Tharsis is a former volcanically and tectonically active region near the Martian equator covering one quarter of the surface of Mars and hosting most of the gigantic Martian volcanoes. This region rises 10 kilometers above the Martian datum, a reference that defines zero absolute altitude since there is no sea level on Mars. Tharsis volcanism is related to multiple volcanic centers of which the oldest one is situated in the Thaumasia region of the southern highlands (visible in the upper right corner of the image). Other, later active volcanic centers are situated in Syria Planum, at Pavonis and Alba Mons. A number of radiating and circumferential dike swarms in the Tharsis region can be traced back to these volcanic centers.