The rock is an example of an accretionary lava ball which forms on the surface of an `a`a flow. The rock grows when a small fragment of solidified lava rolls along the surface of an active flow and lava sticks to its surface, like a growing snowball rolling down a hill.

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Face of Pele in accretionary lava rock in Hawaii Volcanoes National Park

Accretionary Lava Ball - photo: volcanoes.usgs.gov