Integrating Research and Education > EarthChem > Mafic Rocks > Part 3 - Analyze the data

Part 3 - Analyze the data

This part of the exercise consists of a series of activites to complete using the dataset of young mafic volcanic rocks you obtained from NAVDAT. Please work through the questions in order. Additional instructions will be given to help you prepare various maps and plots.

  • Click the "Back" button on your browser to return to the page titled "Total Records Found"
  • Click the button labelled "Create Interactive Map", and then on the next page, "Click Here" to plot the data on an interactive map.
  • You should now see the samples plotted on a map of North America. With the mouse button depressed, drag your mouse over the area with the colored dots in the western U.S. to zoom in on those data and rescale the map.

Spatial distribution of mafic volcanism

  • Question 1. Using the interactive map you have already produced, where has mafic volcanic activity occurred during the last 5 million years in the western United States? Does the pattern of volcanism appear random or ordered in some way?
  • Question 2. Specifically, what are some geographical relationships between the young mafic volcanism in the American West and the various geologic provinces of the United States?

Regional isotopic variations

  • On the scroll-down bar at the top right of the browser window, select "87Sr_86Sr", "Legend", and click "Update Map".
  • The samples plotted as black dots have no Sr isotope data. Remove these from your map by clicking "Hide Null Values" then "Update Map".
  • Question 3. For each of the following regions of volcanism, identify the 87Sr/86Sr ratios as "mostly low", "mostly medium", "mostly high", or "variable":
    • Cascades volcanic chain
    • Yellowstone and the Snake River Plain
    • circum-Colorado Plateau
    • western Great Basin (including eastern Sierra Nevada)
    • southern Rio Grande Rift
  • Click on "Scatterplot" near the top left of the browser window. Choose "SiO2" for Variable 1, and "87Sr_86Sr" for Variable 2. Then click on "Generate Scatterplot" (you may have to do this twice). Notice that SiO2 will vary more than 87Sr/86Sr and is of less significance for determining characteristics of mantle sources since it varies with magmatic fractionation.
  • Question 4. Is there a continuous distribution of Sr isotopic compositions, or is there a clustering of the data about more than one value?
  • Question 5. For each cluster of datapoints you identified, click on an individual datapoint that you think might represent the average value for that cluster. What are the average 87Sr/86Sr values of the clusters?