Blisters

Volatiles in the asphalt can build up as vapor blisters beneath the granule layer.  Eventually, the top of the blister pops or weathers away, exposing an open-centered dome or crater-like cavity.  Heat blisters are not depressions but result from raised domes on the shingle surface. New blisters have sharper, more defined edges where aged blisters can be more worn at their edges.