Elevation feet (m)
Pavonis Mons Elysium Alba Patera lowest point in Valles Marineris Hellas Basin
(Note: Olympus Mons has been widely reported as 17 miles [27 km] in height, but the final MOLA data gives the lower value given here.)
ago, in fact within the first few hundred million years of the planet's existence. Volcanic Martian meteorites formed between 1.3 billion years ago and just a few million years ago, according to dating using radionuclides (see the sidebar "Determining Age from Radioactive Isotopes" on page 30). By studying relationships among flows on the surface of the planet, some geologists believe that the most recent volcanic eruption was as recent as 10 million years ago. The planet is almost certainly still volcanically active.
The Tharsis rise is one of the most prominent features on the surface of Mars. Tharsis is a huge topographic rise that holds four volcanoes, including the largest Martian shield volcanoes.Tharsis ridge stands 5.5 miles (9 km) high and lies on the Martian equator. The lower color insert on page C-4 shows the Tharsis rise and its volcanoes Arsia, Pavonis, and Ascraeus Mons, from bottom to top, and Tharsis Tholis (the Tharsis shield) at the top right; these volcanoes are about 250 miles (400 km) in diameter and reach elevations of 10.6 miles (17 km).
The summit calderas (central depressions) of all four volcanoes probably formed from recurrent collapse following drainage of magma resulting from flank eruptions. Alba Patera in the north center of the image, 1,000 miles (1,600 km) in diameter, far exceeds any other volcano in the solar system in area, covering eight times the area of Olympus Mons but reaching only about 3.8 miles (6 km) in height.
The image also shows at the bottom right Noctis Labyrinthus, the westernmost part of the giant valley network known as Valles Marineris. For more on the terms tholis, mons, and others, see the sidebar "Fossa, Sulci, and Other Terms for Planetary Landforms" on page 102.
Olympus Mons (left center), the largest volcano on Mars and in fact in the entire solar system, lies just to the west of the Tharsis rise. Olympus Mons is 375 miles (600 km) in diameter and 13.16 miles (21,283 m) in height (this volcano has been widely reported as 17 miles or 27 km in height, but the final MOLA data gives the lower value given here). By comparison, the Hawaiian island of Mauna Loa stands only five miles (8 km) above the floor of the Pacific Ocean.The caldera on Olympus Mons (the circular depression in the volcano's peak from which recent eruptions have come) is 50 miles (80 km) across. Olympus Mons was called Nix Olympica ("the Snows of Olympus") by its first telescopic observers, who could only see a white feature.When telescopes improved to the point that they could distinguish the feature as a mountain, it was renamed Olympus Mons.
The image of Olympus Mons below is processed to combine the photographic image of the volcano with topographic (elevation) information from the Mars Orbital Laser Altimeter, producing an image that shows appearance and height. Olympus Mons is surrounded by a well-defined cliff that is up to 3.5 miles (6 km) high. At its highest point, this cliff is more than three times as high as the Grand Canyon is deep.
The great weight and height of the Tharsis region causes stress in the lithosphere, the outer, rigid shell of the planet, much as a person sitting on a bed presses down the mattress and causes it to bend. Faults and ridges surround Tharsis.
Mars's lithosphere must be very thick and stiff to support the great height and weight of Tharsis through most of Mars's history: It has been shown that Tharsis must have formed in the Noachian epoch, very early in Martian history. This is evidence, therefore, that Mars's lithosphere was thick and stiff even then, or Tharsis would have collapsed before now. Whatever theory is called on to explain Tharsis, then, has to work on a planet that already has a very thick, stiff lithosphere, and that is probably already a one-plate planet (any plate tectonic activity that had occurred would have had to be over, and the planet was covered with a single-plate shell).
Wrinkle ridges are globally distributed on Mars, but concentric to Tharsis, the huge volcanic center. Researchers believe that the plains formed first, and then the wrinkle ridges. It has been suggested that wrinkle ridges formed from cooling, either long-term cooling or cooling after volcanism. When solid materials cool, they generally contract.When the interior of a planet cools and contracts, the surface
area is then slightly too large to cover the newly contracted interior, and it may then form wrinkles. Wrinkle ridges may also have been formed by warping the surface of the planet by loading it with ice, as in large glaciers, or with volcanic products, like Tharsis. It is currently thought that only the loading of Tharsis provides the amount of strain observed in the number and size of the wrinkle ridges.
The Tharsis region has also created an odd phenomenon called perched or pedestal craters. These craters are normal impact craters, but they are raised from the surrounding surface.The crater bottom is at a higher elevation than the plain the crater lies upon. There is no cratering mechanism that can cause this; cratering commonly creates a crater bottom that is at a lower elevation than the surroundings when the crater is excavated. The elevations of the crater bottoms have been measured and found to decrease smoothly with distance from Tharsis. The current model for perched craters calls on an immense ash fall from Tharsis that blankets all the surrounding plains with their craters. Later, the violent storms of Mars sweep over the plains and scour away all the ash and regolith from around the craters, but the crater bottoms with their loads of ash are relatively protected from the winds by the crater walls.The ash layers in the craters reflect the thickness of the ash fall from Tharsis and its natural decline with distance, while the plains around the craters have been scoured clean, leaving the craters perched above the new, lower plain level.
There is, at this time, no really convincing theory for the formation of Tharsis.There is no analog on any other terrestrial planet. In some theories, a relatively narrow hot upwelling begins at the bottom of the mantle and moves upward very quickly. This is called a mantle plume:The hot material forms a mushroom-shaped head at the top of the plume, and the remaining hot material trails behind like a thin tail.The hot head of the plume can melt as it reaches shallow depths in the planet, and the resulting melt can erupt onto the surface as volca-noes.The Hawaiian island chain is thought to be an example of a mantle plume on Earth: As plate tectonics moves the Pacific ocean plate over the otherwise stationary plume, the volcanoes emerge through the plate in a line. Such a plume may have formed Tharsis on Mars. Special considerations must be made, though, to explain how a plume could rise far enough to melt under what had to have been a thick lithosphere, and more importantly, why there would be only one Tharsis on the planet. If conditions were right to make a giant plume, why not more than one? Scientists are generally displeased with theories that require what is called "special pleading," arguments on why this is an unusual case, or that rely on coincidence.
A second, intriguing theory on the formation of Tharsis involves Hellas, the huge impact crater that is almost perfectly exactly opposite Tharsis on the planet.What a strange coincidence that is, to have the two largest surface features exactly opposed! Scientists at several universities have created large numerical models to investigate how Mars's mantle might move in response to the impact. Though the impact can make strong mantle convection right underneath its center, at the present time there is no model that indicates a large volcano should form on the opposite side of the planet as a result of the impact.
A third theory for the formation of Tharsis emerges from models for early mantle evolution. Work by Marc Parmentier, Sarah Zaranek, and the author at Brown University indicates that if the planet was initially hot enough to have melted and formed a magma ocean, when it subsequently crystallized denser material would form near the surface (see the section above "Was There a Martian Magma Ocean?" on page 41).This dense material would also contain much of the planet's radioactive elements, which do not fit into common mantle minerals and so are concentrated in the last liquids left as the planet crystallized. This dense, radiogenic material would then sink into the planet's interior. Sarah Zaranek, a scientist at Brown University, hypothesizes that this material would continue to heat up through radioactivity as it lay at depth in the planet. Eventually it would form a warm upwelling through the Martian mantle. If the material was warm enough, it might rise to depths shallow enough to melt and perhaps form the Tharsis rise. The warm conduit formed through the mantle by the rising material might persist over time, allowing continued activity on Tharsis.
Mars has four main shield volcanoes, Olympus Mons and the three shield volcanoes on Tharsis discussed above: Arsia, Pavonis, and Ascraeus Mons.Though volcanism is centered on and around Tharsis, Mars has evidence for volcanic activity over much of its surface. In addition to the shield volcanoes Mars has ancient, shallow volcanoes known as patera volcanoes. Apollinaris Patera, one of the largest, has a caldera that is alone 60 miles (100 km) in diameter. Alba Patera, to demonstrate how shallow these volcanoes are, is 1,000 miles (1,600 km) in diameter but only four miles (6 km) high.
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