Viking Orbiter photography has revealed the importance of volcanism in the geologic evolution of the cratered terrain hemisphere (generally in the southern hemisphere) of Mars. Volcanic units in this region are classified morphologically into four major units: 1) Patera, comprising 2.37¿105 km2 (0.3% of the cratered terrain hemisphere), are large, low profile volcanic structures; some appear to be older shield volcanoes, others apparently represent a unique style of volcanism; volcanoes, others apparently represent a unique style of volcanism; 2) ''Plains'' volcanics occupy 2.3¿106 km2 (2.9% of hemisphere) and represent low volume eruptions that formed cones, low shields and other small scale structures; 3) flood volcanics (3.7 ¿106 km2; 4.7% of hemisphere) are produced by high volume eruptions, post-date the older and more degraded plateau plains, and occur mostly as basin-fill materials; and 4) Plateau plains (28.5¿106 km2; 36% of hemisphere), the martian intercrater plains, contain many wrinkle ridges and floor-fractured craters. The results of study suggest volcanic processes, as well as erosional processes have been important in the obliteration of small (less than 10 km) craters on Mars and that volcanic products may constitute a significant fraction (up to 44%) of the surface rocks in the cratered terrain.