GEOL 1110 Lecture Notes - Lecture 7: Isostatic Depression, Convergent Boundary, Rhyolite
3 Jul 2018
School
Department
Course
Professor
Volcanoes and Volcanic Processes – Chapter 4
Volcanos
TYPE: Shield Composite/
Stratovolcano
Volcanic Dome
(Pyroclastic)
Mafic Intermediate Felsic
Magma Viscosity Low (flows well) Intermediate Higher (flows
poorly)
Gas Content 1-3% 3-4% 4-6%
Shape
Shield
Cinder Cones (Can
be both composite
and dome)
Dome
Cinder Cones
Location Hot Spots (Ex. Hawaii
volcanoes)
Adjacent to
subduction zones
Adjacent to
subduction zones
Eruption Type Effusive Eruption
Because it is low
viscosity, the gasses
in it are able to
escape, and
therefore there is not
as much pressure as
magma that is found
in higher viscosity,
meaning that there is
a non-violent flow of
magma/lava as long
as It is able to break
to the surface
Can have both
Effusive and
Explosive eruptions
Explosive
Eruption
Because Gas is
not able to
escape due to the
high viscosity,
pressure is high
inside the
volcano, because
the magma
doesn’t flow well
it forces pressure
on the rocks of
the volcano,
creating faults/
fractures which
will eventually
become too weak
and the volcano
will beak and
there will be an
explosive
eruption
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Factors determining volcanic explosion types
Magma composition
oSilica content high silica content= high viscosity
oSilica forms chains = limits flow
Magma Temperature
oDetermines the cooling rate
Amount of Dissolved Gas volatiles
oDissolved gasses escape easily from low viscosity magma increases
fluidity
These factors determine viscosity
oLow= easy flow
Temperature
Materials Extruded during eruptions
Lava Flows – mafic lava flows easily
oPahoehoe – braided, rope-like, smooth, fast flowing low viscosity
oAa – vesicular, jagged, slow flowing gas comes out very quickly, lower
viscosity than Pahoehoe
oPillow- enters ocean, globular
The Gas released from the volcano and lava has to do with the viscosity of the
magma
oIf it high viscosity it releases gas very quickly
Gasses
Magma contains gasses (volatiles)
o1-6% of content
oImportant to the composition of the atmosphere
oMixtures of carbon dioxide, sulphur dioxide, water vapor etc.
oEruptions can alter atmospheric concentrations of gasses
find more resources at oneclass.com
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Document Summary
Cinder cones (can be both composite and dome) Magma composition: silica content high silica content= high viscosity, silica forms chains = limits flow. Magma temperature: determines the cooling rate. Amount of dissolved gas volatiles: dissolved gasses escape easily from low viscosity magma increases fluidity. These factors determine viscosity: low= easy flow. Lava flows mafic lava flows easily: pahoehoe braided, rope-like, smooth, fast flowing low viscosity, aa vesicular, jagged, slow flowing gas comes out very quickly, lower viscosity than pahoehoe, pillow- enters ocean, globular. The gas released from the volcano and lava has to do with the viscosity of the magma: if it high viscosity it releases gas very quickly. Magma contains gasses (volatiles: 1-6% of content, important to the composition of the atmosphere, mixtures of carbon dioxide, sulphur dioxide, water vapor etc, eruptions can alter atmospheric concentrations of gasses, effect on climate appears short-term.
