Mike Caplinger, Malin Space Science Systems February 1994
Volcanoes on Mars have been divided into three types.
The montes or large shields are likely basaltic (like the volcanos in Hawaii and Iceland), very large (Olympus Mons is the largest mountain feature known anywhere in the solar system), and have very gentle slopes of six degrees or less.
The tholi are smaller than the montes, generally convex (dome-shaped), and have somewhat steeper flanks, in some cases greater than 8 degrees in slope. There are two possible explanations for the difference between the tholi and the montes. It may be that the material of the tholi was more viscous during the eruptions, or it may be because the eruptions that formed the tholi were of small volume, and hence the flows piled up closer to their source.
Ceraunius and Uranius Tholi
The paterae (patera means "saucer" in Latin) are widely varied in form; in general they have very shallow slopes and scalloped, complex edges in their summit calderas; many have radial channels on their flanks. Some of the paterae (for example, Apollinaris Patera) have been extensively modified by collapse and erosion; others (Biblis Patera, for example) have fairly symmetric structure with collapsed caldera walls. The speculation is that, because of their susceptibility to erosion and collapse, they are composed of loose materials such as ash flows. The extreme state of erosion on some of the paterae leads to the conclusion that these are the oldest of the volcanic constructs.
Alba Patera is unique among the paterae on Mars and indeed, appears to be unlike any other feature in the solar system. It is very large (over 1600 kilometers across), has a central caldera, but rises no more than 3 km from the surrounding plains. Alba Patera appears to have been formed by numerous flow events that were either much higher in rate or much longer in duration than flows seen elsewhere.
central Alba Patera
Mike Caplinger (email@example.com)