Much of the understanding of Martian volcanoes has been derived from photogeological studies of the surface. In addition, many of the plains portrayed here are believed to be of volcanic origin. It is constructed from countless individual flows erupted from the summit area and flanks of the volcano.
The mineralogy of the upper mantle is probably ichercolithic, consisting of olivine, pyroxene, and minor amounts of aluminum (Longhi et al., 1992). Many of the diagnostic spectral features of the classes are explained by differences in the abundance of iron (high red/blue ratio) and iron (low red/blue ratio) components. In situ measurements of the elemental composition of Martian surface materials were made by the Viking X-ray Fluorescence Spectrometer (XRF) and the Pathfrnder/Sojourner Alpha Proton X-ray Spectrometer (APXS).
LARGE SHIELD VOLCANOES 1. Tharsis Region
Due to the extreme temperature dependence of lava rheology, this may allow Martian lava to remain mobile longer. The flanks of the Martian volcanoes have maximum slopes of 5° with shallow top and basal slopes, similar to the subaerial part of the Mauna Loa volcano. Many of the flows have medial lava channels and partially collapsed lava tubes similar to those on terrestrial volcanoes (Greeley, 1973; Oarr and Greeley, 1980).
Some Elysium volcanoes, such as Hecates Tholus, have strongly sheared flanks (Mouginis-Mark et al. Gulick and Baker, 1990). It is also evident that the valley formation events preceded the eruption of the surrounding lava plains because the valleys are cut by the plains (Figure 4.10). However, many valleys form midway on flanks (Mouginis-Mark et al., 1982) and have branches characteristic of surface runoff (Gulick and Baker, 1990).
The summit area at Hecates Tholus has one of the best candidate ash deposits identified on Mars. The calderas on top of Olympus Mons and Arsia Mons are representative of the morphological range of Martian calderas (Grumpier et al., 1996a). Alba Patera, north of the three Tharsis shield volcanoes, is unique on Mars (Carr et al, 1977).
It is larger in area than Olympus Mons, but lacks the relief of the shield volcanoes. This may indicate local topography or large-scale changes to the magma plumbing system of the volcano during late-stage activity. The valleys may be fluvial features formed in unconsolidated pyroclastic deposits produced during explosive eruptions of the volcanoes.
The environment may have played a role in the development of the basal units in several Martian volcanoes.
MARTIAN HIGHLAND PATERAE
Each mechanism requires low shear strength materials to facilitate sliding, and no equivalent sliders have been identified on Earth. In a model for shallow seas on Mars (Baker et ai, 1991), the entire northwest side of Mount Olympus would be submerged, facilitating the formation of submarine landslides, suggesting that a volcano could form from the collapse of submerged basalt materials. Although the lower flanks of Tharsis Tholus are buried by younger flows, remains of a structure can be found northeast of the volcano, indicating that segments of the flanks have moved horizontally.
A large, curved-shaped wrinkled ridge and small dome-shaped features in the eastern part of the caldera, together with the carved rim suggest a complex history. The flanks of the volcano are asymmetric in plan view (—300 x 560 km) and show remnant layers and mesa. The erosional morphology of the channelized flanks of Hadriaca and Tyrrhena Paterae is attributed to a combination of groundwater flow and surface flow, with more extensive surface dissection evident at Hadriaca Patera in the form of V-shaped valley interiors (Gulick and Baker, 1990 ; Crown and Greeley, 1993).
This complex may be the source of the ridge plains of Malea Planum (Greeley and Guest, 1987; Tanaka and Scott, . 1987). A potentially similar MOO km feature found in the Thaumasia region was interpreted as a volcano by Scott and Tanaka (1981) and Scott (1982) but lacks the distinct central vent characteristics of other highland pateras. C) View of a lava flow lobe within the lateral flow southwest of the summit of Tyrrhena Patera. The upper, more irregular part of the flow lobe has a channel in its interior that appears to have fed a wider, ~60 km long, unchanneled area.
The use of the paterae in comparisons with other volcanoes or for inferences about the environment depends on interpretations of their eruptive style. The western rim (left) of the caldera indicates multiple episodes of collapse, while the rim to the north and east may be covered by eruptive products.
SMALL VOLCANIC CONSTRUCTS
If early suggestions that the paterae are composed of liquid lava prove true, lava flows associated with the paterae could indicate significantly different thermal environments, magma bodies, emplacement conditions, or erosion regimes than are typical of the younger volcanoes on the Martian surface.
VOLCANIC PLAINS
However, image resolution and quality are insufficient to map most lowland plains consistently; these plains are probably composed of materials of many different origins, including volcanic. Plains of probable volcanic origin make up more of the Martian surface than any other terrain (Greeley and Spudis, 1981; Greeley and Schneid, 1991). Although they appear to have attained their greatest areal extent in the early Hesperian (Tanaka et al., 1988; Greeley and Schneid, 1919), this is likely an artifact of preservation.
For example, all the northern lowlands are of possible volcanic origin, but burial and subsequent reworking with sediments may have obscured their volcanic origin. Ridge fields that may consist of lava flows are abundant around many volcanic centers, such as Hesperia Planum near Tyrrhena Patera (Figure 4.15). The morphologies of some lava field flows may have been influenced by local changes in the Martian environment.
Each outbreak of a new flow segment appears to have occurred at the distal end of the previous lobe. There is no evidence for lava tubes, covered channels, or lobes emerging from the sides of previous lobes, so an (unknown) limit imposed by the properties of the flow prevented their lateral growth. However, the image resolution for these flows is ~80 m pixel-1 and direct comparison with those in the Elysium Planitia field is not possible.
For example, flat pre-flow terrain, abundant permafrost, or flow pulsation could result in segmented flows (Mouginis-Mark and Tatsumura-Yoshioka, 1998). Many grabens have runoff deposits indicative of surface water flow (Mouginis-Mark, 1985) and the Chaos Zone north of the lava flows appears to have been caused by collapse following the removal of ground ice (Carr and Schaber, 1977), possibly leading to deposition lahar (Christiansen and Greeley, 1981; Christiansen, 1989).
CONCLUSION
Probable Noachian-age volcanic fields, showing the "flooded" appearance of impact craters and mare-type wrinkle ridges typical of volcanic fields on the Moon. Although Viking Orbiter image resolution varies across planets, it appears that segmented lava flows are rare on Mars and may indicate eruptions and/or unusual emplacement conditions. Do the changes in volcanic morphology and inferred styles of volcanism for the highland pateras reflect a change in the Martian environment in which surface or near-surface instabilities are depleted (Greeley and Spudis, 1981), or do the changes represent magma evolution or other factors.
Do channels in some of the volcanoes, such as Hecates Tholus (Mouginis-Mark et al., 1982), represent local climatic regimes that allow precipitation; Alternatively, could the channels represent a previously unrecognized form of volcanism or flow emplacement. As discussed by Greeley and Spudis (1981), mare-type ridges are commonly used to infer marne-like basalts on planetary surfaces with lunar analogs, but the criterion for the presence of such ridges is far from definitive. Most researchers attribute the differences between volcanic features to the interaction of magma with near-surface volatiles or other eruption parameters (Mouginis-Mark et al., 1982; Crown and Greeley, 1993; Robinson et al., 1993; Wilson and Head , 1999).
Thermal emission spectrometer data (Christensen et al., 1992) from the Mars Global Surveyor, other remote sensing data, and measurements by landers could resolve the question of the chemical diversity of Martian volcanic rocks. Similarly, caldera summit volumes and building heights can also be erroneous, with important implications for magma chamber volume, the size of the caldera-forming event (Zuber and Mouginis-Mark, 1992), and the dynamics that triggered the eruption (Wilson and Head, 1994). The International Space Exploration Program focuses on Mars in the late 1990s and into the next century.
Approved and planned missions have the potential to return a wealth of data about the history of Mars and its evolution through time. Combined with missions that will return a series of samples from Mars, laboratory experiments, theoretical considerations and study of relevant terrestrial analogues should ensure that the next decade will provide a significant improvement in our understanding of Mars, its volcanic history and the role of the environment in the control of volcanic processes.
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