Part 2—Investigate the Ages of Sea Floor Rocks

Step 1 – Launch My World and Explore the Map

My World Icon
  1. Launch My World GIS by double-clicking its icon on your desktop or by clicking its icon in the dock (Mac) or Launch Bar (PC).
  2. Choose File > Open Project and navigate to the file, Evidence_1.m3vz. Select it and then click Open.

    When the project opens, a world map displays. It shows Lines of Latitude & Longitude with outlines of Continents underneath. Layers not visible on the map include: States, Countries, and Seafloor age. A separate My World Tour Window opens, skim through the background information shown in this window and then close the window. You can return to it later by selecting Window > Tour Window.
    opening map 1

  3. Practice turning layers on and off, zooming in and out of the map.
  4. When you have finished exploring the map, switch to Visualize mode by clicking the Visualize tab located above the Layer List.

Step 2 – Use the Layer Table to Examine Sea-floor Age

  1. Turn the Sea-floor age layer on, by clicking in the checkbox next to its name.
  2. Click once in the Sea-floor age layer name in the Layer List to make it the active layer. In the image below the Sea-floor age layer is both on and active.
    seafloor on and active

  3. Begin by examining the data within the Seafloor age Layer Table within My World. Choose Layer > Show Layer Table... or click the Show Table of Selected Layer button.
  4. Click the header of the column to Sort the "AGE (m.y.)" field.
  • Layer > Show Layer Table... or click the Show Table of Selected Layer button.
  • Sort a field by clicking on the field name. Each click will sort the field, ascending , descending , and then back to unsorted.

  • Next find the range of values from youngest to oldest.
    It is important to notice where the oldest values (300 m.y.) in the data table are located on the map. To highlight them on the map, once you've sorted the column, hold the control key and highlight each of the seven that have an age of 300. When you have selected all seven, click the "Make Selection From Rows" button at the top of the table. Name the selection: "300 m.y." and click OK. You will be returned to the map.
  • What area of the map represents an age of 300 m.y.?

    • Step 3 – Explore the Data by Identifying the Areas of the Seafloor by Age

    What is the pattern in the data?

    1. Begin by switching the selection back to "Show All (highlighting off)" from "300 m.y." (This switch would be made in the layer list on the left hand side of the window.)
      2. Switching Selections Tip: You can switch between selections within a layer by selecting the radio button radio button As you create selections they are added to this list. Any selection can be deleted by selecting it and clicking the "Delete Selection" delete selection button at the top of the Layer List.
    2. Switch to Construct Mode, choose the Geology Data Library and add the Plate Boundary Types layer to your Layer List. Click and drag the layer above the Sea-floor age layer.
    3. Use the Pointer Tool pointer tool to identify the age of the ocean floor moving away from the plate boundaries. Notice that the age will appear on the Sea-floor age (m.y.) legend located below the map.

    4. While exploring the Sea floor age layer consider the following questions:
      • What color represents the oldest regions of the sea floor and what color represents the youngest regions of the sea floor?
      • If you were to travel in a straight line from Florida to the west coast of Africa, describe the pattern you would see in the age of the sea floor.

    Step 4 – Use Analyze Mode to Create Selections Areas of the Seafloor by Age

    1. Click the Analyze tab.
    2. Then click Select... > By Value.
    3. Select the item from "sea-floor age" whose "Age (m.y.)" is less than "10." Name the result: Young sea-floor 5 m.y.
    4. Return to Analyze mode and create a second selection that is less than "300" and greater than or equal to "170". Name the result of Old sea-floor 170-180 m.y.
    5. Switch between the different selections and make observations of where the oldest and youngest areas are located.
      6. Highlight Mode Tip: After making a selection in Analyze mode, try different Highlight Modes to see different ways to display your selection.

    Step 5 – Observe the Relationship between Plate Boundaries and Areas of the Seafloor

    1. Start by turning on the Plate Boundary Types layer to observe the relationship.
    2. Then, change the legend of the Plate Boundary Types layer to Margin Type.

    Step 6 – Compare the Rate of Seafloor Spreading in the Pacific and Atlantic Oceans

    1. Turn on the Young sea-floor 5 m.y. selection to compare the width of the regions within sea floor of this age.
      2. Measurement Tool Tip: Use the Measurement Tool icon of measurement tool tip to measure the width of the colored age bands. Compare the measurements of the Young sea-floor in the Pacific and Atlantic Oceans.


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