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Volcanoes and volcanism
*Volcanoes represent
venting of the
Earth’s interior
*Molten magma rises
within the Earth and
is erupted either
quietly (lavas) or
violently
(pyroclastics)
Quiet vs. violent activity
*Quiet eruptions
tend to produce
lava flows, which
are not so
dangerous
*Explosive eruptions
produce
fragmental, or
pyroclastic,
material; these are
dangerous
*Two controls on explosivity are
(1) the silica content and (2)
the gas content of the magma
Basalt: 50% SiO2, gas-poor
Andesite: 60% SiO2, gas-rich
Rhyolite: 70% SiO2, gas-rich
*Magmas with higher silica
contents are more viscous
Global distribution of
volcanoes
Magma generation at midocean ridges
In these zones, the
mantle rises and
melts, producing
magma of silicate
composition
the magma continues to
rise, and erupts
mainly as basaltic
lava flows
Magma generation at hot spots
Magmas at hot spots
are derived from
deep within the
mantle
The magmas are fed
by deep mantle
plumes which are
stationary relative to
the drifting tectonic
plates
Magma generation at
subduction zones
During subduction, the
subducted oceanic
plate is heated as it
plunges into the
mantle
At a depth of 80-120 km,
melting begins, and
volcanoes are
produced which
parallel the subduction
zone
Andesitic magmas are
typical of these volcanoes
Plate tectonics and volcanism
Volcanic hazards of North
America
“Active” volcanoes
have erupted at
least once in the
past 10,000 years
The most active
volcanoes (in red)
are those
associated with
subduction zones
Volcanic hazards of
Canada
Canada has “active”
volcanoes (black
triangles) which pose a
potential threat in B.C.
Another major hazard is
ashfall from explosive
eruptions of Cascade
volcanoes in
Washington state
Volcano
types
Volcano types: cinder cones
Cinder cones are
volcanoes which erupt
only during one
episode
They are explosive, but
small in size
The cone is a pile of
pyroclastic debris
which piles up at the
angle of repose
Volcano types: cinder cones
The cinders are generally of basaltic composition
The eruptive activity typically lasts a few months
or years
Cinder cones: Parícutin
Parícutin volcano in
Mexico is a classic
cinder cone
The region contains
many cinder cones
It consists of both
pyroclastics and lava
Parícutin lava flows
These images
shows the
development of
lavas in 1943 and
in 1951-52
Red areas show
new lava flows
Parícutin - five views taken from
Luhr and Simkin (1993)
The eruption was
preceded by about 1½
months of felt seismicity
The eruption began in a
farmer’s field on 20
February 1943
It erupted for a
comparatively long (?)
time (1943-1951)
Parícutin
Here is a photo of the
volcano showing
the classic form of
cinder cones
In the foreground is
the obviously
distressed farmer,
Dionisio Pulido
Parícutin
This is a view of •
the volcano in
March 1944
In the foreground, •
note the flat-lying
lava flows from
the volcano
lava
Parícutin
The partly unfinished
towers of San Juan
Parangaricutico
surrounded by
1944 lava flows
from the volcano
Note how the lava
fills, but does not
destroy, the church
Parícutin
Note how the
percentage of
pyroclastic material
declines steadily
with time…
while the opposite is
observed for lava
The daily mass
eruption rate also
declines steadily
‫‪From:‬‬
‫‪http://www.meteo.mcgill.ca/195-250/2008/ppt/volcanoes.ppt‬‬
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