Highland Paterae
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.
Alba Patera
Amphitite Patera
Hadriaca Patera
Tyrrhena Patera