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 is not a true highland patera, but it is also unique. It is the largest volcano on Mars, and is roughly 1600 km across. Despite its size, however, it is very flat. Alba has a total height of only ~3 km. Alba differs from both the giant martian shields and the highland paterae. Unlike the paterae, there are no signs of any furrowed ash deposits. Also, it is not in the highlands. Rather, it lies north of the Tharsis Volcanoes. Unlike the giant shields, Alba's volcanism is incredibly widespread. Its lava flows look like those on the other shields, but they are not piled as deeply. Also, it sits on a major fault trend that runs north northwards from the Tharsis region. As shown here, the youngest of these faults curve around to neatly frame Alba's summit. Alba also shows a long history of very fluid volcanism. There are both large sheets of mare-like lava flows and hundreds of long, narrow flows with a central channel or lava tube. Most of these flows are over 100 km long, and some are well over 300 km in length. Many are nearly 10 times larger than Earth flows which look similar. This increase in size probably marks larger and longer eruptions than those on the Earth. However, it may also need more liquid lavas than those on Earth.
Amphitrite (AP) also lies on the edge of the Hellas basin. However, it is on the far side from Hadriaca. Although larger, it is also less striking in appearance than Hadriaca. The central ring structure is about 120 km in diameter. The furrowed ash shield is about 300 km across. It seems fairly thin, and it has little apparent relief. Like Tyrrhena Patera, it is located in a large unit of mare-like plains. Little else is known about the volcano.
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).
Hadriaca is younger than Tyrrhena Patera. It is also larger and better preserved. The caldera in the upper left is nearly 60 km across. The furrowed deposits extend off the image to the lower left for over 300 km. However, they extend less than 100 km in the other directions. This is because Hadriaca lies on the edge of a large impact basin, and has mostly flowed down the side of this basin. It has a very low relief as well. Ignoring the slope into Hellas, it is only 1-2 km tall. This volcano also is linked to clear signs of martian ground water. It lies near the sources for a major martian outflow or flood channel. Several of these sources are seen to the right here as large, smooth-floored depressions. These features indicate that a lot of water or ice was buried near the volcano.
(Viking Orbiter image 106A09, from Lunar & Planetary Institute slide set Volcanoes on Mars.)
Tyrrhena is the most striking of the highland paterae. It is about 300 km across, and it has a total relief of about 2 km. The central caldera is 12 km across, but it lies within a larger ring of fractures. This fracture ring may mark an older, buried caldera. It is 45 km in diameter, and it encircles the whole summit. The most striking features, however, are the broad furrows that run away from the fracture ring. These furrows are up to 200 km in length, and they suggest a period of heavy erosion. This erosion is very old and likely dates back to the time of volcano growth. It may mark a time of strong hydrothermal activity. Note that the furrows east of the summit are buried by a smoother set of volcanic deposits.
(Part of Viking Orbiter Mosaic 211-5213.)