A low ridge system of deposited ejecta bisects the crater floor, and includes the steep central peak designated Alphonsus Alpha (α).

The floor is fractured[4] by an elaborate system of rilles and contains four or five smaller craters surrounded by a symmetric darker halo.

These dark-halo craters are cinder cone-shaped and are believed by some to be volcanic in origin, although others think they were caused by impacts that excavated darker mare material from underneath the lighter lunar regolith.

Harold Urey said of a close-up photograph of Alphonsus: The floor is covered with many craters of various sizes, some sharp and hence new, others less distinct and partly filled with fragmented material.

[7] Alphonsus is one of the sites noted for transient lunar phenomena, as glowing red-hued clouds have been reported emanating from the crater.

On October 26, 1956, the lunar astronomer Dinsmore Alter noted some blurring of the rilles on the floor of Alphonsus in the photographs he took in violet light.

", after Louis XIV of France,[13] and Johannes Hevelius called it "Mons Masicytus" after a range of mountains in Lycia.

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Alphonsus.