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Section A: Volcanic Identification -- Data from the Ashfall Fossil Beds

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


                                                                                                                Ash Composition (percentage by weight) from Ashfall

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


                                                                                    General 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
(>1000C); low resistance to flow (thin, runny lava)

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

Low eruption temperature (600C- 900C); 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 LAVA

Andesitic
ANDESITIC LAVA

Dacitic/Rhyolitic
DACITIC / RHYOLITIC LAVA

Dominant Volcano Form

Cinder cones
Shield volcanoes

Composite cones (Stratovolcanoes)

Calderas
Domes


 

Answer the following questions before going to the next section.

 

Type of ash at Ashfall (mafic / intermediate / felsic): _____________________________________________

Type of eruption most likely to have created Ashfall: _____________________________________________

Type of volcano form most likely to have created Ashfall: _________________________________________

 

 

 

 

Section B: Volcano Suspects

 

Data Set #1: Location

 

Use print and Internet resources to determine the approximate distances between the Ashfall Fossil Beds site and each volcano.

    MEGAVOLCANO MAP

 

Answer the following questions before going to the next data set.

  1. Which volcano is located closest to the Ashfall site?

  2. Which volcanoes are farthest away?

  3. Based on the data so far, which volcano seems to be the most likely suspect?

 

Complete the Location information in the Volcano Suspects Table at the bottom of the page.

 

Data Set #2: Description

                                                                                                                                            DATA SET #2: Description

STRATOVOLCANO TYPE VOLCANO Mount St. Helens
Location: Washington
Volcano Type: Stratovolcano
Summit Height: 2,549 m
Crater/Caldera Size: 2 x 3.5 km
Last Known Eruption: 2006
Largest Eruptive Volume: 1 km3

CALDERA TYPE VOLCANO Crater Lake
Location: Oregon
Volcano Type: Caldera
Summit Height: 2,487 m
Crater/Caldera Size: 8 x 10 km
Last Known Eruption: ~2500 BC
Largest Eruptive Volume: 35 km3

STRATOVOLCANO / DOME TYPE VOLCANO Lassen Peak
Location: California
Volcano Type: Stratovolcano/Dome
Summit Height: 3,187 m
Crater/Caldera Size: 213 x 122 m
Last Known Eruption: 1921
Largest Eruptive Volume: 50 km3

CALDERA TYPE VOLCANO Long Valley
Location: California
Volcano Type: Caldera
Summit Height: 3,390 m
Crater/Caldera Size: 17 x 32 km
Last Known Eruption: Pleistocene Era
Largest Eruptive Volume: 600 km3

CALDERA TYPE VOLCANO Valles Caldera
Location: New Mexico
Volcano Type: Caldera
Summit Height: 3,430 m
Crater/Caldera Size: 20 x 22 km
Last Known Eruption: Pleistocene Era
Largest Eruptive Volume: 600 km3

CALDERA TYPE VOLCANO Bruneau-Jarbridge
Location: Idaho
Volcano Type: Caldera
Summit Height: Not Applicable
Crater/Caldera Size: ~80 km
Last Known Eruption: Miocene Era
Largest Eruptive Volume: >1000 km3

CALDERA TYPE VOLCANO Yellowstone Caldera
Location: Wyoming
Volcano Type: Caldera
Summit Height: 2,805 m
Crater/Caldera Size: 45 x 85 km
Last Known Eruption: Late Pleistocene Era
Largest Eruptive Volume: >2000 km3

CALDERA TYPE VOLCANO La Garita
Location: Colorado
Volcano Type: Caldera
Summit Height: Not Applicable
Crater/Caldera Size: 35 x 75 km
Last Known Eruption: Oligocene Era
Largest Eruptive Volume: 5000 km3

 

 

Answer the following questions before going to the next data set.

  1. The eruptive volume corresponds to the explosiveness of a volcano. Which volcano had the most explosive eruption? Which had the least explosive eruption?

  2. Does there seem to be a relationship between the size of a volcano's crater/caldera and an eruption's explosiveness? Why or why not?

  3. Based on the data so far, which volcano seems to be the most likely suspect?

Complete the Description information in the Volcano Suspects Table at the bottom of the page.

 

Data Set #3: Ash Composition

             The following represents a breakdown of the average concentration of major compounds (percentage by weight) in the ash from each volcano.

Ash Sample

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)

Mount St. Helens

14.70

1.81

1.39

2.01

0.46

4.45

0.17

0.04

74.80

Crater Lake

14.79

1.58

1.85

2.77

0.32

5.21

0.43

0.09

72.39

Lassen Peak

13.99

2.03

1.99

3.43

0.88

3.64

0.35

0.10

73.47

Long Valley Caldera

13.24

0.40

0.70

5.06

0.07

2.92

0.07

0.01

72.42

Valles Caldera

12.43

0.45

1.52

4.74

0.05

3.74

0.08

0.01

74.77

Bruneau-Jarbridge

11.90

0.80

2.70

6.10

0.10

2.50

0.10

0.08

75.60

Yellowstone Caldera

11.71

0.56

1.90

5.57

0.15

2.75

0.23

0.01

76.49

La Garita

12.50

0.56

1.10

4.80

0.01

2.38

0.10

0.02

77.10

*combined concentration of FeO and Fe2O3

 

Answer the following questions before going to the next section.

  1. For each suspect volcano, would you characterize the magma that produced the ash sample as mafic, intermediate or felsic? Why?

  2. Which ash seems to be most similar in composition to the Ashfall sample?

  3. Based on the data, which volcano seems to be the most likely suspect?

  4. How confident are you that your answer is correct?

Complete the Ash Composition information in the Volcano Suspects Table at the bottom of the page.

 

 

 

 

Section C: Identifying the Volcano ResponsibleAshfall Fossil Beds,northeastern Nebraska

Now that you have analyzed all three data sets and filled out your Volcano Suspects Table, identify your main suspect and answer each of the questions listed below.

  1. Most of the listed suspect volcanoes have calderas, which are large depressions formed by the collapse of the summit or flanks of a volcano during a large-scale, highly explosive eruption. Why would a caldera-forming eruption be the most likely source of the ash found in Nebraska?

  2. The most explosive volcanoes have magma with a very high silica (SiO2) content. Based on this information, which of the suspect volcanoes is most likely to have had the most explosive eruption?

  3. Which volcano do you think was the most likely source of the eruption that killed the animals in Nebraska? Why?

  4. Did your opinion change as you analyzed more data? Why or why not?

  5. Which data were most relevant?

  6. Which were least relevant?

 

Volcano Suspects Table

Data Set

Mount St. Helens

Crater Lake

Lassen Peak

Long Valley

Valles Caldera

La Garita

Bruneau-
Jarbridge

Yellow-
stone

Location

               

Description

               

Ash
Composition

               

 Remember to send your answers to the six questions above and your completed Volcano Suspects Table!

 


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Copyright 1996 Amy S Glenn
Last updated:   09/13/2017   0130

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