NOVA

Mystery of the Megavolcano

Student Handout

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 Ashfall tephra


Ash Composition (percentage by weight)

Aluminum Oxide
(Al2O3)

Calcium Oxide
(CaO)

Iron Oxide
(FeO)*

Potassium Oxide
(K2O)

Magnesium Oxide
(MgO)

Sodium Oxide
(Na2O)

Titanium Dioxide
(TiO2)

Phosphorous Pentaoxide
(P2O5)

Silicon Dioxide
(SiO2)

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


Mafic

Intermediate

Felsic

Ash
Composition

48-52% SiO2; high in FeO, MgO, CaO; low in K2O, A12O3, Na2O

53-65% SiO2; moderate amounts of all major compounds

>65% SiO2; high in K2O, A12O3, Na2O; low in FeO, MgO, CaO

Magma Characteristics

High eruption temperature
(>1000°C);
low resistance to flow (thin, runny lava)

Medium eruption temperature (900°C-
1000°C); medium resistance to flow (somewhat thicker, sticky lava)

Low eruption temperature (600°C- 900°C); high to very high resistance to flow (very thick, sticky lava)

Eruptive Characteristics

Relatively non-explosive; extensive lava flows

Relatively explosive; pyroclastic flows, ash falls, tephra deposits, volcanic gases, lahars

Highly explosive; enormous dark columns of tephra and gas high into the stratosphere; pyroclastic flows and surges; extensive ash fall

Common Lava/Tephra Type Produced

Balsitic
balsitic

Andesitic
andesitic

Dacitic/Rhyolitic
Dacitic/Rhyolitic

Dominant Volcano Form

Cinder cones
Shield volcanoes

Composite cones (Stratovolcanoes)

Calderas
Domes


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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