By analyzing radioactive minerals in igneous rocks, scientists can tell how much time has passed since rocks solidified from lava. This amount of time defines the age of a rock. This image shows the age of rocks on the Atlantic Ocean seafloor.

  !    Examine the color code to understand how the age of rocks changes from the center to the edges of the ocean floor. Consider how the age of rocks is related to the shape of the seafloor you saw in step 3. Answer the questions below.

3. What areas of the Atlantic seafloor have the youngest rocks? ...the oldest?

4. How old are the very oldest rocks on the Atlantic seafloor? Where are they?

5. Based on the age of the oldest rocks between South America and Africa, when did the two continents split?

The location of the youngest rocks on the seafloor shows that new rocks form in the middle of the ocean, the same location as the shallowest area of the ocean. This line of new, relatively shallow rocks is known as the mid-Atlantic Ridge. New rock material added to the edges of the South American and African Plates at the mid-Atlantic Ridge has separated the two continents.

Imagine what the ocean floor would have looked like 150 million years ago, when all the rocks younger than that did not yet exist. How would things have changed by 80 million years ago? Visualize how the Atlantic Ocean floor started separating the continents and grew to its present size.

6. Based on ages of the oldest rocks in the North and South Atlantic, describe how and when the Atlantic Ocean formed, and how its shape has changed through time.