This 3-Dimentsional picture of Olympus Mons is only one of many other volcanoes in the Tharsis regions (copyright Calvin J. Hamilton).
On Mars, volcanic activity is strongly evident by great landmarks such as the Tharsis and Elysium regions on the surface. In those regions lie the most massive volcanoes known in the solar system. The Tharsis region or Tharsis dome is approximately 2,500 miles (4,000 kilometers) across and 6.2 miles (10 kilometers) high. On the northwest area of this region lie three large shield volcanoes: Ascraeus Mons, Pavonis Mons and Arsia Mons. Venturing further down the flank is the biggest of shield volcano of them all, Olympus Mons.
The second largest volcanic region on Mars is the Elysium Planitia. The dome measures 1,060 by 1,490 miles (1,700 by 2,400 kilometers) and contains a broader classification of volcanoes than Tharsis. Three volcanoes on the plain of Elysium Planitia are Hecates Tholus, Elysium Mons and Albor Tholus.
Looking at a typical Martian volcano, one can see that there is a summit point (or caldera) in which hundreds of lava channels branch out. This is comparable to the shield volcanoes found on Earth and is similar only by structure, not by size. Although most volcanoes on the surface of these two domes are classified as shield volcanoes, the Alba Patera in the northern region of the Tharsis is one in which the horizontal length is comparable to most of the larger scale volcanoes but the height falls far short. Its base diameter is 930 miles (1,500 kilometers) but is less than 7 kilometers (4.3 miles) high.
One of the relatively smaller volcanoes, the Ceraunius Tholus measures only to be about the size of the Big Island of Hawaii. To one observing the appearence of this volcano, ash deposits can be seen all around. The Tyrrhena Patera and Hadriaca Patera volcanoes are similar in this fashion in that both have eroded areas indicating explosive ash eruptions at one point in their life.