MICHAEL CLUTSON / SCIENCE PHOTO LIBRARY MICHAEL CLUTSON / SCIENCE PHOTO LIBRARY
Pumice stone. This soft abrasive igneous rock originated as part of a pyroclastic flow from the 1883 eruption of Krakatoa (Krakatau). It has a vesicular texture, a high silica content and a low density. It can still be found floating in the Sundra Strait between Sumatra and Java.
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