Reykjavik - June 5, 2001
Introduction: The Tharsis Montes on Mars (Figure 1), i.e. Arsia Mons, Pavonis Mons and Ascraeus Mons, are gigantic volcano triplets of a youthful age with evidence of recent volcanism (Hartmann et al., 1999). In the search for lost water on this planet the areas of most recent volcanic activity are probably most likely to reveal the still existing sources of water ice. These volcanoes demonstrate numerous morphological and volcanic features that indicate style of eruption and erosion. Collectively, these features suggest a close relationship between volcanism and age based on counts of young pristine impact craters reported by Hartmann and others (1999). They reported a very young age, or 40-100 million years, of lavas within the Arsia Mons caldera. Inspection of impact crater morphology elsewhere, e.g. at Pavonis Mons, through MOC images shows that small impact craters there have been heavily eroded with only crater remnants present. The present author argues that the erosion of these craters postdates the young lava age within the caldera floor of Arsia Mons. The mechanism of erosion is here believed to be of volcano-fluvial origin. The origin of the fluvial component in this case is believed to be massive ice within the caldera as well as on the volcano flanks (Helgason, 1999). Interaction between magma and ice will have produced a high-density fluid mixture that burst out on the volcano lower flanks. Common with the Tharsis Montes are morphological features, which are thought to be caused by volcanogenic erosion. It is therefore concluded that the heavily eroded impact craters most likely postdate the youngest lava flows dated so far, i.e. within the Arsia Mons caldera. The erosion activity responsible for the large scale morphology, i.e. a network of valleys, is therefore a strong indicator for still younger volcanic activity within in the Tharsis Montes.
Method of approach: In the present study the focus is on relative age of different morphological features, such as ring- and depression-shaped collapse structures and erosional valley systems, and how these provide clues to recent interaction of volcanism and ice within the Tharsis Montes. Attention is given to the relationship between loci of such morphological features and sites where, based on volcano-tectonic relations, eruption of magma is most likely to have occurred. Hartmann et al., 1999 provided relative ages for different segments of Arsia Mons based on crater counts. In the present study, crater morphology and their state of erosion, combined with regional crater count age dating, is used to distinguish age relationships. Basic assumptions in this study are a) that lava surfaces should be visible in the neighborhood of recent volcanism, b) impact craters are stable features and only in the presence of catastrophic erosional processes can carter remnants of the type found at the Pavonis Mons NE-side have formed.
Krafla volcano, northeast Iceland: The Krafla of northeast Iceland is located within a volcanically active rift zone on a plate boundary and is here regarded a analog for Tharsis Montes volcanism (Björnsson et al., 1977). Through the Krafla volcano center runs a fissure swarm made up of numerous near parallel fissures. The swarm extends up to 80 km north and south of the caldera. Beneath the caldera roof is a magma chamber, at some 3-7 km depth, into which magma accumulates from a greater depth. The magma accumulation exerts pressure on the caldera roof that is elevated. Monitoring of the chamber surface provides information on magma accumulation. Eventually, as the magma pressure exceeds the lithostatic pressure the fissure swarm fails and opens up. Then magma hydrofractures the crust and is fed laterally into the fissures, up to 80 km away from the caldera rim. Outside the caldera magma may or may not reach to the surface, depending on the supply and magma pressure. This volcano-tectonic situation is regarded comparable to volcanism within the Tharsis Montes on Mars. The valley systems on the northeast and southwest flanks of the Tharsis Montes are equivalent to the Krafla fissure swarm and are thus the locus of most intense volcanism. Paradoxically, no lavas are seen within the valleys, only sediments. On the other hand lava surfaces are common within the calderas and near the concentric caldera faults. What causes no lavas to be found in the valley systems of the Tharsis Montes?
Location of lava surfaces vs. most plausible eruptive sites: At the end of Viking era imaging had established the distribution of lava fields on Mars. This was achieved locally to the point of differentiating between new and old lavas, flow types and degree of sedimentary cover. With high resolution MOLA measurements detailed numerical data on morphological features became available. Within the Tharsis Montes lava flows were widely detected, such as within the caldera on Arsia Mons and on the concentric caldera rims. The high-resolution MOC images are presently contributing vital images from within the valley networks. Paradoxically, they show no lavas there although earlier studies had reached that conclusion.
Tharsis Montes erosional valley systems: Each of the Tharsis Montes has a caldera and a rift segment extending from it respectively on the southwest and northeast side (figure 2). These volcanoes have a total of six segments that represent a distinct evolutionary stage with regard to rifting, development of erosion and degree of volcanism. Although a high degree of variability exists between the six segments they all share three basic features, i.e. circular to elongated depressions, smooth valley floor without lava surfaces and an extensive fan area down-dip in front of the valley system. The valley systems have been regarded as lava channels. Thus, for the valley system on the southwest side of Ascraeus Mons, Crumpler et al., (1996) state: "pits, pit chanis, and flank channels interpreted as flank vents and lava channels." The present author is in agreement with the channels and ring-shaped features being the locus of volcanic extrusions. However, for the valleys to be lava channels surfaces of lavas need to be observed. This, however, is not the case and in the present paper an explanation is offered for this morphological discrepancy.
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