Exploring Geotherms

Draft Version 11/15/05

Please send comments to Andrew Warnock at

Goals:

1. Measure a temperature gradient and graph it in Excel.

2. Explore the challenges associated with measuring a temperature gradient.

3. Identify variables that can affect the geotherm.

4. Learn about ‘geoneutrinos’ and the source of the Earth’s internal heat.

Model:

Since the ground is mostly rock with a thin layer of soil at the top, we need to focus on how heat moves through rock, if we want to see how heat moves through the ground.

Because we don’t have the tools to drill holes through rocks in our classroom, we need a way to make a “model” that approximates the real world. For our model, we will use a paper coffee cup full of sand. Although the sand is loose, let’s pretend that it is a solid piece of sandstone rock.

We will add water, because water has been present in every deep well drilled on Earth. Add enough water to saturate the sand, but do not let water pool at the surface.

Our model will have the following scale: 1 cm in the model = 0.2 km in the real world. Our heat source will be a coffee cup warmer or a hot plate set to about 120°C.

Procedure:

1. Place cup full of wet sand on cup warmer for at least 10 minutes prior to taking measurements. (Tip: you can put a thermometer all the way into one cup and wait until the temperature reaches about 45°C. Be careful not to poke the thermometer through the bottom of the cup!)

2. Have the students design the experiment and formulate a hypothesis. Will they start with the thermometer all the way down, or all the way up? Will they repeat the experiment? Will they average measurements taken on the way down with ones taken on the way up? How long will they let the thermometer equilibrate before each reading?

3. Perform the experiment.

4. Have student write about their observations. Did they learn anything about how the thermometer measures temperature? Does it measure the temperature at the tip or somewhere else?

5. Enter data into Excel and graph using the X-Y Scatter Plot function. X-axis should be temperature. The Y-axis should be depth in kilometers. They will need to use the model’s scale to convert from cm to km. Also, they should enter their depths as negative numbers.

6. Experiment fitting trend lines through the data points. Are the data linear or curved?

Discussion:

What variables can affect the geotherm in your cup model? What would happen if you tried using dry sand instead of wet sand? What would happen if you tried a cup full of clay-rich soil? We could try this.

What variables might affect the geotherm in the Earth? Type of crust: Continental vs. oceanic? Proximity to volcanoes or mountains? Proximity to active faults? Proximity to hotspots like Yellowstone? Types of rock: siltstones or coal vs. granite? Thickness of the crust: Basin and Range vs. Canadian Shield?

Where does the Earth’s heat originate? What factors keep the Earth hot? Is the Moon just as hot as the Earth? If not, why not? Unlike the Moon, the Earth just happens to be large enough so that the insulating properties of the Lithosphere prevent the heat left over from the formation of the Earth and heat produced by radioactive decay throughout the Earth from escaping too fast. This is a good thing, because the Earth’s internal heat is what drives plate tectonics and plate tectonics is what produces the spectacular scenery such as the Grand Canyon, the Rocky Mountains, Yellowstone, the California coast, etc. In addition, life may be dependent on an active plate tectonic system.

The average continental geotherm increases at a rate of 20-30°C per km to a depth of ~100-150 km, the base of the lithosphere. This is based on extrapolations from measurements taken from boreholes that are only about 9 km deep. Below the lithosphere, we do not yet have a clear picture of the geotherm other than the gradient does not appear to be as steep as near the surface (see figure 2). The study of antineutrinos emanating from radioactive elements (primarily Uranium and Thorium) in the mantle may help improve our understanding of the origin and distribution of the Earth’s heat.

Links:

The Denver Basin Project

Active Mines in Colorado