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Volcanoes
Prepare for Quiz
Print Name: Bill Shields
Lab Section: 12
TA: Ryan
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Signature: #&$*&(*&)*(&
1. Igneous rocks high in silica are dark in color.
A. true B. false
2. Igneous rocks which cool slowly have coarse-grains, (large crystals).
A. true B. false
3. Volcanic igneous rocks are fine-grained, (small crystals).
A. true B. false
4. Extrusive igneous rocks are plutonic.
A. true B. false
5. Mafic igneous rocks are low in iron and magnesium.
A. true B. false
Increasing Fe and Mg
Increasing silica (SiO2)
Igneous Rock
Classification
Phaneritic
light colored
Mafic
intermediate
high in SiO2
dark colored
low in SiO2
Plutonic (magma)
(coarse grained)
Intrusive
Aphanitic
Volcanic (lava)
(fine grained)
Felsic/ Silicic
Extrusive
Granite
Diorite
Gabbro
Rhyolite
Andesite
Basalt
Mount
Vesuvius
Pompeii
Types of Volcanoes
*Based on type of eruption
Nonexplosive – shield volcano
Explosive – composite (strato) volcano
Largely a function of
- viscosity
- dissolved gases
Types of Volcanoes
Shield Volcanoes produce
Non-explosive eruptions
Shield Volcano:
gentle slopes, layered lava flows
Shield Volcano:
gentle slopes, layered lava flows
Fissure Flows
Basalt Flow
14-16 million
years ago
3500m thick
300 separate
flows
Covers 164,000
sq. Km
Composite Volcanoes
Produce explosive eruptions
Composite (Stratovolcano)
Alternating layers of lava, ash, and pyroclastics
Composite (stratovalcano)
steep-sided, alternating layers of lava,
ash, and pyroclastics
Mt. St. Helens –
before eruption
Mt. St. Helens –
after eruption
Volcanism at Divergent
Margins
Few actual volcanoes, lava erupts from
fissures.
Molten results from decompression
melting.
Mafic in chemical composition.
Divergent Plate Boundaries
Pillow
Lava
Submarine eruption of basaltic lava
Volcanism at Convergent
Margins
Volcanoes form in a line parallel to the
subduction zone.
Molten material is generated by heating
"wet" rocks that are being subducted.
Convergent Plate
Convergent plates
of the Northwest
Volcanism at Convergent
Margins
Silicic magma chemistry
Stratovolcanoes
- steep-sided
- alternating layers of lava, ash and
pyroclastics
- explosive (due to high magma viscosity
and gas content
Mantle Plumes (Hot Spots)
Molten material rising from deep within the
Earth.
Mafic magma chemistry.
Can form shield volcanoes – broad and
gently sloping sides, non-violent
eruptions, built by repeated layers of lava
flows (Hawaiian Islands).
Hawaiian Islands
Tephra
- A mixture of hot gas and fragments of all sizes
Ash
Lappili
2-64 mm
Block &
Bombs
Lavas
Pahoehoe
Lavas – A’a
Formation of craters and calderas
Resurgent Dome
Hazardous Volcanic Geologic Processes
1. Tephra Fall
2. Pyroclastic Flows
3. Pyroclactic Surges
4. Explosive Ejection of Ballistic Projectiles
5. Lateral Blasts
6. Lava Flows
7. Lahars
Tephra Fall
Mixture of hot gas and Fragments less dense
than air
Major Hazards of Tephra Fall
1. Impacts
2. Burial of structures
3. Suspension of abrasive aerosols
Pyroclastic Flows
Avalanches of hot, dry, volcanic rock
fragments and gases
• Denser than air
• 600 - 1500O F
• Up to 200 mph
• Tends to channel into valleys
Causes of
Pyroclastic Flow
1. High vertical eruption
columns
2. Boil over
3. Disruption of hot dome
1902 Eruption - Mt. Pelee
St. Pierre, West Indies
Pyroclastic Surges
Avalanches of hot, dry, volcanic rock
fragments and gases
• Less dense than air
• 600 - 1500O F
• Up to 200 mph
• Not confined to channels
Pyroclastic Surge Hazards
•Incineration
•Destruction by high-velocity
ash-laden winds
•Impacts by rock or flying material
•Exposure to noxious gases (SOx) (CO2)
Explosive Ejection
of
Ballistic Projectiles
• Not constrained by wind direction
• Not dependent on eruptions
• Large projectiles thrown 1/2 mile
Ribbon
Fusiform
BOMBS
Rotational
Bread crust
BLOCKS
Lava Flows
• Flow controlled by topography
• Destructive, but not life threatening
• Speed related to slope and viscosity
Pahoehoe Flow
Aa - lava
Pahoehoe
‘A’a Flow
Diversion Barriers
Lahars
• Rapid flow of sediment, rock, and water
• Channelized
• Twice as fast as water
Lahars
Predicting Eruptions
• Ground tilt and displacement
• Increases in surface temperature
• Monitoring earthquakes
• Changes in gas composition
Tragedy at Lake Nyos, Aug, 1986
Nyos Degassing