Besides the large volcanoes, Mars also has many other volcanic features. The most obvious are the mare-like plains near Tharsis and the largest impact basins. However, the Viking mission found other, much smaller features as well. These include Earth-like cinder cones, a few small shields and some very old, rugged mountains in the cratered uplands. Some examples from each of these groups are shown here.
Cinder Cones 2
This image shows a field of larger cinder cones (arrows). Each is 2-5 km in size, and they are located near the edge of the cratered uplands. Like large cinder cones on Earth, most show a lop-sided, horse-shoe like pattern. Also note the many irregular pits within this image. These may be magma drainage features or a mark of volcanically melted ground ice within the region.
(Viking Orbiter image 878A38, from Wichman and Schultz (1989) J. Geophys. Res. vol. 94, p.17343.)
Cinder Cones 1
This image shows one likely group of small volcanic cones on Mars. The features are all less than a kilometer in size, and they are located in the northern lowlands. Each shows a clear central pit, and several lie on or along older faults (arrow). They are similar in size to Earth cinder cones, but the central pits are larger due to MarsÕ lower gravity. Like Earth cones, they mark the sites of small explosive eruptions.
(Viking Orbiter image 070A04, from Wilson and Head (1994) Rev. Geophysics, vol. 32, p. 248)
Highland Volcano (?)
There are also some possible volcanoes in the cratered uplands. Because the cratered uplands are very rough, however, these volcanoes are very hard to find. This is one of the clearest examples. It is about 25 km in size, and it lies south of the Tharsis region. It is clearly an isolated mountain with an apparent (central?) caldera (arrow). It is also heavily furrowed and thus looks like a number of Earth volcanoes. The problem is that it has no clear lava flows. Thus, the inferred caldera could be just another small impact crater. Similarly, the peak itself might be just part of a very old impact crater. However, that seems unlikely in this case.
(Viking image 56A68, from Scott (1982) J. Geophys. Res., vol. 87, p. 9841.)
This image shows a small shield volcano in the Tempe region of Mars. It is about 5 km across (arrow), but it grades smoothly into a plains unit some 20 km in size. These plains may have erupted from the shield, but they also link up with other flows that came in from Alba Patera (upper left). The lava plains in turn bury a heavily faulted section of the old cratered uplands. These faults are part of a large, early deformation event around the Tharsis Montes. Note how both the shield and its summit fissure line up with the buried faults. (Viking Orbiter image 627A28, from NASA SP-460, The Geology of the Terrestrial Planets)
Plains with Calderas
Wind-blown sand and dust cover most surfaces on Mars. Thus, most of the mare-like plains on Mars show little sign of a volcanic origin. This is one of the exceptions. Shown here is a caldera located near the middle of Syrtis Major Planum. This plains unit is over 1200 km in diameter. Unlike the lunar mare, however, it does not fill the floor of a large impact basin. Rather, it covers the cratered uplands just outside the Isidis impact basin.
The caldera itself is 70 km across, and several hundred meters deep. It marks the collapsed roof of an old, shallow magma chamber. To the northwest, it is surrounded by faults and graben. To the southeast, the caldera wall has been buried by younger lava plains. Note the small volcanic cone (arrow) located on the caldera floor.
(Viking Orbiter image 375S13, from Schaber (1982) J. Geophys. Res. vol. 87, p. 9856)
Valles Marineris Pyroclastics
This image shows a small part of the Valles Marineris Canyon System. The upper half of the picture is a canyon wall. The lower half is part of the canyon floor. The band of dark splotches running through the middle is a set of minor volcanic vents. These splotches are fairly thin, and they seem to be centered on faults within the canyon wall. They are also very young. They are younger than almost every other feature in the canyons, and some may be less than 100,000 years old. Thus, they may be the youngest volcanic units on Mars. They probably mark very small pyroclastic eruptions, much like the smallest dark mantling deposits on the Moon.
(Viking Orbiter image 81A04, from Lucchita (1987) Science, vol. 235, p. 566.)