GARY HINCKS / SCIENCE PHOTO LIBRARY GARY HINCKS / SCIENCE PHOTO LIBRARY
Sea floor spreading. Cutaway artworks showing the process of sea floor spreading. New sea floor (upper right) is formed at a mid-ocean ridge by upwelling lava. Over millions of years the new rock fills the gap left as the continents drift apart (red arrows, sequence to lower left), and also helps drive the process. The evidence for this process is found by looking at the magnetism of the rocks. As volcanic rocks solidify, they retain the magnetism present at the time. The Earth's magnetic field reverses periodically, and this is shown by changes in the magnetism of these rocks. This is shown here by the successive colour bands.
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