DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Pebble of andesite, an extrusive volcanic (igneous) rock with a typical porphyritic texture, being that of twinned crystals of feldspar (white) floating in an amorphous matrix of solid lava, here of brick-red colour. This porphyritic texture is caused by exsolution, a process of separation that takes place during successive phases of crystallisation and solidification in the lava. Pebble's actual width is 100 mm.
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