Mountains are widely distributed across the surface of Io, the innermost large moon of Jupiter. There are about 115 named mountains; the average length is and the average height is . The longest is , and the highest is Boösaule Montes, at , taller than any mountain on Earth. Ionian mountains often appear as large, isolated structures; no global tectonic pattern is evident, unlike on Earth, where plate tectonics is dominant. Io is exceptional for the strong tidal heating it undergoes, caused by the eccentricity of its orbit (which results from its resonance with Europa and Ganymede) in conjunction with the proximity and great mass of Jupiter. This leads to widespread and intensive volcanism. Most volcanoes on Io have little relief; those that can be considered mountains are generally smaller than the mountains formed by tectonic processes, averaging only in height and in width. Several geodynamic models of Io exist but the tectonic mountain-building process is still obscure and debatable. However, it is thought to be related to stresses caused by the rapid volcanic resurfacing of the body. Observations To explore the origin of Io's mountains, classification of morphological types and description of morphological features are necessary. Morphological types Four morphological types of mountains have been identified. Mesa: a mountain with flat top and relatively smooth surface. It may be difficult to distinguish mesas from eroded layered plains. Ethiopia Planum is a good example of this morphological type. Eleven mountains on Io are classified as mesas. Plateau: an elevated plain with a rugged surface. There is no steep or prominent peak on plateau. Iopolis Planum is a good example of this type. About 46% of Ionian mountains belong to this morphological type. Ridge: an elevated structure dominated by one or more linear or arcuate rises. 28 (24%) mountains on Io have been cataloged into this type. Massif: an elevated structure dominated by rugged or complex surface and has one or more peaks. Boösaule Montes and Tohil Mons are good examples. Common features Several common features of Ionian mountains have been summarized. Basal scarps: basal scarps always appear as an abrupt boundary of Ionian mountains that separate mountains from volcanic plains. Most Ionian mountains are observed to have this feature. The basal scarps are ten to a few hundred meters high. Sometimes, the scarp is resolved in high-resolution images as the margin of a debris apron. An example is Iopolis Planum.