Intra-patera eruptions occur within volcanic depressions known as paterae, which generally have flat floors bounded by steep walls. Paterae resemble terrestrial calderas, but it is unknown whether they form when an empty magma chamber collapses, like their terrestrial cousins. One hypothesis suggests that they are produced through the exhumation of volcanic sills, with the overlying material either being blasted out or integrated into the sill. Some paterae display evidence for multiple collapses, similar to the calderas atop Olympus Mons on Mars or Kīlauea on Earth, suggesting that they may occasionally form like volcanic calderas. Because the formation mechanism is still uncertain, the general term for these features uses the Latin descriptor term employed by the International Astronomical Union in naming them, paterae. Unlike similar features on Earth and Mars, these depressions generally do not lie at the peak of shield volcanoes and are larger, with an average diameter of . Patera depths have been measured for only a few paterae and typically exceed 1 km. The largest volcanic depression on Io is Loki Patera at across. Whatever the formation mechanism, the morphology and distribution of many paterae suggest that they are structurally controlled, with at least half bounded by faults or mountains.