Volcanoes
Chapters 6 & 7, Hyndman & Hyndman
with input from Richard Sedlock
Department of Geology
San José State University
Origin of “Volcano”
• Word is from the Island of
Vulcano off Sicily
• Ancient people from the area
believed that Vulcano was the
chimney of the forge of Vulcan
(blacksmith of the Roman gods)
• They thought hot lava fragments
and clouds of dust erupting from
Vulcano came from Vulcan's
forge as he beat out
thunderbolts for Jupiter (king of
the gods) and weapons for Mars
(the god of war)
http://www.volcanodiscovery.com/volcano-tours/32.html
VOLCANOES
form where molten rock is vented at Earth’s surface.
BIG Questions
Where do volcanoes form in the
context of plate tectonics?
Volcanoes aren’t equally dangerous....
how do their hazards differ, and why?
What comes out of a volcano?
Fragments of solidified magma
Jagged pieces of tiny rocks and glass
Ash
magnified 200 times
What comes out of a volcano?
Gas
Most common:
H2O
CO2
SO2
HCl
What comes out of a volcano?
Lava
Characteristics of
volcanic rocks
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Illustration by J. Johnson from
http://volcanoes.usgs.gov/Products/Pglossary/VolRocks.html
basalt
contains the least
silica
erupts at the highest
temperature
lowest viscosity (the
least resistance to flow
basalt lava moves over
the ground easily, even
down gentle slopes
dacite and rhyolite lava
tend to pile up around
a vent
form short, stubby
flows or mound-shaped
domes
• lower strata (layers) have
hollow “well” for magma
chamber
• streams of magma rise from
magma chamber, through the
strata
• some streams push up strata
layers
• other streams move to the
top opening(s) of the volcano
• raised opening that emits
magma called the cone
• stream that moves to the top
of the cone called the
Central Vent
• volcano emits magma and a
stream of tephra (ash and
rock)
http://erg.usgs.gov/isb/pubs/teacherspackets/volcanoes/poster/poster.html
Mauna Loa (Hawai’i):
A typical shield volcano
(also the largest volcano on earth)
Photograph by J.D. Griggs on
25 February 1983
Characteristics:
• broad gentle slopes
• formed from basalt lava of low viscosity
• eruption is mostly lava rather than pyroclastic material
• eruptions are not explosive (unless water gets into vents)
• lava pours out of vents or as fountains
Mt. St. Helens:
A typical composite volcano
(stratovolcano)
Characteristics
• steep sides, symmetrical cones
• formed from magma of moderate viscosity (andesite)
• alternating layers of lava flows, volcanic ash, fragments of debris
• erupts explosively repeatedly from summit crater
• often tall and snow covered, therefore subject to mudlfows (lahars)
Mt. St. Helens after its 1980 eruption
Volcanologists recognize many types of
volcanoes, but we only need to worry about
two:
Explosive
(“Clogged”)
Andesitic composition
Composite volcanoes
Non-explosive
Basaltic composition
Shield volcanoes
(“Runny”)
Volcanic Hazards
Lava flows
Ash fall
Pyroclastic flows
Mudflows
Volcanic Gases
Tsunami
Mt. Pinatubo http://pubs.usgs.gov/fs/1997/fs115-97/
Ash
• Composed of bits of pumice less than 2 mm across
• Can drift long distances on the wind
• Can stay suspended in atmosphere blocking sun and
ruining crops
• Crop and roof damage - 20 cm can cause roofs to
collapse
• Ash eruptions generate “volcano weather” – rain
• Irritates lungs and eyes - long term exposure can
cause lung disease
• Reduced visibility making evacuation difficult
• Planes flying into plumes of ash can have engine
trouble
Mount St. Helens
Ash Fallout
http://vulcan.wr.usgs.gov/Volcanoes/MSH/Maps/may18_ashmap.html
Plate-tectonic setting of volcanism
Explosive (andesitic) volcanoes form at
subduction zones.
Plate-tectonic setting of volcanism
At spreading centers, low pressure triggers
mantle melting—fluid basaltic magma rises.
Plate-tectonic setting of volcanism
Within plates, rising plumes of hotter mantle feed
hot spots; varied volcanoes result (basaltic on Hawaii).
Evidence of Volcanoes in
our own Backyard
• Calistoga, Petrified Forest
• Redwood trees, 8 ft in diameter
• Blown over and covered with ash from
volcano 3 million years ago
• molecules of silica in the ash replaced
molecules of wood
• wood turned to solid
stone
http://www.petrifiedforest.org/