<P> The three Tharsis Montes are about 700 kilometres (430 mi) apart . They show a distinctive northeast--southwest alignment that has been the source of some interest . Ceraunius Tholus and Uranius Mons follow the same trend to the northeast, and aprons of young lava flows on the flanks of all three Tharsis Montes are aligned in the same northeast--southwest orientation . This line clearly marks a major structural feature in the Martian crust, but its origin is uncertain . </P> <P> In addition to the large shield volcanoes, Tharsis contains a number of smaller volcanoes called tholi and paterae . The tholi are dome - shaped edifices with flanks that are much steeper than the larger Tharsis shields . Their central calderas are also quite large in proportion to their base diameters . The density of impact craters on many of the tholi indicate they are older than the large shields, having formed between late Noachian and early Hesperian times . Ceraunius Tholus and Uranius Tholus have densely channeled flanks, suggesting that the flank surfaces are made up of easily erodible material, such as ash . The age and morphology of the tholi provide strong evidence that the tholi represent the summits of old shield volcanoes that have been largely buried by great thicknesses of younger lava flows . By one estimate the Tharsis tholi may be buried by up to 4 km of lava . </P> <P> Patera (pl . paterae) is Latin for a shallow drinking bowl . The term was applied to certain ill - defined, scalloped - edged craters that appeared in early spacecraft images to be large volcanic calderas . The smaller paterae in Tharsis appear to be morphologically similar to the tholi, except for having larger calderas . Like the tholi, the Tharsis paterae probably represent the tops of larger, now buried shield volcanoes . Historically, the term patera has been used to describe the entire edifice of certain volcanoes on Mars (e.g., Alba Patera). In 2007, the International Astronomical Union (IAU) redefined the terms Alba Patera, Uranius Patera, and Ulysses Patera to refer only to the central calderas of these volcanoes . </P> <Ul> <Li> <P> 2001 Mars Odyssey THEMIS mosaic of Uranius Tholus (upper volcano) and Ceraunius Tholus (lower volcano). The latter is about as high as Earth's Mount Everest . </P> </Li> <Li> <P> 2001 Mars Odyssey THEMIS mosaic of Tharsis Tholus . </P> </Li> <Li> <P> Western part of Jovis Tholus, as seen by THEMIS . </P> </Li> <Li> <P> Neighboring Biblis and Ulysses tholi (THEMIS daytime IR mosaic). </P> </Li> <Li> <P> Ulysses Tholus, with its location in relation to other volcanoes shown (photo by THEMIS). </P> </Li> </Ul>

Where are most of the extinct volcanoes located on mars