Download PDF file of Chapter 5 lecture - Volcanoes

Dr. James Wittke
[email protected]
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Explosiveness mainly controlled by magma
viscosity
Viscosity measures a material’s resistance to
flow
◦ Low viscosity magma flows easily
◦ High viscosity magma is very “sticky”
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Magma viscosity controlled by…
◦ Composition (mainly SiO2 content)
◦ Temperature
◦ Dissolved gases
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Composition
◦ Higher silica  higher
viscosity
◦ Lower silica  lower
viscosity
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Temperature
◦ Cooler  higher viscosity
◦ Hotter  lower viscosity
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Gas held in magma by
pressure
Gas expands as magma
rises (decreasing
pressure)
◦ Bubbles form and decrease
magma density
◦ Lower density  increases
rate of rise
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Eruption violence related
to how easily gas can
escape from magma
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Hot, low viscosity,
gas-poor magma
(basalt)  quiet
eruptions
Cooler, high
viscosity, gas-rich
magmas (rhyolite or
andesite) 
explosive eruptions
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Pahoehoe has twisted
or ropey texture
A’a’ has rough, jagged
blocky texture
Lava transported in
tubes through partially
cooled flows; erupted at
edges of flow
Pillow lavas form when
lava contacts water
Examples of lava
flows in Hawaii
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Thick, steep-sided, obsidian flows
Thurston Lava Tube, Hawaii
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Gas mainly water vapor and carbon dioxide
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High gas content  pyroclastic rocks
◦ 1-6 % of magma (by weight)
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Pumice: porous rock
(“frothy” lava)
Ash and dust: fine, glassy
fragments
Lapilli: walnut-sized
material
Particles larger than lapilli
◦ Blocks: hardened or cooled
lava
◦ Bombs: ejected as hot lava
Ash
Ash Fall
Lapilli
Block
Bomb
A place where magma reaches the surface.
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Vent – connected to
magma chamber by
a pipe or conduit
Crater – steepwalled depression at
volcano top
Caldera – very large
depression formed
by collapse after
massive eruption
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Parasitic Cone –
small vent on side
of volcano (fed by
branch off main
conduit)
Fumarole – vent
on side that emits
only gasses
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Volcanic landforms
◦ Shield volcanoes
◦ Composite volcanoes
◦ Cinder cones
Volcanic pipes
 Fissure eruptions
 Large caldera
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Broad, slightly domed-shaped
Primarily basaltic lava
Mild eruptions of large volumes of lava
(generally cover large areas)
Example: Mauna Loa (Hawaii)
Curtain of
Fire
Fissure eruptions
associated with
Pu’u O’o’
Summit
caldera
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Built from ejected lava
(mainly lapilli-sized)
fragments
Steep slope angle
Rather small size
Occur as parasitic vents
on shield volcanoes or
in groups
Also called scoria
cones
SP Crater,
located
north of
Flagstaff
Sunset
Crater
SP
Crater
Paracutin
Pu’u O’o’
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Large, cone-shape volcano (1000s ft. high & miles wide at
base)
Most next to Pacific Ocean in “Ring of Fire” (e.g.,
Fujiyama, Mt. St. Helens)
Alternating lava flows and layers of pyroclastic debris
Most violent type of activity (e.g., Mt. Vesuvius)
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Produced by composite
volcanoes
◦ Associated with felsic &
intermediate magma
◦ Consist of hot gases (up to
800 C), ash, pumice, and
other debris
◦ Flow down sides of a
volcano at speeds up to
200 km/hr
◦ Also called nueé ardente
May 8, 1902
eruption killed
29,000 people
and destroyed St.
Pierre
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Volcanic mud
flow
Very often due to
reactivation of
volcanic deposits
by heavy rains or
snow melt
Rapid movement
(up to 100 kph)
Results of lahars
associated with
Mount St. Helens
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Large roughly circular depressions with
diameters >1 km
Form by three processes
◦ Collapse of summit of large composite volcano (Crater
Lake)
◦ Gradual subsidence at top of shield volcano due to
magma withdrawal (Kilauea)
◦ Collapse of large area due to eruption of large
amounts of silica-rich magma along ring fractures
(Yellowstone, Long Valley)
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Fluid basaltic lava
erupts from
fractures in Earth’s
surface (fissures)
Lavas cover very
large areas  flood
basalt plateaus
◦ Columbia River
Plateau (northwest
USA
◦ Deccan Traps
(central India)
Laki
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Bulbous mass of
congealed silicarich lava
Form after main
explosive
eruptions, blocking
vent
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Solidified magma
conduit
Resistant rock left
after erosion has
removed the
overlying volcano
Examples, Devil’s
Tower (WY),
Shiprock (NM)
Aglatha Peak,
Navajo Nation
dikes
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Form when magma hits
groundwater
Steam explosion forms circular pit
Examples, Coliseum (AZ),
Kilbourne Hole (NM)
Coliseum, Hopi Buttes, Navajo
Nation
Weinfelder Maar, Eifel Volcanic
Field, Germany
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Deep volcanic conduits
into mantle
Gas-rich magma brings
up pieces of mantle very
rapidly
In kimberlite pipes,
magma may carry
diamonds
A maar forms at surface
Kimberley, South Africa
Mirny, Russia, East Siberia
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Global distribution of volcanism is not
random
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Most volcanoes within or near ocean basins
Basalt common in both oceans and
continents
Granite/rhyolite rarely found in oceans
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Greatest volume
produced at
oceanic ridges
◦ Lithosphere pulls
apart (mid-ocean
ridges, continental
rifts)
◦ Decompression
melting of mantle
◦ Large quantities of
basalt
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Places of subduction
marked by deep
oceanic trenches
Descending plate
adds water to mantle
 hydration melting
Rising magma can
form either…
◦ Island arc (in ocean)
◦ Volcanic arc
(continental margin)
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Activity inside plate (not at
edge)
Localized volcanic region
inside plate called hot spot
Columns of hot rising
material in mantle (plumes)
 decompression melting
◦ Basaltic magma in oceanic crust
(e.g., Hawaii, Iceland)
◦ Flood basalts
◦ Basalt intrusion of continental
crust  granitic melts (e.g.,
Yellowstone Park)
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Pyroclastic flows
Lahars
Collapse of flank
or dome of
volcano
Ash (hazard to
aircraft)
Lava flows