Geology
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.
The total mass of the Tharsis bulge is approximately 1021 kg, about the same as the dwarf planet Ceres. Tharsis is so large and massive that it has likely affected the planet's moment of inertia, possibly causing a change in the orientation of the planet's crust with respect to its rotational axis over time. According to one recent study, Tharsis originally formed at about 50°N latitude and migrated toward the equator between 4.2 and 3.9 billion years ago. Such shifts, known as true polar wander, would have caused dramatic climate changes over vast areas of the planet.
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