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The Tharsis region is an extensive volcanic plateau on Mars. The region is centered close to the Martian equator and is home to a few of the largest volcanoes in the Solar System. The area is dominated by the three enormous shied volcanoes that make up the Tharsis Montes: Arsia Mons, Pavonis Mons, and Ascraeus Mons. Just off of the western edge of the Tharsis region is Olympus Mons, the largest volcano in the Solar System. Another feature that is close to the region, and often associated with it, is the largest and deepest valley in the Solar System: Valles Marineris.
Tharsis does not have defined boundaries, making exact measurement of the area difficult, but the bulge is generally defined as being about 5,000 km across and up to 7 km high. That 7 km height does not include the additional height of the volcanoes. It is bordered by Amazonis Planitia in the west and Chryse Panitia in the east. In the north it begins at Alba Patera and ends in the south at the Thausmasia highlands. That gives it a surface area of between 10 and 30 million km2. To put that in context, it covers roughly 25% of Mars.
Tharsis is a large enough that it has been broken down into geological subprovinces by NASA scientists. The subprovinces are of differing ages, volcanic activity, and possible tectonic activity. The northern rise subprovince is dominated by Alba Patera, a vast, low-lying volcanic construct unique to Mars. The oldest part of the northern rise consists of the highly fractured terrain of Ceraunius Fossae. This area could contain some of the oldest features on Mars. The southern rise area has a western border that is defined by Daedalia Planum. In the east, the enigmatic Syria-Thaumasia block, a rectangular slab of uplifted crust about 3,000 km wide.
Central Tharsis is defined by the three massive volcanoes mentioned above, a number of smaller volcanoes, and adjacent plains consisting of relatively new lava flows. The lava flows slope toward a system of immense northwest-oriented valleys that are up to 200 km wide. These northwestern slope valleys (NSVs) are separated by a parallel set of gigantic “keel-shaped” promontories and may be relics of catastrophic floods. The floods would have occurred several billion years ago when Mars was warm enough to support liquid water. Olympus Mons needs to be mentioned here because, despite being a distinct sub region, it is clearly related to the volcanic activity that formed the Tharsis area. When looked at in that light, Olympus Mons would be the youngest of the Tharsis Montes.
This is normally where I would cut an article off to allow a reader to digest the content, but I have not even mentioned the areas geology. Hopefully, you will read on or bookmark this page and come back to it.
Tharsis is commonly called a volcano-tectonic province. All that means is that the crust in the area was shaped by volcanoes and tectonic processes. Tharsis overlies a magma ”hot spot”. A hot spot is caused by massive columns of hot, low-density material rising through the mantle and erupts as basaltic lava. Since Mars lacked plate tectonics that would have closed the weak area of the crust, the eruptions lasted for millions of years and produced enormous volcanic constructs. It is thought that on Mars, as it does here on Earth, not all of the magma erupts onto the surface. This stalled magma slowly cools and produces large intrusive complexes. These features can be linear: dikes, fossae, and catenae(crater chains) or horizontal: sills and laccoliths.
Geologic evidence indicates that the bulge was largely in place by some 3.7 billion years ago, but 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. One estimate shows that Tharsis contains around 300 million km3 of igneous material. That material most likely contained CO2, water, sulfur, and chlorine. Given the volume of the material, it probably released a substantial atmosphere as well as a global layer of water 120 m deep. These elements would have combined to produce acids that broke down primary rocks and minerals. The total mass of the Tharsis bulge is approximately the same as some dwarf planets. The area is so massive that it may have caused a change in the orientation of the planet’s crust to the rotational axis. One recent study claims that the area originally formed at about 50°N latitude and migrated toward the equator between 4.2 and 3.9 billion years ago, causing dramatic climate changes over vast areas of the planet.
Some scientist have come forth to say that the entire Tharsis region, including Olympus Mons, may be just one enormous volcano with multiple eruption points. This requires a complete rethinking of the current view of what a volcano is. When dealing with a feature that encompasses 25% of a planet’s surface, that might be the only solution to understanding it.
We’ve also recorded several episodes of Astronomy Cast about Mars. Here’s a good one, Episode 52: Mars.