More than 135 mountains have been identified on the surface of Jupiter's moon Io. Despite the extensive active volcanism taking place on Io, most mountains on Io are formed through tectonic processes. These structures average 6 km (4 mi) in height and reach a maximum of 17.5 ± 1.5 km (10.9 ± 1 mi) at South Boösaule Montes. Mountains often appear as large (the average mountain is 157 km (98 mi) long), isolated structures with no apparent global tectonic patterns outlined, in contrast to the situation on Earth. To support the tremendous topography observed at these mountains requires rock compositions consisting mostly of silicate, as opposed to sulfur.