Volcano Lecture
Volcanic Rock types
Basalt—lava flow
Andesite—lava flow
Rhyolite—lava flow
Obsidian—lava flow (glass)
Pumice—pyroclastic
Scoria—pyroclastic
Ash Tuff—pyroclastic
Welded Tuff—pyroclastic
Physical properties of magma
How can the physical properties of magma vary?
Temperature
% silica
% of water
All of the above can affect the viscosity of a magma
Increasing viscosity
Lower temperature
Increase silica
Lower the amount of water
water interrupts the silica structure and allows easier flow
Silica and Temperature: important for type of volcanic eruptions
Low silica = high temp = low viscosity
allows gasses dissolved in solution to easily escape
creates fluid lava flows or fire fountains
ex//
High silica = low temp =high viscosity
water or gas dissolved in magma can’t escape
pressure builds and explosion occurs
ex//
Types of volcanoes
Shield Volcanoes
created from basaltic lava flow
high temperature and fluid flows
Cinder cones
Basaltic in composition
created by fountaining effect
Composite cone
silicic volcanism (andesite and rhyolite)
alternating layers of ash and lava
conical looking
Pumic cone: Silicic volcanism
Volcanic products
Lava
basalt, andesite, rhyolite & obsidian
Pyroclastic material
ash,
cinders, bombs, blocks
Gasses : primarily H2O, CO2, N and SO2
Pyroclastic flows
ignimbrites
or nue ardents
Lahars
Silicic Volcanism
Creates andesite and rhyolite volcanoes
associated with subduction zones
very viscous lava flows
short flows
very explosive
Products of Silicic volcanoes
Ash and lapilli
Composite cones
(strato volcanoes)
Lava domes
volcanic plumes
pyroclastic flows
collapsed calderas
lahars
NW Silicic Volcanism
Cascade volcanoes
3 episodes of eruption
Western Cascades (20 mybp)
Ancestral Cascades (7 mybp)
High Cascades (1 mybp)
recent activity
South Sister (2001 to present)
Basaltic Volcanism
Basaltic rock is most abundant rock on earth
eruptions are often non-violent
Low viscosity
flows travel long distances
Magma is produced by melting mantle (peridotite)
What is the intrusive form of basalt?
Where is basaltic volcanism found?
Structures
Shield volcanoes
Cinder cones
Features of Basaltic volcanism
Lava flows
pahoehoe
‘a’a
channelized flowsà leads to lava tubes
How do lava tubes form?
Channelized flow forms
Cuts down into bed
Crusts over on top as lava cools
Continues to flow inside of sealed tube
Eruption ends…tube drains and empty tube remains.
Flood basalts: ex//Columbia River Flood Basalts
Columular joints
tree casts
pillow lavas
bombs
NW Basaltic Volcanism
Flood basalts due to continental extension
17-6 million years ago
originated near Idaho/Oregon/Washington border
flowed to the coast in thick flows (haystack rock)
Boring lavas
small shields & cinder cones
4-1 million years
Newberry volcano
about 1300 years since last eruption
shield volcano with many cinder cones
Also obsidian
Volcanic Hazards
Basaltic Volcanism?
Silicic Volcanism?
How can Volcanic eruptions help change the climate?
1815 Tambora
year without a summer