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It is the largest volcano on Mars in terms of area, with volcanic flow fields that extend for at least 1350 km (839 mi) from its summit. Although the volcano has a span comparable to that of the United States, it reaches an elevation of only 6.8 km (4.2 mi) at its highest point. This is about one-third the height of Olympus Mons, the tallest volcano on the planet.
http://en.wikipedia.org/wiki/Alba_Mons