Lunar and Planetary Science XXIX 1127.pdf

THARSIS MONTES AS COMPOSITE VOLCANOES?: 1. THE ROLE OF EXPLOSIVE IN EDIFICE CONSTRUCTION AND IMPLICATIONS FOR THE VOLATILE CONTENTS OF EDIFICE-FORMING . James W. Head1 and Lionel Wilson1,2. 1Dept. of Geological Sciences, Brown University, Providence, RI 02912, 2Environmental Science Dept., Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, U.K. [email protected], [email protected].

Introduction and background: Discovery of the Tharsis hawaiian cones should have diameters that are about a factor Montes during Mariner 9 [1] led to their detailed characteri- of two larger and heights that are correspondingly about a zation during Viking [2-5] and comparison to shield volca- factor of four smaller than on Earth; central craters in these noes on Earth [6,7], primarily because of their shapes, abun- edifices should also be broader than on Earth, by a factor of dant flows, distinctive shield-like complexes, up to at least five [9]. Most of the erupted pyroclasts will be and apparent distinctiveness from other edifices interpreted to incorporated into these near-vent edifices and into lava ponds represent pyroclastic eruptions [6-8]. In this contribution, around vents feeding flows; however, a few percent of the we discuss the theoretical basis for pyroclastic volcanism on volume of the edifice (a factor of about 2 to 3 more than on Mars [9] and show that hawaiian and even plinian eruptions Earth) will be emplaced as relatively widely dispersed pyro- should be very common, even in eruptions of basaltic compo- clastic deposits from this type of activity. sition. We conclude that there is a strong theoretical basis to Plinian eruptions: Because of the high degree of consider reinterpretation of the Tharsis Montes as composite fragmentation induced by the low martian atmos- volcanoes. Elsewhere we assess specific lines of evidence pheric pressure, only a small amount of additional gas is that hawaiian and plinian eruption products derived from needed in a martian magma (e.g. 0.2 % versus 0.15 %, both Tharsis Montes summit and flank eruptions have contributed common for terrestrial magmas), to cause a plinian-style, to the formation of the Tharsis Montes edifices [10] and far- rather than hawaiian-style, eruption to occur, thus leading to field deposits [11]. very wide dispersal of the pyroclasts, on and adjacent to the Definitions: A shield is defined as 'a broad, gen- volcano. Basaltic plinian eruptions, rare on Earth, should tly sloping volcanic cone of flat domical shape...built chiefly therefore be relatively common on Mars [9]. The production of overlapping and interfingering basaltic lava flows. Typi- of large-scale plinian deposits need not signal the presence of cal examples are the volcanoes Mauna Loa and Kilauea on more silicic compositions, but rather may be linked to the the island of Hawaii.' [12]. Alternatively, a composite vol- enhanced fragmentation of basaltic magma in the martian cano or stratovolvano is 'a volcano that is constructed of al- environment [9], or the interaction of basaltic magma with ternating layers of lava and pyroclastic deposits....viscous, ground water [8]. Martian air-fall deposits may be recog- acidic lava may flow from fissures radiating from a central nized as areas of mantled topography with widths ranging vent, from which pyroclasts are ejected.' In terrestrial volca- from several tens to a few hundred km. The thorough magma noes, composite volcanoes are commonly inferred to have fragmentation means that martian plinian deposits of any more 'acidic' compositions, while shield volcanoes are domi- magma composition will be systematically finer-grained than nated by ; the implied viscosity and volatile content those on Earth by a factor of about 100, thus being almost (both inferred to be higher in magmas on entirely sub-cm in size. formation is clearly Earth) are viewed to be an important factor in the increased inherently more likely to occur on Mars than on Earth, since explosivity of the magma on Earth [13]. eruption cloud instability occurs at a lower mass eruption rate Theoretical analysis: Mars atmospheric pressure causes for a given magma volatile content [9]. For a given initial enhanced explosive volcanism. We have investigated the magma volatile content, eruption speeds are a factor of at theory of the ascent and eruption of magma under martian least 1.5 higher on Mars, and so the fountains feeding pyro- conditions [9] and here we assess the implications of these clastic flows will be more than twice as high as on Earth. conditions for the formation of edifices and deposits. The Pyroclastic flow travel distances may be a factor of about combination of lower gravity and lower atmospheric pressure three greater, leading to values up to at least a few hundred on Mars ensures that both nucleation of volatiles and subse- km [9]. quent disruption of magma occur at systematically greater The exact gas content marking the transition between the depths than on Earth. In addition, for a given set of condi- plinian and hawaiian eruption styles depends on the volume tions, pyroclastic eruptions should be more common with eruption rate and the atmospheric pressure (and hence vent increasing altitude due to the decrease in atmospheric pres- altitude). The observed lengths of martian lava flows (~100 sure. to ~1000 km) imply magma eruption rates in the range 100 to Hawaiian eruptions: Atmospheric pressure throughout 1000 m3 s -1 [9; Fig 8] which correspond to mass fluxes in the the entire altitude range of the Tharsis Montes is sufficiently range 3 x 10 5 to 3 x 106 kg s-1. At the 600 MPa Mars surface low to cause any magma with a total volatile content of more reference pressure, these mass fluxes would lead to plinian than about 0.03 wt% (very low by terrestrial standards) to be eruption plumes (rather than to hawaiian lava fountains disrupted into pyroclasts to produce hawaiian-style explosive feeding lava ponds surrounded by spatter cones, the ponds in eruptions. Grain sizes in martian hawaiian edifices should be turn feeding flows) if the magma gas contents exceeded the at least an order of magnitude finer than in terrestrial equiva- minimum values given in lines 1 and 3 below [9; Fig. 18]. At lents because of enhanced magma fragmentation. Martian ~20 km elevation, where the atmospheric pressure is ~10 Lunar and Planetary Science XXIX 1127.pdf

THARSIS MONTES COMPOSITE VOLCANOES?: 1: J. W. Head and L. W. Wilson

times less, the minimum gas contents needed to ensure of the Tharsis Montes, then the implication is that the mag- plinian activity are given in lines 2 and 4, in each case a fac- mas producing an edifice had low volatile contents: <~0.2 wt tor to 2 to 3 times less than at the lower elevation. Thus, % in the early stages (when the edifice was at or below the whatever the actual magma volatile contents, the trend is surface reference pressure) and even lower in the later stages towards being more likely to have plinian eruptions at higher as the volcano built to higher elevations and lower atmos- elevations. Given that the actual magma volatile contents are pheric pressures. indeed likely (by Earth analogy at least) to lie within the The distinctive veneer of lava flows on the surfaces and ranges given below, this makes it entirely plausible that mar- flanks of Tharsis Montes [1,2,4,5], and the evidence for effu- tian shield volcanoes do indeed show this trend towards sive flows contributing to their construction throughout their having a larger proportion of plinian eruptions as their sum- histories [14], means that the lava flow eruptions of the Thar- mit elevations increase. sis Montes are characterized by magma volatile contents of <0.2 wt %, similar to those of recent Kilauea eruptions [15]. Erupted magma mass flux in kg s-1: 3 x 105 3 x 106 Evidence has been cited for early pyroclastic eruption depos- its in the construction of Olympus Mons [16] and Alba Patera 1. minimum CO2 content at 600Pa level: 0.2 wt% 1.0 wt % 2. minimum CO content at 60Pa level: 0.1 wt% 0.3 wt% [17], as well as dominating in the early volcanic history of 2 Mars [18], and we hypothesize that it has also occurred dur- 3. minimum H2O content at 600Pa level: 0.15 wt% 0.4 wt% 4. minimum H2O content at 60Pa level: 0.07 wt% 0.15 wt% ing the formation of the Tharsis Montes. We are investigat- ing several factors that might be responsible for the change in Summary and conclusions: On the basis of theoretical eruption style with time, including: 1) changes in the compo- treatments of the ascent and eruption of magmas on Mars [9], sition and/or volatile content of mantle derived magmas, 2) 1) hawaiian-style volcanic eruptions should be very common changes in the contamination of ascending magmas by crustal and should produce more widespread and finer-grained frag- rocks or volatiles (e.g., groundwater), and 3) changes in the mental deposits than their terrestrial equivalents, 2) plinian- behavior of the evolving magma resevoir. style (including basaltic plinian) eruptions should be very References: 1. M. Carr, JGR, 78, 4049, 1973. 2. M. Carr et al., common, 3) plinian-style eruption products should be very JGR, 82, 3895, 1977. 3. L. Crumpler and J. Aubele, Icarus, 34, 496, fine grained and very widely dispersed as flow and fallout 1978. 4. J. Zimbelman, NASA TM-88784, 271, 1984. 5. J. Zimbel- deposits, 4) the presence of abundant plinian-style deposits man and K. Edgett, PLPS 22, 31, 1992. 6. R. Pike, PLPS 9, 3239, would suggest magma volatile contents in excess of 0.2 wt % 1978. 7. R. Pike, USGS PP 1046-C, 1980. 8. R. Greeley and D. at mean elevations, and 5) plinian-style eruptions are in- Crown, JGR, 95, 7133, 1990. 9. L. Wilson and J. Head, Rev. Geo- phys., 32, 3, 1994. 10. J. Head and L. Wilson, LPSC 29, Tharsis creasingly favored at higher altitudes. On the basis of these Montes as composite volcanoes?: 3. Lines of evidence for explosive considerations, we propose that the Tharsis Montes may have volcanism in edifice construction, this volume. 11. L. Wilson, J. had a significant component of explosive volcanism during Head, and K. Mitchell, LPSC 29, Tharsis Montes as composite vol- their history of eruption and growth and that they may actu- canoes?: 2. Lines of evidence for explosive volcanism in far-field deposits, this volume. 12. R. Bates and J. Jackson, Dictionary of ally be composite volcanoes rather than shield volcanoes. Geological Terms, AGI, 1984. 13. R. Cas and J. Wright, Volcanic Elsewhere we analyze possible lines of evidence for Tharsis successions, Allen and Unwin, 1987. 14. G. Schaber et al., PLPSC 9, Montes as composite volcanoes for edifice-related activity 3433, 1978. 15. T. Gerlach, JGR, 91, 12177, 1986. 16. J. King and J. [10] and for far-field deposits (more distal than the topo- Riehle, Icarus, 23, 300, 1974. 17. P. Mouginia Mark et al., Bull. graphic base of the volcano) [11]. If pyroclastic activity can Volc., 50, 361, 1988. 18. R. Greeley and P. Spudis, Rev. Geophys. Space Phys., 19, 13, 1981. be shown NOT to be relatively common in the construction