Olympus Mons levees along their margins (pictured). The cooler, outer margins of the flow solidify, leaving a central trough of molten, flowing lava. Partially collapsed lava tubes are visible as chains of pit craters, and broad lava fans formed by lava emerging from intact, subsurface tubes are also common. In places along the volcano's base, solidified lava flows can be seen spilling out into the surrounding plains, forming broad aprons, and burying the basal escarpment. Crater counts from high-resolution images taken by the Mars Express orbiter in 2004 indicate that lava flows on the northwestern flank of Olympus Mons range in age
Tectonics of Mars in elevation than the southern highlands, and have a much lower crater density, indicating a younger surface age. The underlying crust is however thought to be the same age as that of the southern highlands. Unlike the southern highlands, magnetic anomalies in the northern plains are sparse and weak. The Tharsis plateau, which sits in the highland-lowland boundary, is an elevated region that covers roughly one quarter of the planet. Tharsis is topped by the largest shield volcanoes known in the solar system. Olympus Mons stands 24 km tall and is nearly 600 km in diameter. The adjoining Tharsis Montes