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.
    1. Launch My World GIS. A blank map appears.
      1 My World Opens
    2. Choose File > Open Project. 2 File Open Project
    3. Navigate to the file, Evidence_1.m3vz. This may take several steps depending on where you located the file.
    4. Select the file, Evidence_1.m3vz. Then click Open.
      file 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.
  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.

  5. 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.
  6. 5 million years to 300 million years.
  7. What area of the map represents an age of 300 m.y.?
  8. The areas that represent the oldest rock are located in parts of the Pacific Ocean, east of the coast of China. The oldest rocks are also found near the coast of eastern United States, and off the coasts of both sides of Africa.

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.
  3. 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.
  4. 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.

  5. 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?
    • The oldest regions of the seafloor are represented by dark red. The youngest regions of the seafloor are represented by beige.
    • 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.
    • The seafloor would go from very old (on Florida's coast) to very new (in the middle of the Atlantic Ocean) and back to very old (on the coast of Africa). The plate boundary in between the two coasts causes this pattern to occur.

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.
    • Is there any symmetry to the pattern within an ocean basin?
    • Yes, for the younger parts of the sea floor age, there is some symmetry along the edge of the plate boundaries.
    • What might this pattern tell you about the movement of Earth's plates?
    • This pattern demonstrates that the plates are moving in a manner that causes the sea floor to spread apart from each other.
    • Compare the width of the Young sea-floor 5 m.y. in the North Atlantic to the Pacific Ocean.
    • The amount of young seafloor in the Pacific is much larger than the amount of young sea floor in the Atlantic.

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.
  3. image of plate boundary types
    • Is there a relationship between sea floor age and margin type?
    • Yes, the divergent plate boundaries compliments the young sea floor age.

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.
    • Which spreading center is opening faster?
    • The boundary in the Pacific Ocean.
    • What does the width of the colored bands in the sea floor represent?
    • It represents the width of the sea floor (in kilometers).
    • Why do the widths vary from one ocean basin to the other?
    • The sizes and intensities of the plate boundaries vary.

      Summary Question:
      For each plate boundary type, list evidence you've discovered that describes plate movement.

      Divergent boundaries are near young sea floor. Plates are moving most rapidly in the Pacific Ocean.



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