Mars

Mars—the Red Planet, the fourth planet from the Sun—gets its color from iron oxide (rust) in the sand and dust covering most of its surface. The most prominent dark area near the equator, Syrtis Major, is exposed basalt rock not covered by sand and dust. The light-colored round spot north of the equator in the large desert area is Olympus Mons, an extinct volcano with an altitude of 69,841 feet above the mean altitude. Why so big? Olympus Mons is made by a volcanic hot spot, just like some of those on Earth. But on Earth we have moving tectonic plates, so a hot spot on Earth made a string of Hawaiian Islands as the Pacific plate moved. Mars had no crust movement, so the hot spot there made just one enormous volcano. Its base is 370 miles wide.