The highland paterae on Mars are unique. First, they are not part of the volcanoes in Tharsis and Elysium. They mostly lie in the Cratered Uplands far from other large volcanoes. They also are much older than the Tharsis and Elysium shields. Second, these paterae do not look like Earth volcanoes. There is no sign of actual lava flows. Rather, their central calderas are surrounded by sets of radial furrows. Third, these volcanoes are very flat. They typically are only 1-2 km high and 200-300 km across. These volcanoes are sometimes called ash shields. They seem to be (thin) piles of easily eroded volcanic ash. In contrast to the Earth, however, this ash seems to be composed of basalt. It probably formed when magmas met underground water and exploded into ash and steam. Such explosions help to explain the low height of these paterae. First, large ash eruptions tend to trap air beneath the ash clouds. This air helps support the ash and lets it spread out over wide areas. Second, Mars' gravity is about 1/3 the Earth's. Thus, an eruption on Mars can also carry ash much further than on the Earth.
NOTE: The circular feature PP may also be a highland patera. However, it shows no sign of any furrowed ash units. Thus, it may just be a large impact that was partly buried by plains lavas. (Viking orbiter images 94A74, 94A75, & 94A76, from Tanaka & Leonard (1995) J. Geophys. Res., v. 100).
(Viking Orbiter image 106A09, from Lunar & Planetary Institute slide set Volcanoes on Mars.)
(Part of Viking Orbiter Mosaic 211-5213.)