This crater is particularly interesting because, although it was formed by an impact, it has fractures on its floor and there are symmetric dark patches around some of these fractures. The dark patches are believed to be volcanic in origin, and were most likely produced by explosive eruptions. Alphonsus is a very old impact crater, dating from the immediate post-Nectarian age.
Alphonsus crater was one of the primary alternative landing sites considered for both the Apollo 16 and the Apollo 17 missions. The Ranger 9 probe impacted in Alphonsus crater, a short distance to the northeast of the central peak.