You're describing magnetic stripes. These are long, linear features on the ocean floor where the Earth's magnetic field has been imprinted in the rocks. Here's a breakdown:

* How they form: As magma rises from the mid-ocean ridges (where new ocean floor is created), it cools and solidifies. The iron minerals within the solidifying rock align themselves with the Earth's magnetic field at the time.

* Alternating polarities: Over time, the Earth's magnetic field flips its polarity (north becomes south and vice versa). This means that the rocks laid down during different periods will have opposite magnetic polarities.

* Striped pattern: The alternating magnetic polarities create a striped pattern on the ocean floor, symmetrical about the mid-ocean ridges.

* Evidence of seafloor spreading: These magnetic stripes are a key piece of evidence for the theory of plate tectonics, specifically seafloor spreading. The symmetrical stripes indicate that the ocean floor is being created at the ridges, pushing the older seafloor away.

Let me know if you have any other questions!