Lava balls are formed from pieces of breakdown that float and roll along in a stream of moving lava, accreting mass as they move. It is much like a snowball rolling downhill and growing larger as more snow sticks to it.
Lava balls tend to roll along until being wedged at a constriction, or often, are stuck to a ceiling during a surge in the flow. They may get quite large, as shown in the second photo. Sometimes they are lifted by upwelling lava through a window or skylight and are deposited around the edges before the lava drains back in, as in the third photo.

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Created: August 4, 2000
Author: Dave Bunnell