P52A-01 INVITED
Transfer Rates of Magma From Planetary Mantles to the Surface.
We discuss the speed at which magma can be transferred to a planetary surface from the deep interior. Current literature describes a combination of slow percolation of melt in the mantle where convection-driven pressure-release melting is occurring, concentration of melt by source region deformation, initiation and growth of magma-filled brittle fractures (dikes) providing wider pathways for melt movement, additional growth and interconnection of dikes with decreasing depth, rise of magma to storage zones (reservoirs) located at levels of neutral buoyancy at the base of or within the crust, and transfer from the storage zones in dikes to feed eruptions or intrusions. We do not take issue with these mechanisms but think that their relative importance in various circumstances is poorly appreciated. On Earth, preservation of diamonds in kimberlites implies very rapid (hours) transfer of melts from depths of 100-300 km, and there is strong geochemical evidence that magmas at mid-ocean ridges reach shallow depths faster than is possible by percolation alone. On the Moon, the petrology of pyroclasts involved in dark-mantle-forming eruptions implies rapid (again probably hours) magma transfer from depths of up to 400 km. The ureilite meteorites, samples of the mantle of a disrupted asteroid 200 km in diameter, have compositions only consistent with the rapid (months) extraction of mafic melt from the mantle. All of these examples imply that brittle fractures (dikes) can sometimes be initiated at depths where mantle rheology would normally be expected to be plastic rather than elastic, and that melt can be fed into these dikes extremely efficiently. Further evidence for this is provided by the giant radial dike swarms observed on Earth, Mars and Venus. The dikes observed (on Earth) and inferred from the presence of radiating graben systems (Mars) and radiating fracture and graben systems (Venus) are so voluminous that they can only be understood if they are fed from extremely large magma reservoirs, probably located at the base of the crust, that are supplied from the mantle (i.e. buffered) while the dikes are being emplaced, again implying extremely efficient melt extraction from mantle source regions.
P52A-02
Volcanism on Io: Insights from Global Geologic Mapping
NASA's Galileo Mission (1996-2003) acquired excellent images of the antijovian (or far side) hemisphere of Jupiter's volcanic moon Io, which are complementary to the subjovian (or near side) images obtained by the 1979 NASA Voyager Mission. In 2005 the U.S. Geological Survey produced a set of global image mosaics of Io (spatial resolution 1 kilometer/picture element and full color) that enable for the first time production of a complete global geologic map. We have mapped Io using ArcGIS software to assess the types and abundances of process-related geologic material units and structures, to gain further insights into the types and styles of activity that shape this hyperactive volcanic moon. We find that lava flow fields make up about 28% of the surface, in which bright (presumably sulfur) flows are twice as abundant as dark (presumably silicate) flows. Many of the bright flows do not have adjacent dark flows, perhaps indicative of extensive primary rather than secondary sulfur volcanism (i.e., effusion of crustal sulfur magma, rather than sulfur-rich country rock melted by adjacent silicate magma). Ephemeral, diffuse pyroclastic plume deposits mantle about 18% of the surface at any time, and include condensed sulfur and sulfur dioxide gases and silicate ash. Patera (i.e., caldera) floors contain lava flows and/or some lava lakes, and cover only 2.5% of the surface, but are the source of most of the active hot spots. Restriction of effusive resurfacing mostly to caldera-like topographic depressions, and the ephemeral nature of plume deposits, explains the relatively small amount of surface changes observed between the Voyager and Galileo missions. Tectonic mountains, rising up to 17 km, cover about 3% of the surface, but close association of about one-third to one-half of the mountains with paterae suggest linkage of volcanic and tectonic processes. About 67% of Io is covered by plains, thought to consist of silicate crust covered with accumulations of lava flows and pyroclastics whose boundaries are not discernable. No impact craters have been found on Io, indicating a surface age of less than a few tens of millions of years. We will discuss the implications of these results for Io's volcanism.
P52A-03
Cryovolcanism on Titan: Interpretations from Cassini RADAR data
Several surface features interpreted as cryovolcanic in origin have been observed on the surface of Titan by both Cassini's RADAR (in SAR mode) and VIMS instruments throughout the Cassini prime mission. These include large flows, an eroded volcanic dome or shield, and calderas associated with flows. The Titan flyby T41 of 22 February 2008 includes a SAR image of part of Hotei Arcus, a semi-circular albedo feature, some 650 km in length along the arc, centered at 26S 79W. A second SAR image of Hotei was acquired May 12, 2008 on flyby T43. These images show that the arcuate southern boundary of Hotei, also seen in ISS data, appears somewhat mountainous in the SAR imagery, and 5 distinct narrow channels, presumably fluvial, flow radially inwards. In the center of the arc, the images reveal lobate, flowlike features that embay surrounding terrains and cover the channels. Analysis of these features suggest that they are of cryovolcanic origin and younger than surrounding terrain. Their appearance is superficially similar to a region in western Xanadu at 10S 140W imaged by RADAR on flyby T13, on Apr 30, 2006. These two regions are morphologically unlike most of the other cryovolcanic regions so far seen on Titan. Both regions correspond to those identified by the Cassini VIMS as having anomalous and variable infrared brightness, probably due to recent cryovolcanic activity. The RADAR images provide morphological evidence that is consistent with cryovolcanism.
P52A-04
Low-Temperature Volcanism and Pyroclastic Flows on Comet Tempel 1
The Deep Impact mission discovered several smooth terrains and repetitive outbursts on comet 9P/Tempel 1 suggestive of cryo-volcanic activity in its interior. We present new measurements of the extent of the smooth terrains, the slopes along their centerlines, and the areas of their likely source regions and vents. Our analysis of these features indicates that they may be only a few orbits old and the result of an ongoing process. Based on the source locations of repetitive outbursts, we propose that the smooth terrains originate from different regimes of fluidization and gas transport in a weakly bound particulate mixture of ice and dust above an amorphous to crystalline water ice phase transformation boundary where CO and/or CO2 is released. The stresses due to gas pressure extrude, at low velocity, fluidized and dilated, gas-laden cometary material onto the surface leading to downhill flow and subsequent collapse of the evacuated cavity. The most prominent smooth terrain is longitudinally striated and slopes monotonically downward from an apparent source crater with an average gradient of about 3 deg. It resembles terrestrial catastrophic rock avalanches such as the Alaskan Sherman Glacier landslide of 1964. However, in the case of Tempel 1 the fluidizing agent is CO or CO2 gas. The gas-charged material erupted onto the surface and, as the gas slowly diffused out of the moving mass, moved downslope as a mobile debris flow, similar in concept to terrestrial mudflows that are fluidized by water (the pressure in both obeys precisely the same equation, but this differs from a terrestrial pyroclastic flow). The mass of material that remains on the surface comprises about 10E10 kg of fine particulates in the best-imaged example. Due to their high density and thus relatively high viscosity, these flows traveled in a laminar regime and halted abruptly as the fluidizing gas escaped, leaving a steep terminal scarp. Nevertheless, the flow viscosity was not high enough to seriously impede its motion, which was nearly frictionless during most of its travel. Several methods of estimating the flow viscosity agree in placing it between 30 and 100 Pa-sec. The time scale for emplacement was a few hours and the maximum velocity about 0.3 m/sec.