Mars is only about one-half of the size of Earth and yet the red planet has several volcanoes that surpass the scale of the largest terrestrial volcanoes. The four brown spots visible on the northern (upper) portion of the planet are four enormous volcanoes on the Tharsis region of Mars. Olympus Mons (slides #2 and #3) is the northernmost volcano and the three aligned volcanoes are the Tharsis Montes (slide #4). This view emphasizes the fact that the volcanoes of Mars are significant not only as localized centers of eruptive activity but also as prominent landforms on a planetary scale.
Viking Orbiter Views of Mars, NASA SP-441, p. 14, U.S. Govt. Printing Office, Washington, DC.