GARY HINCKS / SCIENCE PHOTO LIBRARY GARY HINCKS / SCIENCE PHOTO LIBRARY
Magnetic reversal in volcanic rocks. Cutaway artwork of a shield volcano (upper left) forming successive layers (grey) of volcanic rock after eruptions over hundreds of thousands of years. As volcanic rocks solidify, they retain the magnetism present at the time of the eruption. The Earth's magnetic field reverses periodically, and this is recorded as changes in the magnetism of these rocks (dark grey and light grey layers). This is also shown in the inset at lower right, where bar magnets (red and black) represent the reversing magnetic field.
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