page_title: Tharsis
section_title: Volcanism
hierarchical_section_title: Tharsis / True polar wander on Mars / Volcanism
caption_attribution_description: English: Diffuse, water-ice clouds, a hazy sky and a light breeze. Such might have read aweather forecastfor the Tharsis volcanic region on Mars on 22 November 2016, when this image was taken by the ExoMars Trace Gas Orbiter. Clouds, most likely of water-ice, and atmospheric haze in the sky are coloured blue/white in this image. Below, 630 km west of the volcano Arsia Mons, the southernmost of the Tharsis volcanoes, outlines of ancient lava flows dominate the surface. The dark streaks are due to the action of wind on the dark-coloured basaltic sands, while redder patches are wind blown dust. A handful of small impact craters can also be seen. The Trace Gas Orbiter, a joint effort between ESA and Roscosmos, arrived at Mars on 19 October last year. Since March it has been repeatedly surfing in and out of the atmosphere, generating a tiny amount of drag that will steadily pull it into a near-circular 400 km altitude orbit. It is expected to begin its full science operational phase from this orbit in early 2018. Prior to this ‘aerobraking’ phase,several test periodswere assigned to check the four science instrument suites from orbit and to refine data processing and calibration techniques. The false-colour composite shown here was made from images taken with the Colour and Stereo Surface Imaging System, CaSSIS, in the near-infrared, red and blue channels. The image is centred at 131°W / 8.5°S. The ground resolution is 20.35 m/pixel, and the image is about 58 km across. At the time the image was taken, the altitude was 1791 km, yielding a ground track speed of 1.953 km/s.
context_page_description: Tharsis is a vast volcanic plateau centered near the equator in the western hemisphere of Mars. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons, which are collectively known as the Tharsis Montes. The tallest volcano on the planet, Olympus Mons, is often associated with the Tharsis region but is actually located off the western edge of the plateau. The name Tharsis is the Greco-Latin transliteration of the biblical Tarshish, the land at the western extremity of the known world.
context_section_description: Spacecraft exploration over the last two decades has shown that volcanoes on other planets can take many unexpected forms. Over the same time period, geologists were discovering that volcanoes on Earth are more structurally complex and dynamic than previously thought. Recent work has attempted to refine the definition of a volcano to incorporate geologic features of widely different shapes, sizes, and compositions throughout the Solar System. One surprising and controversial conclusion from this synthesis of ideas is that the Tharsis region may be a single giant volcano. This is the thesis of geologists Andrea Borgia and John Murray in a Geological Society of America special paper published in 2010.
  The key to understanding how a vast igneous province like Tharsis can itself be a volcano is to re-think the notion of volcano from one of simple conical edifice to that of an environment or "holistic" system. According to the conventional view in geology, volcanoes passively build up from lava and ash erupted above fissures or rifts in the crust. The rifts are produced through regional tectonic forces operating in the crust and underlying mantle. Traditionally, the volcano and its magmatic plumbing have been studied by volcanologists and igneous petrologists, while the tectonic features are the subject for structural geologists and geophysicists. However, recent work on large terrestrial volcanoes indicates that the distinction between volcanic and tectonic processes is quite blurry, with significant interplay between the two.
  Many volcanoes produce deformational structures as they grow. The flanks of volcanoes commonly exhibit shallow gravity slumps, faults and associated folds. Large volcanoes grow not only by adding erupted material to their flanks, but also by spreading laterally at their bases, particularly if they rest on weak or ductile materials. As a volcano grows in size and weight, the stress field underneath the volcano changes from compressional to extensional. A subterranean rift may develop at the base of the volcano where the crust is wrenched apart. This volcanic spreading may initiate further structural deformation in the form of thrust faults along the volcano's distal flanks, pervasive grabens and normal faults across the edifice, and catastrophic flank failure (sector collapse). Mathematical analysis shows that volcanic spreading operates on volcanoes at a wide range of scales and is theoretically similar to the larger-scale rifting that occurs at mid-ocean ridges (divergent plate boundaries). Thus, in this view, the distinction between tectonic plate, spreading volcano, and rift is nebulous, all being part of the same geodynamic system.
  According to Borgia and Murray, Mt. Etna in Sicily is a good terrestrial analogue for the much larger Tharsis bulge, which to them is one immense volcano they call Tharsis Rise. Mt. Etna is a complex spreading volcano that is characterized by three main structural features: a volcanic rift system that crosses the summit in a north-northeast direction; a peripheral compression belt (thrust front) surrounding the base of the volcano; and an east-northeast trending system of transtensional (oblique normal) faults that connect the summit rift to the peripheral thrust front. The volcano's peak contains an array of steep summit cones, which are frequently active. The entire edifice is also peppered with a large number of small parasitic cones.
  The structural similarities of Etna to Tharsis Rise are striking, even though the latter is some 200 times larger. In Borgia and Murray's view, Tharsis resembles a very large spreading volcano. As with Etna, the spreading has produced a rift through the summit of the rise and a system of radial tear faults that connect the rift to a basal compression belt. The tear-fault system on Tharsis is represented by the radial fossae, of which Valles Marineris is the largest example. The thrust front is visible as the Thaumasia Highlands. Unlike on Earth, where the rifting of plates produces a corresponding subduction zone, the thick lithosphere o
caption: Images of ice water clouds over Tharsis taken by the ExoMars Trace Gas Orbiter, 2016
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