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10/6/2015
The Nature of Volcanic Eruptions
GEOL 110: GEOLOGY
Mt. St. Helens
GEOL110WSP2015
MGEOLROUGHLEY52385
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Chapter 5:
VOLCANOES AND
VOLCANIC HAZARDS
•https://www.youtube.com/watch?v=-H_HZVY1tT4
•David Johnston 6 miles away
Not for Distribution. For study purposes only for related class.
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© 2014 Pearson Education, Inc.
Volcanoes
The Nature of Volcanic Eruptions
The Nature of Volcanic Eruptions
• All volcanic eruptions involve magma
• Mobility of magma is called viscosity
• The more viscous the material, the greater its
resistance to flow
• Ex: honey is more viscous than oil
3 Types of Volcanoes
• Cinder Cones: Cinder Cone
• Shield: Prospect Peak
• Stratovolcano (composite): Mt. Tehama
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Photo reference cited in notes
Photo reference cited in notes
The Nature of Volcanic Eruptions
Viscosity is dependent upon the magma’s:
• Temperature: hotter =less viscous; cooler =more viscous.
• Chemical composition: more Si =more viscous
• Dissolved gases: Dissolved water in magma reduces
viscosity;
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The Nature of Volcanic Eruptions
Explosivity of
Volcanoes
•Si-rich magmas create
explosive eruptions as
gases become “stuck” in
the viscous fluid of the
magma.
•Fluid basaltic lavas
create quieter eruptions
due to gases readily
escaping the magma at
eruption.
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© 2014 Pearson Education, Inc.
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The Nature of Volcanic Eruptions
The Nature of Volcanic Eruptions
Different compositions result in volcanic
landforms
Quiescent vs. Explosive Eruptions
• Quiescent Hawaiian‐Type Eruptions
Pyroclastic
flows; lava
domes
Stratovolcano
• Involves fluid basaltic lavas
• Eruptions are characterized by outpourings of lava that can last weeks, months, or even years
Shield
volcanoes,
basalt plateaus,
cinder cones
• Explosive Eruptions
• Associated with highly viscous magmas
• Volatiles released; not major explosives
• Eruptions expel particles of fragmented lava and gases at high speeds that evolve into eruption columns.
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Photo reference cited in notes
Materials Extruded During an Eruption
What materials are
extruded during
eruptions?
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Materials Extruded During an Eruption
Lava on Earth
• ~1 percent of lava is rhyolitic lava
• <10 percent of lava is andesitic lava
• ~90 percent of lava is basaltic lava
• Lava Flows
• Gases
• Pyroclastic Materials
(pulverized rock and lava
fragments)
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Materials Extruded During an Eruption
Basaltic Lava Flows
Very fluid; flow in thin, broad sheets
or stream-like ribbons; move quickly.
• Aa flows have surfaces of rough
jagged blocks
• Pahoehoe flows have smooth
surfaces and resemble twisted
braids of rope
• Pillow Lavas are composed of
basaltic lavas extruded
underwater
•
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Materials Extruded During an Eruption
Andesitic and Rhyolitic Lava Flows
•Thick flows
•Andesitic eruptions are intermediate between basaltic and
rhyolitic.
• Block Lava Flows are composed of andesitic and rhyolitic
lava; upper surface consists of massive, detached blocks
https://www.youtube.com/watch?v=0yWYlv3x
G-c
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Materials Extruded During an Eruption
Gases
• Gases make up 1%‒6% of the
total weight of a magma
• Mostly water vapor with smaller
amounts of CO2, S, Cl, and
other gases
• Gases are held in molten rock
by confining pressures
• Gases are released as the
magma reaches the surface and
the pressure is reduced
• Relative proportion of gases
varies by volcanic region
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Materials Extruded During an Eruption
Materials Extruded During an Eruption
Pyroclastic Materials:
Pulverized rock and lava
fragments ejected from a
volcano
5 Types by Size:
• Volcanic Ash – fine, glassy
fragments; welded tuff is
created when hot ash falls and
glassy fragments fuse together.
• Lapilli – pyroclasts 2-64mm in
size; walnut size
• Cinders – pyroclasts 2-64mm
in size; pea size
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Materials Extruded During an Eruption
Pyroclastic Materials
Pyroclastic Materials
5 Types by Size (con’t):
2 Types by Composition:
• Scoria – reddish-brown
porous rock from frothy
basaltic and andesitic lava;
many voids (vesicles); more
dense than pumice
• Blocks hardened or cooled
lava fragments >64mm
(2.5”)
• Bombs ejected hot lava
blobs that harden tin
streamlined shapes mid-air
>64mm (2.5”)
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Anatomy of a Volcano
General Features of a Volcano
• Pumice – vesicular rock
made from “frothy” lava;
many very small vesicles;
less dense than scoria;
andesitic to rhyolitic
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Anatomy of a Volcano
General features at the summit
Classified by eruptive patterns and landform
General features at the summit
• Conduit – a fissure that magma
moves through to reach the
surface
• Vent –surface opening of a conduit
• Volcanic cone – cone shaped
structure created by eruptions of
lava and pyroclastic material at
vent
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• Crater – a funnel-shaped depression at
the summit of most volcanic cones;
generally less than 1 km in diameter;
formed by accumulated materials
• Caldera – a volcanic crater >1 km in
diameter; produced by a collapse
following a massive eruption
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Anatomy of a Volcano
Volcanoes
General features on the sides
3 Types of Volcanoes
• Parasitic cones – a flank vent that
emits lava and pyroclastic
material
• Shield
• Cinder Cones
• Composite
(Stratovolcano)
• Fumaroles – a flank vent that
emits gases (Lassen)
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Shield Volcanoes
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Shield Volcanoes
Shield Volcanoes
• Mauna Loa is the largest shield volcano on Earth
• Produced by mild eruptions of large volumes of
basaltic lava
• Broad, slightly dome-shaped
• Most begin on the seafloor as seamounts; only a few
grow large enough to form a volcanic island
• Gentle slopes formed from very hot, fluid lava that
travels far from the vent
• Often have a large, steepwalled caldera at summit
from magma chamber collapse
• Examples include the Hawaiian Islands, the Canary
Islands, the Galapagos, and Easter Island
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Cinder Cones
Cinder Cones
• Lassen NP
• Built from basaltic ejected lava that hardens in flight
(scoria)
• Material is commonly pea- to walnut-sized
• Can produce large lava flows in final eruptive stages
• Steep slope angle: 30-40°
• Frequently occur in groups; sometimes associated
with extensive lava fields
• Can be produced as parasitic cones, or in calderas
of large volcanoes
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Shield Volcanoes
Mauna Loa
•Earth’s largest shield
volcano
•9km (6mi) tall
•1 of 5 overlapping shield
volcanoes of Hawaiian
Islands
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Cinder Cones
Paricutin in Mexico City
• Grew over short period (9 years)
• Produced ash and volcanic fragments
• Ended with aa flows 30 meters thick that covered
nearby villages
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Composite Volcanoes
Composite Volcanoes
Composite Volcanoes (Stratovolcanoes)
Composite Volcanoes (Stratovolcanoes)
• Large, classic-shaped volcano (symmetrical cone,
thousands of feet high and several miles wide at the
base)
• Composed of alternating layers of cinders and ash
interbedded with lava flows
• Viscous nature of eruptive materials
• Made from silica-rich, andesitic magmas
• Many will also emit basalt lava and rhyolitic
pyroclasts
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• Most erupt explosively with huge quantities of
pyroclasts
• Fine ash Is deposited in a thin layer over large area
• Course fragments accumulate near the vent and produce
steep slopes of summit
• Long lava flows common in early stages of growth; become
shorter with time
• Many have secondary vents that create ciner cones
or other volcanic structures on flanks
• Many are located adjacent to the Pacific Ocean in
the Ring of Fire
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Composite Volcanoes
Volcanic Hazards
Volcanic Hazards
Mount St. Helens, WA
Fujiyama, Japan
Mt. Shasta, CA
Mount Etna, Italy
• Pyroclastic Flows
• Lahars
• Other
•
•
•
•
•
Fujiyama
Ash and pyroclasts deposits
Tsunami
Airborne Ash
Volcanic Gases
Ash and gases in atmosphere
Mt. Shasta
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Volcanic Hazards
Volcanic Hazards
Pyroclastic Flows
• Mixture of hot gases infused
with ash and lava fragments that
flows down a volcanic slope
• Cloud of hot gas and ash with
ground-hugging mixture of
pumice and pyroclasts
• Moves very quickly >60mph
• Driven by gravity, mobilized by
expanding gases, follows valleys
and other topographic lows.
Photo reference cited in notes
Lahars
https://www.youtube.com/watch
?v=Cvjwt9nnwXY
• A mudflow created as
volcanic debris becomes
saturated with water
• Follows stream valleys
downslope
• Common on glacier and
snow-covered volcanoes
and heavy rainfall areas
• Some lahars are triggered
when magma nears the
surface of a volcano
covered in ice and snow
and causes it to melt
https://www.youtube.com/watch?v=kznwnpNTB6k
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Volcanic Hazards
Other hazards
• Volcano-related tsunamis due to sudden
collapse of volcanic flanks into the ocean
• Volcanic ash – a hazard to airplanes; destroy
plant life
• Volcanic gases – a respiratory health hazard
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Other Volcanic Landforms
Other Volcanic Landforms
Other Volcanic Landforms
•
•
•
•
•
Calderas
Fissure Eruptions
Lava Domes
Volcanic Necks
Volcanic Pipes
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Formation of Crater Lake-Type Calderas
Calderas are circular, steep-sided depressions
with a diameter >1 km
Crater Lake-type
calderas:
Three different types
• Crater Lake-type calderas:
• Hawaiian-type calderas:
• Yellowstone-type calderas:
Form from the collapse
of the summit of a large
composite volcano
following an eruption;
these calderas eventually
fill with rainwater
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Hawaiian-Type Calderas
Hawaiian-type calderas: Form gradually from the
collapse of the summit of a shield volcano following the
subterranean drainage of the central magma chamber
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Formation of Yellowstone-Type Calderas
Yellowstone-type calderas: Form from the collapse
of a large area after the discharge of large volumes of
silica-rich pumice and ash; these calderas tend to
exhibit a complex history
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Other Volcanic Landforms
Fissure Eruptions
• Long, narrow cracks in Earth that emit fluid basaltic
lava
• Creates large, flat, broad igneous provinces (basalt
plateaus)
• Also called flood basalts
• The Colombia Plateau and the Deccan Traps are
two examples
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Other Volcanic Landforms
Lava Domes
• Viscous silica-rich, ryholitic lavas “squeezed” out of a
vent, creating a dome-shaped mass
• Usually only a few 10’s of meters high
• Can collapse and produce pyroclastic flows
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Other Volcanic Landforms
Volcanic Necks
• The remains of magma that solidified in a volcanic
conduit
• Weathering erodes surrounding volcanic structure,
leaving resistant conduit
• Ex: Devil’s Tower, WY
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Other Volcanic Landforms
Volcanic Pipes
• Conduits of magma from deep within the
mantle that traveled upward through the
crust
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Plate Tectonics and Volcanic Activity
Plate Tectonics and Volcanic Activity
Volcanic activity not random, most commonly associated
with plate boundaries
• Convergent Boundary
• Divergent Boundary
• Intraplate
Plate Tectonics and Volcanic Activity
Convergent Plate Boundaries
• Partial melting of subducting plates creates
buoyant magma that rises in crust
• Most common along Pacific Ocean Ring of Fire
• Eruptions tend to be explosive and associated
with volatile-rich, andesitic magma
http://earth.rice.edu/mtpe/geo/geosphere/topics/plates2.gif
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Plate Tectonics and Volcanic Activity
Convergent Plate Boundaries
• Volcanic arcs develop parallel to the associated
subduction zone trench
• The Aleutians, the Tongas, and the Marianas are
examples of volcanic island arcs
• The Cascade Range is an example of a continental
volcanic arc
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Plate Tectonics and Volcanic Activity
Divergent plate
boundaries
• Mid-Atlantic Ridge
• ~60% of Earth’s yearly output
of magma is created at
oceanic ridge systems
• Mantle material melts through
decompression melting as it
rises to fill rift
• Basaltic magma creates
oceanic crust, pillow basalts
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Plate Tectonics and Volcanic Activity
Intraplate volcanism
• Occurs when a mantle plume ascends towards
the surface
• Occur thousands of kilometers from plate
boundaries
• Ex. include the Hawaiian Islands, the Columbia
River Basalts, and the Galapagos Islands
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