Tasks for today

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  Geologic Maps quiz
  
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  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)
  

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  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
  

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:

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  it determines, in part, how rapidly a magma can move from its source region to the surface of the earth;
  
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  it influences how fast crystals can rise or sink in magma;
  
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  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
  
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  it also is an important factor in determining the explosiveness of an eruption.
  

There are two different unit systems for viscosity:

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.

In other words: fastest lava = lowest viscosity;

slowest lava = highest viscosity.

A. Setup

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  Get into a group of four students.
  
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  Each group needs these supplies:
  

-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.
  
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  Put the lid inside of the box, wax paper side up, so that it forms a slope.
  
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  Draw a starting line and finish line on the slope, about 20 cm away from each other.
  

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  Each group should complete four “lava races.”
  
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  You can do the races in any order.
  
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  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.
  
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  For each race:
  

-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.

Prepare 3 lavas:

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  Corn syrup only
  
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  Corn syrup and small amount of sand (sand represents phenocrysts)
  
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  Corn syrup and larger amount of sand
  

Prepare 3 lavas:

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  Corn syrup only
  
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  Corn syrup and very small amount of water
  
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  Corn syrup and larger amount of water
  

Prepare 3 lavas:

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  Corn syrup only
  
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  Corn syrup and some bubbles
  
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  Corn syrup and a lot of bubbles
  

Prepare 3 lavas:

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  Corn syrup at room temperature (control)
  
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  Cold corn syrup (bottle from fridge)
  
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  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!!!)
  

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.

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: