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LAB 7: VOLCANOES AND MAGMA VISCOSITY Name____________________________

GEOL110 Physical Geology
TASKS FOR TODAY:

  • Geologic Maps quiz

  • Part 1 of lab (in groups of 4 students) then clean up!

  • Practice quiz for part 1 (Results & conclusions section C of Part 1)

  • Part 2 of lab

  • Practice quiz for part 2 (In part 2 below)


OBJECTIVES OF LAB

  • Evaluate the effect of crystal growth, bubble growth, water, and temperature on magma viscosity

  • Relate the viscosity of magmas to viscosity of everyday food items


AN OVERVIEW OF VISCOSITY

By definition, viscosity is the “resistance to change of form” (Handbook of Chemistry

and Physics, 61st Edition) or the “resistance to flow within a fluid” (Fowler, C.M.R, 1990, The Solid Earth, Cambridge University Press, p. 462). The viscosity of a substance is a property that determines how rapidly the substance will respond to shear stress; for liquids, this is most obviously reflected in how rapidly a liquid will flow down an inclined surface. For example, water has a low viscosity compared to honey; honey has a high viscosity compared to water. Liquids that are viscous, or have a high viscosity, flow very slowly. Liquids that are non-viscous, or have a low viscosity, flow relatively more rapidly.
Viscosity is an important property of magmas for several reasons:


  • it determines, in part, how rapidly a magma can move from its source region to the surface of the earth;

  • it influences how fast crystals can rise or sink in magma;

  • it helps volcanologists predict whether the lava flow from a volcano will move very quickly, and hence be life threatening, or will move very slowly and not be life threatening; and

  • it also is an important factor in determining the explosiveness of an eruption.


UNITS OF VISCOSITY

There are two different unit systems for viscosity:



Poise: 1 poise = 1 gram per cm distance per second

Pascal-second (Pa-s): 1 Pa-s = 1 kilogram per meter distance per second

10 poise = 1 Pa-s (Pascal-second)

Neither of these unit systems is intuitive- in other words, if someone tells you a lava flow has a viscosity of 105 Pa-s, it’s hard to have a sense of how viscous that really is compared to everyday materials. Part 2 of this lab will help us relate lava flow viscosities to viscosities of everyday food items.



PART 1. PHYSICAL FACTORS THAT AFFECT VISCOSITY: EXPERIMENTS WITH TEMPERATURE, DISSOLVED H2O, SOLIDS, AND BUBBLES
Purpose: By completing the activities described below, you will learn about four important factors that can affect the viscosity of magmas: temperature, dissolved water, percent of solid material contained in the magma (e.g., phenocrysts, visible crystals in the lava), and the bubble content of the magma.
Key principle for Part 1: For lava flows of the same thickness, the speed at which the lava flows downhill is directly related to the viscosity of the lava.

In other words: fastest lava = lowest viscosity;

slowest lava = highest viscosity.

A. Setup


  • Get into a group of four students.

  • Each group needs these supplies:

-One plastic box and lid

-Four sheets of wax paper that fit over the lid (with extra length to tape to back)

-A marker to draw a starting and finishing line

-A bottle of corn syrup

-A fork or a spoon

-A stopwatch or watch that measures seconds

-14 plastic cups

-sand (put in one of your paper cups)

-water (put in one of your paper cups)

-paper towels



  • Attach one piece of wax paper to the inside of the lid by folding top and bottom edges over to the outside of the lid and taping. Try to keep the surface of the wax paper even (no folds, no holes). It won’t be completely flat, because the inside of the lid isn’t flat, but that’s ok. You will change this wax paper after each “lava race” or as often as needed to keep one lava flow from colliding with another.

  • Put the lid inside of the box, wax paper side up, so that it forms a slope.

  • Draw a starting line and finish line on the slope, about 20 cm away from each other.


B. Experiments

  • Each group should complete four “lava races.”

  • You can do the races in any order.

  • Each lava race has three lava flows competing against each other: a “control” lava flow (regular corn syrup), and two other flows. Note: if you use identical amounts of corn syrup for ALL of your lava flows, you only need to do the “control” once.

  • For each race:

-Fill three plastic cups with equal amounts of corn syrup (I suggest about 1/3 full)

-You may add color to your lava flows- keep in mind that a tiny amount of food coloring goes a LONG WAY. Use sparingly- one drop is more than enough.

-Add additional ingredients (i.e., bubbles, “crystals”) to two of the paper cups.

-Measure the time it takes for each lava flow to move from the starting line to the finish line. You should pour the lava flows above the start line and let them flow down over the line (don’t pour them directly on the line). It’s easiest to pour one lava flow at a time, rather than all three at once.



-Record the time and flow characteristics for each lava flow in Table 1 and answer the questions for each lava race.
Table 1:

Lava Race

Lava Characteristics

Time (seconds) to flow from start to finish

Describe relative width of flow and any other observations

1

Corn Syrup only (control)








1

Small amount of sand (phenocrysts)







1

Larger amount of sand (phenocrysts)







2

Corn Syrup only (control)








2

A very small amount of water








2

A larger amount of water








3

Corn Syrup only (control)








3

Some bubbles (vesicles)








3

Lots of bubbles (vesicles)








4

Corn Syrup at room temperature (control)







4

Cold (from fridge)








4

Hot (from microwave; ask TA for help)








Lava Race 1: Effect of phenocrysts (mineral crystals)

Prepare 3 lavas:



1. During this experiment, pick one of the sand grains near the front of the syrup flow and watch how it moves down the slope. Describe with words (or a sketch) the movement of the particle:
Lava Race 2: Effect of water

Prepare 3 lavas:



  • Corn syrup only

  • Corn syrup and very small amount of water

  • Corn syrup and larger amount of water

2. Which do you think has a greater effect on viscosity (for the same amount added): phenocrysts or water? How did you choose that answer?


Lava Race 3: Effect of bubbles

Prepare 3 lavas:



You can make bubbles by beating the corn syrup with a fork.

3. Do any of the bubbles change shape as the lava flows down the slope? Check at the front, sides, and middle of the lava flow.

Lava Race 4: Effect of temperature

Prepare 3 lavas:



  • Corn syrup at room temperature (control)

  • Cold corn syrup (bottle from fridge)

  • Hot corn syrup (from microwave- ask for help. A TA will be “cooking” batches of syrup. You can get burned by hot syrup-use caution!!!)

4. What texture do you observe on the surface of the hot lava flow (especially at the bottom of the slope)? Describe the features and give it a proper geologic name.


C. Results and conclusions: Practice Quiz for Part 1

Summarize your results by filling in Table 2 and answering the questions below. Check with instructor / TAs for correct answers when you are done. You should be able to fill in a table like this and answer questions about a combination of variables acting on viscosity simultaneously (question 5) next week.


Table 2.

A lava containing more phenocrysts is ________________ (more or less?) viscous than a lava containing little or no phenocrysts.


A lava containing a lot of water is __________________(more or less?) viscous than a lava containing little or no water.


A lava containing more vesicles is ________________ (more or less?) viscous than a lava containing little or no vesicles.


A lava at high temperatures (hotter) is ________________ (more or less?) viscous than a lava that is relatively cool.




5. Evaluate lava flow hazards: Pretend you are a volcanologist working for the US Geological Survey in Washington State. Your studies reveal that Mount Rainier could erupt at any time and send lava down its slopes into the town of Tacoma. Rank the relative danger to human life of each type of lava flow (high, medium, low) based on your results above. The faster a lava flow travels, the more dangerous it is because authorities need time to evacuate citizens.

A. A lava flow with a lot of vesicles and little or no water (circle one):
High risk Medium risk Low risk
B. A lava flow containing a large amount of water and few phenocrysts (circle one):
High risk Medium risk Low risk
C. A hot lava flow with a lot of phenocrysts (circle one):
High risk Medium risk Low risk

6. How does viscosity affect the width of the lava flow?

Please CLEAN UP everything (including the plastic containers) before continuing.

PART 2: COMPARING VISCOSITIES: LAVA AND FOODS

A recent study (Baker et al. 2004 J Geosci Ed, 52, 363-367) measured viscosities of common food items and compared these to viscosities of lavas. Here are the results:





7. What is the viscosity of the rhyolite magma in the diagram above? Give your answer in Pa-s and in poise:
8. What is the viscosity of the basalt magma in the diagram above? Give your answer in Pa-s and in poise:
9. ACH Foods Inc. reports that Karo Light Corn syrup has a viscosity of 22-30 poise at 25° C. Plot corn syrup on the diagram above. Taking into account the fact that this is a log plot, corn syrup has a viscosity most similar to which other food or magma?


Practice quiz for Part 2 (you may use the diagram above for this practice quiz; you will not have the diagram for the real quiz).
10. A typical rhyolite has a viscosity that is most like which food?


11. A typical basalt has a viscosity that is most like which food?

12. Which of these viscosities (in Pa-s) is the most reasonable value for a magma? (circle best answer):
0.001 Pa-s
1000 Pa-s
100,000,000 Pa-s






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