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Volcanism on Mars ri VOLCANISM ON MARS JAMES R. ZIMBELMAN Center fer Earth aiiä PUmttary Studies, Smithsonian Institutimt I. Introduction Fossae Descriptor applied to an aligned series of fractures I. Background in a planetary surface. III. Large Central Volcanoes Hesperian The intennediate geologic epoch on Mars iden- IV. Paterae and Tholi tified through geologic mapping of superposition rela- V. Highland Paterae tions, and the areal density of impact craters. VI, Small Constructs Icelandite Volcanic rock with silica content comparable to VII. Volcanic Plains an andésite, but which did not form as a result of subduc- VIII Compositional Constraints tion or orogenic processes, IX. Volcanic History of Mars Mons Descriptor applied to a large isolated mountain on X. Future Studies a planetary surface, Noachian The oldest of the geologic epochs on Mars iden- tified through geologic mapping of superposition rela- tions, and the areal density of impact craters. Patera Descriptor applied to an irregular or complex crater GLOSSARY with scalloped edges, surrounded by shallow flank slopes. pseudocrater A "rootless vent" created by the interaction Amazonian The youngest of the geologic epochs on Mars of lava with groundwater or wet sediments beneath identified through geoiogic mapping of superposition the flow. relations, and the area! density of impact craters. shield volcano A broad volcanic construct consisting of caldera An irregular collapse feature formed over the evac- multitudes of lava flows. Flank slopes are typically •5°, uated magma chamber within a volcano. or less than half as steep as the flanks on a composite central volcano Emplacement of volcanic materials from a volcano, centralized source vent rather than along a distributed SNC meteorites .A group of meteorites thought to originate line of vents, on Mars, due to a relatively young age and trapped composite volcano A volcano consisting of intermixed effu- gases very like the atmosphere of Mars. The acronym sive lava flows and pyroclastic materials. Flank slopes is derived from the names of the three meteorites that are tyjjically >10'', or more than twice as steep as the define the major subdivisions identified within the group: flanks of a shield volcano. S, Shergotty; N, Nakhta; C, Chassigny. Copyright © 200C by .\cüdem!C Press Encycloptdia of Vüktmots 771 All rights of reprixliittion in any form restrveii. 772 VOLCANISM ON MARS Tholus Descriptor applied to an isolated domical smali body in the solar system, but this activity is powered mountain or hill, usually with slopes much steeper than by unique gravitational tidal forces within the Jovian that of a patera. system; see "Volcanism on lo"). Next we review some volcanic plains Planar mappable units interpreted to consist important background information before examining of volcanic materials, usually with individual flow mar- the types of volcanic features present on Mars. gins resolvable within the unit. yardang A rounded erosional landforiu produced by wind- driven sand. Some yardang fields on Alars are interpreted to be formed in wind-eroded pyroclastic deposits. 11. BACKGROUND The first direct evidence of the importance of volcanism on Mars came during the Mariner 9 orbital mission. A I. INTRODUCTION massive global dust stonn ob.scured the entire Martian surface when Mariner 9 arrived at Mars on November The advent of spacecraft exploration revealed that Mars 14, 1971, but as the weeks passed, the dust slowly began possesses some of the most dramatic volcanic features to settle. The first surface features at equatorial latitudes found any\\'here in the solar system. How did a planet to become visible through the dust pall were four dark half the size of the Earth generate volcanoes like Olym- "spots" in the region of the planet that telescopic ob- pus Mons, which is several times larger than the largest servers called Tharsis. Gradually these spots were re- volcanoes on Earth? This question is just one example vealed to each be topped by a complex crater assemblage, of the issues currently being investigated as part of the the first clue that four huge volcanoes were present in relatively recent scientific endeavor called "Comparative the Tharsis area. By chance, the three previous fly-by Planetology." This article will summarize the basic in- Manner missions failed to image this area in detail suffi- formation presently known about volcanism on Mars. cient to show these large volcanoes. Mariner 9 continued The reader can then compare the Martian volcanoes to to map the entire Martian surface until October 12, the numerous volcanoes studied in great detail here on 1972, revealingthatTharsis was not alone in containing the Earth as well as to volcanoes observed on other impressive volcanic constructs. The Mariner 9 view of planetary bodies. Mars was updated significantly with the improved reso- Two generalizations can be made before we investi- lution of the cameras on the two Viking Orbiter space- gate the Martian volcanoes in greater detail. Volcanoes craft that operated at Mars from June 19,1976, to August on Mars appear to be broadly similar in overall morphol- 17, 1980. The -55,000 Viking Orbiter images led to ogy {although different in scale) to volcanic features on refined interpretations for many Martian features, but Earth, so there is no direct evidence for Martian eruptive they did not significantly alter the first global perspective processes that are somehow significantly different from of Mars revealed by the Mariner 9 ¡mages. the volcanic styles and processes known on Earth. This The distribution of volcanoes on Mars is not uniform; inference strengthens our confidence in using the basic there are four regions where central volcanoes are con- approach of attempting to understand the Martian vol- centrated (Fig, 1). Twenty-three volcanic constructs canoes in terms of traditional volcanology. Next, the have been given names (Table I) by the International Martian volcanoes occur on terrains of various relative Astronomical Union, the only organization that can of- ages (discussed below), so that volcanism represents a ficially designate names for planetary features. The dominant geologic process throughout much of Martian largest concentration of Martian volcanoes occurs in the history. This is in contrast to volcanism on the smaller Tharsis region, where the volcanoes are distributed on body of Earth's Moon, where volcanic activity was con- and around a 40i)(}-km-diameter bulge in the Martian fined to roughly the first third of lunar history (see crust, centered on theet]uatorat 11Ü°W longitude. This "Volcanism on the Moon"). Comparative planetologj' bulge has lifted the crust to an elevation •10 km above thus supports the concept that volcanism is the primary the Martian datum, twice as high as the highest portions mechanism for a planetary body getting rid of its internal of the cratered highlands that dominate the southern heat, where smaller bodies tend to lose their internal hemisphere of Mars (shaded portion of Fig, 1), The heat more rapidly than larger bodies (Jupiter's moon lo "Tharsis Bulge" is surmounted by three volcanoes col- appears to contradict this trend because this object, the lectively called the Tharsis Montes (5, 6, and 16 in size of Earth's Moon, is the most volcanically active Fig. 1), aligned along a N40E trend that includes addi- VOLCANISM ON MARS 773 FIGURE Í Location map for named central volcanoes on Mars, The black areas Indicate various volcanic centers, v^ith numbers keyed to Table I. The shaded area represents densely cratered materials typical of the highlands that dominate the southern hemisphere. Letters indicate landing sites (VI, Viking I ; V2, Viking 2, P, Mars Pathfinder) and the Medusae Fossae Formation (MFF). Mercator projection. tional volcanic centers northeast of the Tharsis Montes. late in Martian history, hut ages for documented samples The Tharsis Montes and nearby Olympus Mons (15 in collected from regionally expansive terrains are needed Fig, I) are the four tallest features on Mars, the summit to constrain the precise time range covered by the crater- calderas of which were first imaged by Mariner 9 above ing record on Mars. In spite of this uncertainty in abso- the dusty lower atmosphere. Outside of the Tharsis area, lute age, the geologic stratigraphy provides a robust Martian central volcanoes are confined to the Elysium, framework in which the relative ages of various materi- Syrtis Major, and Hellas regions (Fig. 1), all named for als, including both central volcanoes and regional volca- broad bright or dark surface markings visible with Earth- nic plains, can be evaluated. based telescopes. In addition to the named central volca- Remote-sensing data provide information about Mar- noes, broad expanses of regional plains exist across Mars, tian materials that is complementary to the photogeo- many of which include compelling evidence that they logic interpretation of spacecraft images. Reflected were formed by the emplacement of numerous individ- visual and infrared light provides compositional infor- ual volcanic flows. mation by the wavelengths at which some of the light The Mariner 9 and Viking images showed the physio- is selectively absorbed. Earth-based telescopic studies graphic makeup of the entire surfaceof Mars. Through can not resolve individual volcanoes but reflectance careful observation of the superposition and cross-cut- spectra indicate that bright regions are covered with a ting relationships of distinctive material units, a global dust
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