Paleomagnetism in sea floor spreading. Artwork showing the changes in magnetism in sea floor volcanic rocks as the Earth's magnetic field changes over time. These changes are evidence for sea floor spreading. This, combined with plate tectonics, helps to explain how the continents move relative to each other (continental drift). This is the Atlantic Ocean. The youngest rocks are at the centre (red), forming the Mid-Atlantic Ridge. Older rocks are found either side of the Ridge, with the oldest rocks (purple) found at the continental margins either side. The youngest rocks were formed 0-2 million years ago. The oldest were formed over 200 million years ago, when the Atlantic Ocean began to form.