DR KEN MACDONALD / SCIENCE PHOTO LIBRARY DR KEN MACDONALD / SCIENCE PHOTO LIBRARY
Geomagnetic reversals. Image of the ocean floor showing part of the East Pacific Rise, an ocean spreading ridge. The vertical stripes of colour show changes in the Earth's magnetic field over the last few million years. The colours indicate crustal magnetisation; white, red and yellow indicate one polarity, green and blue the opposite polarity. These polarities are given to the rocks as they form at the mid-ocean ridge (centre-line of red areas). As the rocks continue to move apart, they become a valuable record by which recent geological timescales may be set. Part of the ridge in the foreground is offset by a large lateral fault, so appears shifted to the left.
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