Complex Lavas in Nili Patera
Not all Martian volcanoes are cut from the pattern that produced Olympus Mons with its summit elevation of 22 kilometers (14 miles). Among the volcanoes with lower profiles is Syrtis Major.
This dark, triangular feature, visible even in backyard telescopes from Earth, was the first Martian surface marking to be detected with the crude telescopes of the 1600s. Syrtis spans some 1,300 km (800 mi) wide, yet rises only about 1 km (3,300 ft) high. On most Mars globes, it's just a low bump lying west of the Isidis impact basin. Geologists call such features shield volcanoes, and they are built from a great many thin lava flows.
The image here, assembled from dozens of individual frames taken at visible wavelengths by the Thermal Emission Imaging System (THEMIS) shows Nili Patera. This is Syrtis' summit caldera, the collapsed volcanic mouth from which the lavas flowed.
The floor of Nili Patera lies about 1,800 m (roughly 6,000 ft) lower than its western rim, which is marked by curving fractures and faults. Nili spans about 50 km (30 mi) wide and forms part of a bigger caldera complex stretching some 400 km (250 mi) in a north-south direction.
What gives Nili Patera special interest for planetary geologists is that it contains two kinds of lava flow: basalt and dacite. This is unusual for Mars, where most lavas are basalt, and the discovery showed that the planet is a volcanically complex world.