The formation of new crust adds to the plates on either side of the mid-ocean ridge and results in sea floor spreading, in which the two plates slowly move apart. This slow spreading pushes on the "conveyor belt", moving along the plates and the continents they carry. The North American Plate is moving slowly northwestward at the rate of about 1 inch per year. Although this doesn't sound like very much, this process seems to have been in effect for the history of the earth, and over time has had a great effect on the movement and position of the continents. The colors on the map show the ages of the rocks that make up the ocean floor. The youngest rocks are those that are being formed at the mid-ocean ridge. As you move away from either side of the ridge, the rocks become progressively older because they formed earlier and have slowly moved away from the spreading center.