Mars - A large highland volcanic province revealed by Viking images

Many of the mountains in the rugged highland terrain of the Phaethontis and Thaumasia quadrangles are believed to be of volcanic origin. Those provisionally mapped as volcanoes have diagnostic characteristics such as lobate flow fronts around their bases, depressed central areas, or have massive, bulbous accumulations of material of no determinable origin other than volcanic. Most of the volcanoes are younger than materials forming the highlands but are older than early lava flows from Arsia Mons. Many are aligned along older fault and ridge systems that are transverse to the more recent and prominent faults transecting the region. The older faults are generally buried by plains lava flows but their traces are visible in several places in the highlands. These faults are relatively short in length.