P34A-01 INVITED
Volcanism at Aramaiti Corona, Venus
Aramaiti Corona (26° S, 82° E) is a 375 km diameter volcano-tectonic structure on Venus that is characterized by 1) an annulus of concentric fractures that crosscut a suite of radial fractures and 2) a >1100 km2 flow field that consists of small (1-20 km diameter) shield edifices, long (>100 km2) lava flows, and three overlapping steep- sided domes. The small shields occur locally within the corona interior, but are predominantly associated with the radial fractures and comprise about 75% of the flow field; timing of the shields is unconstrained. Comprising the rest of the flow field are the long lava flows and domes, which erupted from the concentric fractures, and, as such, we interpret them to represent some of the latest stages of Aramaiti volcanism. The three domes occur at the origin of the longest lava flow suggesting that the flow and domes are part of the same eruptive episode. Analysis of the spatial and temporal relation of the fractures and volcanic features reveals insight into the volcanic processes that have operated at Aramaiti. To illustrate, based on interpretations for the formation of terrestrial shield fields, the large distribution of small shield edifices suggests that volcanism at Aramaiti was dominated by the discontinuous rise of small batches of magma along the radial fractures. Further, based on their association with the concentric fractures, eruption of the late stage long lava flows and domes may be due to concentric fracture formation. To illustrate, if the center of a corona collapses (thus forming the concentric fractures) into an underlying shallow magma chamber, magma would rise through the concentric fractures to the surface. This process would be broadly akin to terrestrial caldera collapse and predicts that the domes may represent a crystal-rich mush that was squeezed from the bottom of a fractionating magma chamber; such a scenario seems to be consistent with previous interpretations of steep-sided dome formation through low effusion rates. Reconnaissance mapping of Bhumidevi (17° S, 343° E) and Zemire (32° N, 312° E) Coronae reveals similar spatial and timing relations between fractures, shields, flows, and domes suggesting that the volcanic processes that occurred at Aramaiti may be common among Venusian coronae.
P34A-02 INVITED
Eruption Conditions of Mare Lavas
The fact that lava flows spread down-slope away from their sources to a very much greater extent than they spread laterally has long been recognized as an indication that they behave as non-Newtonian fluids, having a yield strength as well as a plastic viscosity (and possibly other complications such as strain rate- and time- dependent rheology). However, a significant criticism of the earliest models of lava flow emplacement is that they assumed that all parts of the flow - channel and levees - had the same non-Newtonian rheology. Theoretical expectations that channels and levees should have significantly different rheologies are confirmed by analyses of field measurements on terrestrial flows. Here I revisit morphometric measurements on lava flows on the Moon and used these to deduce volume eruption rates for some mare flows. The Gratz number criterion can be used to determine the volume effusion rate of a cooling-limited flow from its length, width and thickness. In so far as the flow may be supply-volume-limited rather than cooling-limited, this method provides a lower limit on the effusion rate. Dividing the volume flux by an estimate of the length along strike of the active part of the fissure feeding the flow gives a value for the product of the magma rise speed and the width of the feeding dike. Using petrological indications of the magma source depth in the mantle, the pressure gradient driving the magma rise through the dike system can be estimated, and balancing wall friction against driving stress then gives the dike width. The requirement that the magma should not cool excessively while rising from the mantle provides a limit on the permutations of feeder dike width and horizontal length of active fissure that can produce a given flow unit. Some values of these parameters for typical mare lava flows will be presented and their implications for the geometries of the dikes feeding them will be discussed.
P34A-03 INVITED
High-Resolution Imaging of Lava Flow Terrains on Mars by MRO HiRISE
Lava flows account for a significant fraction of the surface of Mars and HiRISE has imaged a diverse selection of these areas at resolutions comparable to "air photos" on Earth. Although some lava are extensively impact "gardened" or mantled, many areas retain primary geomorphologic information at meter-scale. Primary characteristics similar to that of late Cenozoic lava flows in arid areas of Earth are visible in the HiRISE images of even some Hesperian lava plains. The extensive mantling by wind-blown deposits in some of the more youthful volcanic terrains (e.g., Tharsis) may, like terrestrial counterparts, be a result of the excessively rough surface trapping mobilized sand. Evidence for lava flow inflation, including lava-rise plateaus and pits and deformation both vertically and laterally of lava crusts, is documented in areas of flood lavas. Other lava flows are clearly fed from surface channels resulting in dispersive flow surface pattern. Lava flows with hummocky surface textures are comparable to terrestrial flows with auto-brecciated and disturbed surfaces, but the Martian flows are often much larger than typical terrestrial examples. Kilometer-scale areas of puzzle-work plates characterize some flood lava flows. The abundance of plate-like and rubbly deformation styles observed at many localities on Mars is consistent with examples of rubbly pahoehoe seen in situ by the Spirit rover. Rubbly pahoehoe may be a common primary surface texture for many areas of plains-like lava flow emplacement such as the Hesperian lava plains. Sinuous rill-like channels headed at distinct vents and collapse pits suggest significant lava erosion and correspondingly high effusion rates. While the Athabasca Valles channel bed forms appear to be associated with major aqueous outflows, the entire region is draped with lava.
P34A-04 INVITED
Fire and Ice: Lavas on Io, Cryolavas on Titan
Volcanic flows in our solar system are remarkably varied. Io and Titan are particularly good examples of relatively large bodies that have erupted a variety of flows, ranging from basaltic and possibly sulfur and ultramafic lavas on Io to mixtures of water and possibly ammonia and methanol on Titan. These exotic extraterrestrial flows can be much different from the examples we see on Earth, but the similarities are also striking. Understanding their eruption mechanisms is important for better constraining how effusive eruptions behave on Earth under present and past conditions. Io has exceptionally long lava flows, but these are rare compared to the most common form of Ionian volcanism; lava lakes and lava flows that are confined within calderas [Lopes et al., 2004, Icarus; Gregg and Lopes, Icarus, in press]. The largest lava flows on Io can be considered analogues to continental flood basalts on Earth, being hundreds of km long and containing many different flow units. The composition of these flows on Io is thought to be either basaltic or ultramafic. Galileo results showed the largest active flow in the Solar System at Amirani [300 km long; Kezthelyi et al., 2001, JGR 106] and recent observations by the New Horizons spacecraft showed a new flow at Masubi that is about 200 km long. Ionian flows at volcanoes such as Masubi, Maui, and Prometheus generate persistently active plumes and the movement of the Prometheus plume has been related to the growth of the lava flow [Kieffer et al. 2000, Science 288]. Sulfur flows are thought to exist on Io, but are largely a by-product of silicic volcanism. On Earth, sulfur flows are rare but have formed from melting hydrothermal sulfur deposits. Flows around Emakong on Io are thought to be sulfur flows [Williams et al., 2001, JGR 106], but to date there are no measurements that can confirm their composition. Ra Patera's flows at the time of the Voyager encounter was thought to be a site of sulfur volcanism [Pieri et al., 1984, Icarus 60], but the deposits have since been covered over. Cryovolcanism is a process that has no terrestrial analogue but which appears to be widespread in the outer solar system. Features thought to be cryovolcanic have been shown on Titan's surface [Lopes et al., 2007, Icarus 186]. Cryolavas on Titan are thought to be water and ammonia, with possibly some methanol, and to have rheologies similar to those of andesites or more evolved flows on Earth. As we continue our exploration of the solar system, we need to redefine "lava flows" as a geologic process, using a planetary rather than terrestrial context.
P34A-05
Insight Into Lava Sheet Inflation and Deflation Events From the McCartys Flow, New Mexico, and Implications for Planetary Lava Flow Emplacement
Basaltic lavas typically form channels or tubes during flow away from a vent. However, the importance of sheet flow in the development of basaltic terrains has gained attention within the last 15 years. The McCartys lava flow field (NM) is among the youngest (~3000 yrs) basaltic lava flows in the continental United States. It was emplaced over slopes of < 1 degree, displaying features suggested to represent lava sheet inflation and deflation. Therefore, it among the most pristine examples of sheet flow morphologies in the United States. Here we present field observations of this flow field. At the meter scale the interior flow surface typically forms smooth, undulating lobes that appear to represent breakouts from adjacent lobes. These features display grooved surfaces and occasional squeeze-ups along lobe contacts. At the scale of 10s to 100s of meters the flow comprises multiple topographic platforms and depressions. Some depressions display level floors with surfaces as described above, while some are bowl shaped with floors covered in broken lava slabs. The boundaries between platforms and depressions are also typically smooth, grooved surfaces that have been tilted to angles sometimes approaching vertical. The upper margin of these tilted surfaces typically displays large cracks parallel to the boundary, sometimes containing squeeze-ups. The bottom boundary with smooth floored depressions typically shows embayment by younger lavas. The superposition relationships between platforms, depressions, and small lava flows within depressions are complex. It appears that this style of terrain represents the emplacement of an extensive, sheet, likely as one large unit. The sheet experiences inflation episodes within preferred regions, which produce platforms of varied elevations. Inflation events appear to be associated with breakouts of lava which flood the floors of accessible depressions. Depressions are the result of non-inflation, or collapse of an inflated surface as lava is drained from the sheet interior during breakout events. Such an event would comprise an extensive sheet experiencing multiple, possibly contemporaneous, dynamic episodes of inflation and deflation throughout the flow field. The resultant morphology might be analogous to surface textures seen in Tharsis province plains regions adjacent to the Tharsis Montes, Mars.
P34A-06
Lunar lava flows and related eruption styles: Relation to dike emplacement processes
Lava flows are the traditional evidence for volcanism on the Moon, but the nature of volcanic vents can provide important information about the range of eruption conditions and the context in which effusive flows occur. The wide variety of morphologic features representing a range of eruption styles has been documented on the Moon, including steep-sided domes, small shields, cones, crater chains, dark halo craters of internal origin, dark mantle deposits, dark halo rings, linear rille-related deposits, extremely long lava flows, and sinuous rilles and their related deposits. No large Hawaii-like shield volcanoes have been observed. The main path for the ascent and eruption of magma from mantle source regions is through magma-filled cracks or dikes. We assess the relationship between the nature of dike intrusion to shallow depths within the crust and the resulting landforms and deposits. The surface manifestation of a dike that does not actually reach the surface can take a range of forms. If the dike stalls at a sufficiently great depth, there will be some undetectably small amount of surface extension and uplift. If it penetrates to shallower depths there may still be no noticeable topographic effects at the scale of available images, but incipient failure or activation of pre-existing fractures may generate pathways along which gas (probably mainly carbon monoxide) formed by carbon-metal oxide "smelting" reactions in magma in the shallowest parts of the dike can reach the surface. Still shallower penetration will lead to a larger volume of melt being exposed to the relatively low pressure environment near the surface and will encourage the generation of a greater mass of CO since the chemical reaction producing it is pressure-dependent. Subsequent loss of this gas, coupled with a magma volume decrease on cooling, may lead to collapse features (or even explosion craters) forming on the surface above the dike. Very shallow intrusion may lead to further development of a graben and will encourage the formation of small secondary intrusions and possible eruptions; we have developed criteria to distinguish between graben formed by dike emplacement and those resulting from tectonic deformation alone. The shallow stalling of a dike wide enough to allow spontaneous convection to occur during the early stages of its cooling can expose so much magma to low pressure degassing that it leads to major gas buildup and propagation of a crack to the surface, resulting in an Io-like eruption plume and the formation of a dark ring deposit (such as the 154 km diameter feature surrounding a vent in southern Orientale). Also assessed is the deep generation of magmatic gas on the Moon at dike crack tips and the implications for pyroclastic eruptions and pyroclastic glass provenance. Models of the ascent of magma feeding steep-sided domes suggest high-viscosity magma, consistent with their distinctive shapes. Together, the relationship between the vent characteristics and models of the behavior of dikes provide important insight into the generation, ascent and eruption of magma on the Moon, and the role of lava flows. New data scheduled to be acquired in the next several years will provide important advances in these studies.
P34A-07
Flow Fields at Tooting Crater, Mars
HiRISE images of the impact crater Tooting (~29 km dia., located at 23.4oN, 207.5oE) on Mars have revealed a remarkable series of lobate flows on the southern rim, wall and floor of the crater. The origin of these flows has not yet been determined, but their spatial distribution and morphology could indicate that they are flows of impact melt, mudflows, or lava flows. Tooting crater shows numerous signs of being very young (very few superposed impact craters, very high depth/diameter ratio, high thermal inertia ejecta, and a well preserved set of secondary craters), and so allows detailed analysis of these unusual flows, which appear to be almost pristine. We have developed a 2-meter digital elevation model of Tooting using stereo HiRISE images to characterize the flows, which in general are <10 m thick. Four distinct flow fields have been identified: (1) an extensive flow field on the S rim that appears to be solidified melt sheet ~2.5 km x 1.7 km in size that has four 200 to 600 m long flows with festoon ridges on their surface. (2) A single lobate flow on the SW rim that originates from a smooth "catchment area" of low relief close to the crater rim crest. Five discrete segments of this flow exist, including a 1.3 km segment with a discrete 15 m wide central channel and three lobate distal margins. (3) A set of 7 lobes ~700 m long on the inner S wall. These lobes have very well defined central channels ~25 m wide and levees <4 m wide. (4) A lobe complex on S floor that includes lobes >30 m thick and 300 m wide. These flows no doubt formed in an unusual environment, probably including extensive amounts of impact melt, volatiles released from the substrate, and highly unstable slopes on the crater rim. Tooting crater therefore displays a novel planetary flow field; the correct identification of the origin of these flows holds significance for understanding the role of volatiles in the impact cratering process, the potential of thermal anomalies existing within the crater cavity for extended period of time, and the emplacement of the ejecta. We are therefore developing numerical models, based on the rheology of lava flows, in order to help to resolve the origin of this flow field.
P34A-08
Lava Flow Emplacement at Pillan, Io in 1997: Implications for Massive Basaltic Flow Emplacement on Earth and Mars.
Only two bodies in the Solar System currently exhibit high-temperature volcanic activity: the Earth, and the Jovian satellite, Io. Massive flow emplacement has played a significant role in the volcanic history of both Earth and Mars. The largest lava flows on Earth by volume in historic times were emplaced at Laki, Iceland, in 1783-84. These flows were dwarfed by the flows emplaced at Pillan, Io, in 1997, which may be the largest effusive eruption ever witnessed. At least 31 km3 of lava were erupted in ~100 days [1] with an additional 25 km3 erupted shortly thereafter, yielding a total of 56 km3 [2]. In addition to the lava flows, a pyroclastic deposit of unknown thickness covering over 125,000 km2 was laid down. This eruption is particularly important as it sheds light on the emplacement of very large, voluminous flows that were emplaced millions of years ago on Earth (flood basalts) and also on Mars. Temporal and areal constraints allow application of models of varying effusion rate [3] to determine peak effusion rate, and also the varying rate of surface coverage. From this information, thermal emission profiles have been constructed that show that the thermal emission measured by the Galileo Near Infrared Mapping Spectrometer (NIMS) [e.g., 4] can be produced by insulated flows, implying that turbulent flow emplacement was probably not the dominant emplacement regime. Peak effusion rates at Pillan most likely exceeded 104 m3 s-1. The resulting insulating crust, with a small crack fraction, appears to be similar to that on terrestrial flood basalts that formed inflated sheet flows with a 'rubbly' surface [5]. Our new results from analysis of the Pillan data lend additional quantitative support to the idea that terrestrial rubbly pahoehoe flows and Martian "platy-ridged" flows are diagnostic of similar high eruption rates [5]. Additionally, NIMS thermal emission measurements can be reproduced with a basaltic, rather than ultramafic, magma composition. We note that even more extensive flow fields are seen on Io. Lei Kung Fluctus has an area of 1.25 x 105 km2. This work was carried out at the Jet Propulsion Laboratory-California Institute of Technology, under contract to NASA. AGD is supported by a grant from the NASA PG&G Program. References: [1] Williams, D.A. et al. (2001) JGR, 106, E12, 33,105-33,119. [2] Davies, A. G. et al. (2006) LPSC 37, abstract 1155. [3] Wadge, G. (1981) JVGR, 11, 139-168. [4] Davies, A. G. et al. (2001) JGR, 106, E12, 33079-33104. [5] Keszthelyi, L. et al. (2004) G3, 5, 2004GC000758.