Mystery of the Megavolcano
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Student Handout
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Volcanic Identification
Your
job is to identify the type of volcano that created the Ashfall Fossil Beds
site. First compare the "Ashfall Data" with information about three main types
of magma (mafic, intermediate, felsic) listed in the "Magma and Eruption
Characteristics" chart. Then fill in the "Ashfall Conclusions" section with
your conclusions.
Ashfall Data
Ashfall tephra
Ash
Composition (percentage by weight)
Aluminum
Oxide
(Al2O3)
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Calcium
Oxide
(CaO)
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Iron
Oxide
(FeO)*
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Potassium
Oxide
(K2O)
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Magnesium
Oxide
(MgO)
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Sodium
Oxide
(Na2O)
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Titanium
Dioxide
(TiO2)
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Phosphorous
Pentaoxide
(P2O5)
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Silicon
Dioxide
(SiO2)
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11.80 |
0.60 |
2.80 |
6.30 |
0.10 |
2.60 |
0.20 |
0.08 |
75.50 |
*combined
concentration of FeO and Fe2O3
Magma and Eruption Characteristics
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Mafic |
Intermediate |
Felsic |
Ash
Composition
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48-52%
SiO2; high in FeO, MgO, CaO; low in K2O,
A12O3, Na2O
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53-65%
SiO2; moderate amounts of all major compounds
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>65%
SiO2; high in K2O, A12O3,
Na2O; low in FeO, MgO, CaO
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Magma
Characteristics
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High
eruption temperature
(>1000°C);
low resistance to flow (thin, runny lava)
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Medium
eruption temperature (900°C-
1000°C); medium resistance to flow (somewhat thicker, sticky lava)
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Low
eruption temperature (600°C- 900°C); high to very high resistance to
flow (very thick, sticky lava)
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Eruptive
Characteristics
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Relatively
non-explosive; extensive lava flows
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Relatively
explosive; pyroclastic flows, ash falls, tephra deposits, volcanic gases, lahars
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Highly
explosive; enormous dark columns of tephra and gas high into the stratosphere;
pyroclastic flows and surges; extensive ash fall
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Common
Lava/Tephra Type Produced
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Balsitic
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Andesitic
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Dacitic/Rhyolitic
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Dominant
Volcano Form
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Cinder
cones
Shield volcanoes
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Composite
cones (Stratovolcanoes)
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Calderas
Domes
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Ashfall Conclusions
Type
of ash at Ashfall: (circle one) mafic | intermediate | felsic
Type of eruption most likely to have created Ashfall: ___________
Type of volcano form most likely to have created Ashfall: ___________
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