Backscatter electron microscope image of light and dark growth rings in a plagioclase crystal from a prehistoric Augustine eruption. Image brightness reflects the mean atomic mass. Brighter zones have more calcium (higher anorthite, “An”) and darker zones have more sodium (lower An). Round inclusions of melt trapped during crystal growth are visible, and can be used to measure the water content of magmas before eruption. Image by Matt Loewen, USGS/AVO.
Image courtesy of AVO/USGS.Please cite the photographer and the Alaska Volcano Observatory / U.S. Geological Survey when using this image.
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