Mars has a number of volcanoes with diameters between 50 and 150 km. These volcanoes are thus the same size as many large Earth volcanoes. Most are found within the Tharsis and Elysium regions. However, they are all older than the giant shield volcanoes. Thus, many have been partly buried by younger lavas. These volcanoes are divided into two general types. The first type are called "paterae," after the Greek word for a shallow saucer or bowl. They are fairly flat, they generally have little relief, and they often have large calderas. The second class of volcanoes are called "tholi." These volcanoes have much steeper sides, and they typically are taller than the patera structures. Both types apparently formed mostly from basaltic lava flows. Thus, they are much like Earth shield volcanoes. Their different appearances may mark either different depths of burial or changes in the nature of volcanism.

Biblis Patera


Biblis Patera is one of two volcanoes located near the center of Tharsis volcanism. With Ulysses Patera, it lies almost halfway between Olympus Mons and the southern Tharsis Montes. It is nearly 170 km long by over 100 km wide, and it has a central caldera nearly 55 km in diameter. It appears to be some 2 to 3 km tall. Biblis Patera is probably between 2 and 2.8 billion years old. However, it is surrounded by much younger lava flows. These flows come from Pavonis Mons to the east, and are clearest in the lower left of the image. The elongate shape of Biblis partly reflects the slope of these flows. Biblis is also clearly cut by a number of faults and graben. Note that some of these faults are buried by younger lavas along the edge of the volcano. This suggests that Biblis was modified by several episodes of tectonism and volcanism. (Viking Orbiter images 44B48 & 44B50, from Plescia (1994) Icarus, vol. 111.)

Ceraunius Patera


Ceraunius Tholus lies on the northeast edge of the Tharsis region. It is roughly 120 km long by 95 km wide, and it is about 2-3 km in height. It is probably close to 3 billion years old. Although running water formed the vast majority of channels on Mars, Ceraunius also shows two likely lava channels. One is the large channel in the upper center. Note the size of this feature. It is much larger than any of the other (water-carved) gullies on Ceraunius. It also appears to feed a volcanic cone in the old impact crater at its base. The second example is the chain of lunar-like pits in the left center. This pit chain links up to a clear channel downslope and probably reflects a collapsed lava tube. Both of these features strongly resemble the sinuous rilles on the Moon. (mosaic of Viking Orbiter images 516A24, 622A56, 622A58, 622A59 & 622A60, from Gulick and Baker (1990) J. Geophys. Res., vol. 95, No. B9)

Jovis Tholus


Jove's Tholus lies nearly due east of Olympus Mons and northwest of Ascreus Mons. Thus, it lies on the very northern edge of the lava plains surrounding the Tharsis Montes. It is roughly 80 km long by 60 km wide, and it has a large off-center caldera complex some 40 km in diameter. The total height is probably close to 2 km. Its age is very poorly constrained. The volcano could be anywhere from 2.3 to 3.5 billion years old. Like most tholi, Jovis Tholus is surrounded by younger lava flows. These lavas also cover a set of buried graben , however. The graben cut the easternmost and westernmost edges of the volcano. Jovis also shows signs of a long eruptive history. Its sprawling caldera contains no less than 5 craters. These calderas march southwest from the first, central caldera. Each younger caldera has a lower floor. (Viking Orbiter images 41B17 & 41B19, from Plescia (1994) Icarus, vol. 111.)

Ulysses Patera


Ulysses Patera lies just east of Biblis Patera. It is also located near the middle of Tharsis volcanism, and it has been buried by lavas from Pavonis Mons. It is about 100 km in diameter and is about 2-3 km tall. The central caldera is nearly 56 km across . Like Biblis Patera, Ulysses Patera is surrounded by younger lava flows. It is also cut by a few graben. The most striking features on this volcano, however, are two large impact craters. These craters overlap the caldera and clearly postdate all major volcanism at this site. Large craters are rare outside of the cratered uplands on Mars. Thus, the presence of two such craters on Ulysses Patera is highly unusual. This volcano appears to be very old. It has an estimated age of ~3.4 billion years. (VIking Orbiter images 49B68, 49B70 & 49B85, from Plescia (1994) Icarus, vol. 111.)

Uranius Tholus


Uranius Tholus is located just north of Ceraunius Tholus. It is ~57 km in diameter, and it shows a clear cone several kilometers high. It is heavily cratered and it seems to be quite old. It is probably over 3 billion years in age. It is also surrounded by fairly old volcanic plains, much older than those seen at Biblis and Jovis. These plains record a long history of faulting, partially seen at the upper left. Such fault systems have been buried closer to the giant Tharsis shields, but they may have strongly influenced the locations of both shield volcanism and of the smaller tholi. (Viking Orbiter image 516A23)