Download Introduction to volcanoes, volcanic eruptions, and volcanic

Introduction to volcanoes, volcanic
eruptions, and volcanic landforms
What mainly controls eruptive style?
• Gas content of magma
• Viscosity of magma
Viscosity in magma
2. Eruptive style:
Explosive or effusive?
• High viscosity magma= high silica
content=more explosive
• Low viscosity magma= low silica
content=less explosive (effusive)
Why are viscosity and gas important?
How would they affect volcanic hazards?
Effusive eruption:
advancing lava flow
Felsic (high SiO2) = high viscosity
and more explosive (e.g. dacite)
Mafic (low SiO2) = low viscosity
and effusive (e.g. basalt)
Explosive eruption:
pyroclastic flow
Unzen Volcano, Japan, 1991
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Where do volcanoes erupt—and how does
this relate to plate tectonics?
Types of volcanoes and volcanic landforms
Shield volcano
ex: Mauna Loa; Larch Mtn, OR
Composite volcano (stratovolcano)
ex: Vesuvius,
Mount Rainier, Popo, Mount Fuji
Domes
ex. Goat Mountain, Black Buttes & Mt. Shasta (CA)
Cinder cones and spatter cones
ex: Paracutin, Puu O
(Kilauea), Sunset Crater (AZ)
Maar volcano
Caldera
ex: Battleground Lake, Ubehebe (CA)
ex: Yellowstone, Toba, Crater Lake
Flood basalt
ex: CRBs 17–15 Ma; Deccan Traps--India 65
Ma; Siberian Traps ~250 Ma)
Lower viscosity basaltic lava (mafic)
is ~45% to 54% silica
Shield Volcano (typically
basaltic!)
Shield Volcanoes
Effusive eruptions (less explosive)
Gentle relief + cinder cones.
Volcano = chiefly lava flows.
Below: aerial view of Hawaii
Kupaianaha lava pond, Kilauea: photo by Pat Pringle, July 1987
Humongous landslides
from Hawaiian
volcanoes (dark gray =
islands). The landslide
deposits (stippled area)
are found on the sea
floor (note scale in km!)
2007 gps velocities ~ 7 cm/yr
So what? There is more to
see below the surface of the
ocean!
Pacific Plate moves over the Hawaiian hot spot; ages of the
Hawaiian Islands + geologic factors reflect plate movement—
older to the NW
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Basaltic volcanism in the Cascade Range
Larch Mtn shield, OR
<= Tumac Mtn shield,
NE of White Pass;
view to East
Clayton (1983) estimated
the age of Tumac Mtn at
30 to 20 ka.
ka = kiloannum
Underwood Mtn
shield, WA
Mount Adams composite volcano is
constructed on top of a broad shield
Composite volcanoes
(aka stratovolcanoes)
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Where are Composite
volcanoes?
~55 – 65 % silica (intermediate SiO2) andesite, dacite
Intermediate viscosity
Moderately violent explosions
~50/50 lavas/ fragmental deposits
Thick lavas (to 20 or even 100 m)
Steep cones; can have domes
Lahars! (volcanic debris flows)
Photo by Pat Pringle
Mount Rainier from
Glacier View Wilderness
• Ring of Fire (Circumpacific belt)
• Mediterranean belt
Glacial erosion increases relief and supplies groundwater => hydrothermal alteration and weakening
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Composite volcano
Below: isopach
map of Vesuvius’
AD 79 tephra
Mount Hood
Left: tectonic setting of Italian
volcanoes; right above: thickness of
deposits from AD 79 Vesuvius eruption
Composite Volcano
Mount Baker
aka stratovolcano
May 18, 1980 Plinian
eruption column Looking
NNW
Photo by Bob Krimmel,
USGS
Mount St. Helens =>
changed the way we
think about composite
volcanoes!
Vesuvius,
1944
Monte Somma –
remnant of pre79 AD edifice
Crater walls of Mount St. Helens: note abundant fragmental debris!
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Martha Sabel, USGS,
1982
Incised canyon on the north flank of Mount
St. Helens near north end of 1980 Crater
floor; note great percentage of fragmental
debris! View looks NNE; photo by Pat
Pringle, September, 1982
Summit dome dacite of Kalama age
(post AD 1479)
Basalt of Castle
Creek age
(~1900 yr B.P.)
basaltic dike
Crater walls of Mount St.
Helens; photos by Pat Pringle; above
from south crater rim; right, from top of
Lava Dome.)
Dacite dome rock of Pine
Creek age (~2500 yr B.P.)
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