixing with volcanic material after
eruption.


Buries entire villages under mud.
Lahars are the most dangerous volcanic
hazard!
 Lahars are no laughing mater!
 Lahar, har, har.

Nevado del Ruiz, Columbia
Armero
Armero

Gualí River. Houses and towns located high enough above river channels escaped damage from the
lahars. In the Gualí River valley, at least two lahar pulses were reported by eyewitnesses, separated by 5
to 15 minutes depending on distance from the volcano. Eyewitnesses reported that the noise created by
the passage of each pulse made their houses and the ground shake and that conversation, even by
shouting, was impossible. Photograph by R. Janda on November 26, 1985
Mount Pinatubo

Like thousands of other buildings downstream from Mount Pinatubo, this
school house was buried by a lahar after the enormous eruption on June
15, 1991.
Mt. St. Helens
Mount Rainier, WA
Year-round snow and ice.
 100,000 people live in the valleys around
Rainier.
 Many homes built on lahar deposits.


Mount Rainier and Tacoma, Washington, as seen from shore along Commencement Bay.
USGS Photograph taken on August 20, 1984, by Lyn Topinka.


Spray Park. Photo by John Titland
Mount Rainier. Photo by John Wald


Mt. Rainier
Hazard
Map - ash
http://images.google.com/imgres?imgurl=http://pubs.usgs.gov/fs/2002/fs03402/images/fig3.jpg&imgrefurl=http://pubs.usgs.gov/fs/2002/fs03402/&h=672&w=520&sz=83&hl=en&start=46&tbnid=N3ZbtHAMIbo8dM:&tbnh=138&tbnw=107&prev=/images%3Fq%
3DMount%2BRainier%26start%3D40%26gbv%3D2%26ndsp%3D20%26svnum%3D10%26hl%3Den%26safe%3Dactive
%26sa%3DN

Mt. Rainier
Hazard Map Lahars
Plate Tectonics and Igneous Activity REVIEW

Global distribution of igneous activity is not
random
Igneous activity along plate margins
 Divergent
boundaries AKA spreading
centers
 The
greatest volume of volcanic rock is
produced along the oceanic ridge system.
• Two oceanic plates pull apart
• Mantle material and new lavas rise up to
fill the gap
• Large quantities of basaltic magma are
produced
Pillow Lavas
Remember: most volcanic activity is at MORs
at divergent plate boundaries.
 When lava outpours on the ocean surface, the
outer skin quickly cools.
 Lava breaks through the hardened surface,
producing pillow lavas, or pillow basalts.
 Only occur in underwater environments.

 Convergent
 Subduction
boundaries
zones
• Descending plate releases volatiles,
lowering the melting point of rock in the
mantle wedge.
• Magma slowly moves upward
• Rising magma can form either:
• An island arc if in the ocean
• A volcanic arc if on a continental
margin
“Ring of Fire”
 Around
the Pacific Ocean
 Most of the world’s explosive volcanoes
are found here
 Due to convergent plate boundaries
Intraplate Volcanism
Intraplate Volcanism
 A hot
spot is a fixed place where there
are eruptions above a mantle plume.
 Mantle plume: hotter than normal mantle
material (remember: solid) that ascends
to the surface.
 Mantle
plumes
have a bulbous
head followed
by a narrow tail
beneath it.
 Start as deep as
the D” layer
(core-mantle
boundary).
 When
it nears the top
of the mantle,
decompression
melting occurs,
making basaltic
magma.
 The head reaches the
surface, creating a
large outpouring of
lava called flood
basalts.
After the head is
emptied, the tail keeps
going, produces a chain
of volcanoes as the
plate moves above it.
 As the plate moves,
islands move off the
hot spot and are no
longer “active”.
 Examples:
• Hawaii
• Yellowstone
(Snake River
Plain)

Volcanism on a tectonic
plate moving over a hot spot
Figure 5.41
Hot Spot Volcano Tracks
Hawaii
Predict the Future…




What will happen to the “Big
Island?”
What will there be in the future?
There is a new volcano, Loihi,
forming off the coast of the big
island! (still underwater)
There will soon* be a new island!
*Soon being about 10,000 years! Buy your tickets now! 
Loihi
Yellowstone
Hot spots can occur anywhere with a plume
under it.
Through Oceanic Crust
(basaltic)
Through Continental Crust
(andesitic and rhyolitic)
Hot spots help determine past plate movement
Study for exam!