page_title: Tharsis
section_title: Geology
hierarchical_section_title: Tharsis / Geology
caption_attribution_description: Uploaded from wikipedia. Original caption is/was here. Original Caption Released with Image: The volcanoes of the Tharsis region are highlighted by this color image mosaic obtained on a single martian afternoon by the Mars Orbiter Camera (MOC) onboard the Mars Global Surveyor (MGS) spacecraft. Olympus Mons dominates the upper left corner -- it is one of the largest known volcanoes and is nearly 550 km (340 miles) wide. The grayscale image on the right shows the name of each volcano in the scene. The white or bluish-white features are clouds. Clouds are common over the larger Tharsis volcanoes in mid-afternoon. The four largest volcanoes are more than 15 km (9 mi) high. Viewed from Earth by telescope before any spacecraft had visited the planet, astronomers often described a "W"-shaped white cloud over the Tharsis region. This "W" was actually the result of seeing the combined effects of bright clouds hanging over the Ascraeus, Pavonis, Arsia, and Olympus volcanoes. The clouds result when warm air containing water vapor rises up the slopes of each volcano, cools at the higher altitude, and causes the water vapor to freeze and form a cloud of ice crystals. Pavonis Mons lies on the martian equator, north is up, and sunlight is illuminating the scene from the left. The picture is a mosaic of red and blue filter images taken on three consecutive orbits. The slightly blurred appearance of the left side of Arsia Mons results from distortion toward the edges of the images used to make the mosaic. To remove the blur, an image obtained on another day would be added to the mosaic--however, this image would not match well because the cloud patterns will have changed by the next day. Mosaics such as the one shown here are used to monitor changes in martian weather and to plan future observations. JPL Image Policy Credit line: "Courtesy NASA/JPL-Caltech."
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: Tharsis is commonly called a volcano-tectonic province, meaning that it is the product of volcanism and associated tectonic processes that have caused extensive crustal deformation. According to the standard view, Tharsis overlies a hot spot, similar to the one thought to underlie the island of Hawaii. The hot spot is caused by one or more massive columns of hot, low-density material (a superplume) rising through the mantle. The hot spot produces voluminous quantities of magma in the lower crust that is released to the surface as highly fluid, basaltic lava. Because Mars lacks plate tectonics, the lava is able to build up in one region for billions of years to produce enormous volcanic constructs.
  On Earth (and presumably Mars as well), not all of the magma produced in a large igneous province erupts at the surface as lava. Much of it stalls in the crust where it slowly cools and solidifies to produce large intrusive complexes (plutons). If the magma migrates through vertical fractures it produces swarms of dikes that may be expressed at the surface as long, linear cracks (fossae) and crater chains (catenae). Magma may also intrude the crust horizontally as large tabular bodies, such as sills and laccoliths, that can cause a general doming and fracturing of the overlying crust. Thus, the bulk of Tharsis is probably made of these intrusive complexes in addition to lava flows at the surface.
  One key question about the nature of Tharsis has been whether the bulge is mainly the product of active crustal uplifting from buoyancy provided by the underlying mantle plume or whether it is merely a large, static mass of igneous material supported by the underlying lithosphere. Theoretical analysis of gravity data and the pattern of faults surrounding Tharsis suggest the latter is more likely. The enormous sagging weight of Tharsis has generated tremendous stresses in the crust, producing a broad trough around the region and an array of radial fractures emanating from the center of the bulge that stretches halfway across the planet.
  Geologic evidence, such as the flow direction of ancient valley networks around Tharsis, indicates that the bulge was largely in place by the end of the Noachian Period, some 3.7 billion years ago. Although the bulge itself is ancient, volcanic eruptions in the region continued throughout Martian history and probably played a significant role in the production of the planet's atmosphere and the weathering of rocks on the planet's surface.
  By one estimate, the Tharsis bulge contains around 300 million km³ of igneous material. Assuming the magma that formed Tharsis contained carbon dioxide (CO₂) and water vapor in percentages comparable to that observed in Hawaiian basaltic lava, then the total amount of gases released from Tharsis magmas could have produced a 1.5-bar CO₂ atmosphere and a global layer of water 120 m thick. Martian magmas also likely contain significant amounts of sulfur and chlorine. These elements combine with water to produce acids that can break down primary rocks and minerals. Exhalations from Tharsis and other volcanic centers on the planet are likely responsible for an early period of Martian time (the Theiikian) when sulfuric acid weathering produced abundant hydrated sulfate minerals such as kieserite and gypsum.
caption: Orographic water ice clouds hover over the volcanic peaks of the central Tharsis region in this color image mosaic from Mars Global Surveyor. Olympus Mons dominates at upper left.  At center are the three Tharsis Montes: Arsia Mons at bottom, Pavonis Mons at center, and Ascraeus Mons at top.
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