Volcanoes
What do you know about
volcanoes?
What you need to understand
about volcanoes
• Where and why volcanoes occur?
• Source of lava
• Composition of lava and name of rock
associated with each tectonic setting
• Causes of melting: addition of volatiles;
decompression melting
– Geothermal gradient
– Lithostatic pressure
• Why and how volcanoes erupt?
Tectonic setting
Convergent with
oceanic and
continental crust
Convergent with
two oceanic crusts
Divergent
Transform
Physical
characteristics
Locations on
Earth
Type of
crust
Lava source
Composition of
crust
Volcanic rock name
Volcanic
landforms
Composition
• Continental crust
– Higher % Si, O
– Lower % Fe, Mg
• Oceanic crust
– Higher % Fe, Mg
– Lower % Si, O
Tectonic Setting
• Convergent plate boundary with an
oceanic crust
Continental
crust +
Oceanic
crust +
Sediments
+
Water
Tectonic Setting
• Convergent plate boundary
– Two oceanic crusts
Water +
Sediments
+
Oceanic
crust +
Oceanic
crust =
Volcanic rocks formed in
subduction zones
• Water + sediments + oceanic crust + the opposing
crust (oceanic or continental)
• Intermediate composition
Andesite
Extension
• Extension: continental crust thins; magma
moves to surface through faults and
“weak” crust
Tensional stress
Tensional
stress
Extension: Kilimanjaro
Andesite
Long Valley Caldera
Basin and Range province
Rhyolitic eruption:
760,000 years ago
Ash produced by the eruption of rhyolitic lava (melted continental crust
Inyo and Mono Craters: LVC
Bimodal volcanism:
rhyolite and basalt
Basaltic eruptions
producing cinder cones
Extension: basalt, andesite and
rhyolite may be produced
Andesite
Rhyolite and basalt
Divergent Plate Boundary
• Basaltic lava produced
Asthenosphere
Asthenosphere
Oceanic crust source is the asthenosphere.
Rift Valley,
Iceland
Notice the similarity
of landforms
Shield volcano
Tectonic Setting
• Hot spots under oceanic crust
Oceanic
crust +
Asthenosphere
Volcanic rocks formed at mid-oceanic
ridges, hot spots under oceanic crust,
and areas of extension
• High % Fe, Mg;
• Low % Si, O
Basalt
Pillow Basalts: are formed as
basalt is extruded under water.
Hawaii
How can the concept of
uniformitarianism be applied
to the observation of pillow
basalts forming?
Basalt that contains pillow basalts
was originally formed under water.
Wales, Great
Britain
Hot spot under continental crust
• Rhyolite is formed
Melting of
continental
crust
Extension
• Rhyolite
Melting of
continental
crust
Hot spot under continental crust and
areas of extension cause melting of
continental crust
Rhyolite: highest percentage
of Silicon and oxygen
Geothermal
Gradient
Increase of
temperature
with depth
Geothermal Gradient
•
•
•
•
Increase temperature with depth
Varies depending on tectonic setting
Average increase is 25 degrees C/KM
Yellow dotted line indicates 500 degrees
centigrade
Geothermal Gradient
• Rocks in the lower crust are near their melting
point
• Any addition heat may induce melting
– From superheated or ascending melted rock
– What is an example of this process (see above
diagram)?
Causes of Melting
• Addition of volatiles:
– Water and carbon dioxide, lowers melting
temperature, triggers melting
– Where within the plate boundaries we have
discussed, does this occur?
Adding water, lowers the rock’s
melting temperature.
Basalt is hydrated from processes
such as black smokers.
Addition of volatiles lowers melting
temperature
Lithostatic Pressure
• There is an increase pressure with depth
Decompression Melting
• As hot material near the melting point
ascends, pressure is decreased
• The lowering of pressure causes the hot
material to melt
The decompression of hot
water drives a geyser.
Hot water is circulated through the upper
crust due to temperature differences.
Hot water remains liquid at
depth, due to high pressure.
Geyser Eruption
• Super heated water is
under pressure at
depth
• Remains liquid
• Rises because it is
very hot
• Gas comes out of
solution as pressure
is reduced
Geyser eruption
• Plumbing is correct
• Allows a balance
between liquid and
gas to collect
Geyser Eruption
• When enough gas is
“collected” within the
“plumbing”
• The gas escapes, along
with the fluid
As super heated water rises, pressure is
decreased. Gases come out of solution. When
plumbing is correct, gases “drive’ the fluid out of
the system.
Gas release drives volcanic
Eruptions
Explosive
volcanic
eruption
Volcanoes
• Composition of oceanic and continental
crust and how this relates to lava
composition
• Where volcanoes occur and how this
relates to lava composition
• Geothermal gradient and lithostatic
pressure
• Why volcanoes erupt