Issues in Earth Science
Eww, Theres Some Geology in my Fiction!
Issue 5, Dec 2015
Suggestions for Activities and Discussions to accompany a Reading of
Shifting Fortunes by S.Q. Eries
Is the on-again-off-again geyser that provides a key story element to Shifting Fortunes geologically reasonable? If so why?
How does a geyser work, anyway?
The key to understanding a geyser is to understand the boiling temperature of water. The boiling temperature of water is an appropriate investigation for the secondary science classroom!
Deep underground in the plumbing system of a geyser, the pressure is higher due to the weight of all of the overlying water. Will the boiling temperature at that depth be greater than, less than, or equal to 100°C?
You can figure it out! Let's consider a graph showing possible ways that the boiling temperature depends on pressure. Note that pressure on this graph increases downwards, like pressure increases downward in a geyser system due to increasing weight of overlying water.
Does the boiling temperature decrease as pressure increases, like line A?
Does the boiling temperature stay constant as pressure changes, like line B?
Does the boiling temperature increase as pressure increases, like line C?
Does the boiling temperature change even if pressure stays constant, like line D?
You can determine which line is correct by making two measurements of boiling at two different pressures.
Measure the temperature at which water boils at 1 atmosphere pressure, using a thermometer. If you are not at sea level, you might be able to measure that the boiling temperature is not exactly 100°C! Which way is it off? Do you suppose the pressure is higher or lower than one atmosphere at your location?
A second measurement can be made by determining whether increasing or decreasing pressure will cause water to boil at room temperature. Put 20ml water in a 60ml syringe. Make sure all air is removed from the syringe. Hold a balloon over the end of the syringe with your thumb to seal it up. Then alternately pull or push on the syringe (decrease or increase pressure in the syringe), and see which works to boil the water. Plot your two points on the graph. From this, you can determine which graph line must be correct.
Water boiling at room temperature.
A second key to understanding geysers is to think about what is going on at depth to cause geysers to erupt and then stop erupting. Unlike hot springs, geysers have a constriction in their piping which prevents circulation of heat from deeper in the geyser to the surface. Since heat can't escape from the deeper parts of the geyser system, water deeper in the geyser system slowly gets hot enough to boil. This is shown below.
Below are the steps for one conceptual model for how a geyser works. Try to match up each step in the model with one of the arrows on the temperature-pressure diagram for the boiling temperature of water, shown below. Be sure to think about what the temperature-pressure graph means!
1) Constrictions in the piping allow magmatic heat from deeper in the crust to slowly heat up the water in the lower part of the geyser plumbing until it reaches boiling temperature
2) When the water starts to boil, the rising and expanding water vapor pushes water out at the surface, which decreases the pressure lower down, which decreases the boiling temperature lower down, which causes more boiling and more throwing water out at the surface (the geyser erupts). As pressure falls and water boils, heat is lost to the water vapor escaping to the surface.
3) Once the water cools off enough, the boiling stops, and the geyser plumbing slowly fills back up with cool water
So, what might the earthquake have done in Shifting Fortunes to cause the geyser to start working again?
The experimental activity and the modeling activity above support the following NGSS performance expectations: MS-ESS2-1, MS-PS3-4, HS-ESS2-2, HS-ESS2-5, HS-PS3-4.
Students can exercise skills in the practices of 1) Developing and using a model, 2) Analyzing and interpreting data, and 3) Constructing explanations.
Students experience the crosscutting concepts of 1) Cause and effect: mechanism and explanation, 2) Systems and system models, and 3) Energy and matter: flows, cycles and conservation.
The Teacher Resources for Shifting Fortunes are written by Russ and Mary Colson.
Shifting Fortunes by S. Q. Eries
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