John E. Guest

Venus volcanism: Classification of volcanic features and structures, associations, and global distribution from Magellan data

A preliminary analysis of a global survey of Magellan data covering over 90% of the surface and designed to document the characteristics, location, and dimensions of all major volcanic features on Venus has revealed over 1660 landforms and deposits. These include over 550 shield fields (concentrations of small volcanoes <20 km in diameter), 274 intermediate volcanoes between 20 and 100 km diameter with a variety of morphologies, 156 large volcanoes in excess of 100 km diameter, 86 calderalike structures independent of those associated with shield volcanoes and typically 60–80 km in diameter, 175 coronae (annulus of concentric ridges or fractures), 259 arachnoids (inner concentric and outer radial network pattern of fractures and ridges), 50 novae (focused radial fractures forming stellate patterns), and 53 lava flood-type flow fields and 50 sinuous lava channels (all of which are in excess of 102–103 km in length). The vast majority of landforms are consistent with basaltic compositions; possible exceptions include steep-sided domes and festoons, which may represent more evolved compositions, and sinuous rules, which may represent more fluid, possibly ultramafic magma. The range of morphologies indicates that a spectrum of intrusive and extrusive processes have operated on Venus. Little evidence was found for extensive pyroclastic deposits or landforms, consistent with the inhibition of volatile exsolution and consequent disruption by the high surface atmospheric pressure. The large size of many volcanic features is evidence for the presence of very large magma reservoirs. The scale of resurfacing implied by individual features and deposits is typically much less than 125,000 km2. The areal distribution, abundance, and size distribution relationships of shield fields, arachnoids, novae, large volcanoes, and coronae strongly suggest that they are the surface manifestation of mantle plumes or hot spots and that the different morphologies represent variations in plume size and stage and thermal structure of the lithosphere. Maps of the global distribution of volcanic features show that they are broadly distributed globally, in contrast to the plate boundary concentrations typical of Earth. However, they are not randomly distributed on the surface of Venus. An observed deficiency of many volcanic features in several lowland areas of Venus may be due to an altitude-dependent influence of atmospheric pressure on volatile exsolution and the production of neutral buoyancy zones sufficient to form magma reservoirs; this would favor lava floods and sinuous channels at low elevations and edifices and reservoir-related features at higher elevations. A major concentration of volcanic features is observed in the Beta/Atla/Themis region, an area covering about 20% of the planet and centered on the equator. This region is unique in that it is the site of local concentrations of volcanic features with concentrations 2–4 times the global average, an interlocking network of rift and deformation zones, several broad rises several thousand kilometers in diameter with associated positive gravity anomalies and tectonic junctions, and evidence for volcanically embayed impact craters. Although the region as a whole does not appear to be anomalously older or younger than the rest of Venus, there is evidence that the most recent volcanic activity on the planet occurs here, and the presence of this series of concentrations suggests that the mantle in this region is anomalous. Analysis of the impact crater population shows that it cannot be distinguished from a completely spatially random population (Phillips et al., this issue), and several end-member models for this distribution are possible: (1) single production age or “spasmodic or catastrophic volcanism” model: craters have accumulated subsequent to a global volcanic resurfacing event about one-half billion years ago (Schaber et al., this issue); (2) vertical equilibrium or “leaky planet” model: craters are removed by slow accumulation of lava over the whole planet leading to a range of volcanic degradation states for craters; (3) regional resurfacing or “collage” or “cookie-cutter” model: craters are removed largely instantaneously by superposition of features and deposits; the horizontal scale of resurfacing does not exceed the horizontal scale of randomness of the crater population. Our data on the scale and location of resurfacing are consistent with the regional resurfacing model and with the catastrophic resurfacing model. The nature and abundance of impact craters definitely degraded by volcanism also favor these two models, although uncertainty exists as to whether all such craters have been detected. Although a process toward the regional resurfacing end-member model presently seems most plausible, distinction between the three models requires an understanding of the mode and timing of emplacement of the volcanic plains that make up the majority of the surface and which are not clearly related to the edifices and features mapped in this study. In addition, the resurfacing mechanisms involved in the catastrophic resurfacing models are not yet explicitly enough formulated to test with the existing data. An equilibrium resurfacing model implies a volcanic flux of 0.5 km3/yr, a value similar to the present rate of intraplate volcanism on Earth (0.3–0.5 km3/yr). This value is broadly comparable to that implied by the edifices and deposits on Venus mapped in this study. Geologically recent volcanism on Venus is dominated by features interpreted to be related to mantle plumes.

The evolution of lava flow-fields: observations of the 1981 and 1983 eruptions of Mount Etna, Sicily

The eruptions of Mount Etna in 1981 on the north flank and 1983 on the south flank of the volcano were of strikingly different character. The former was a short duration, high effusion rate eruption producing for the most part a simple flow-field; the latter was of relatively long duration and low effusion rate, producing a compound flow-field of overlapping flows.Despite the differences between the eruptive behaviour of these two events and the way in which the flow-field developed, both the flow-fields achieved about the same maximum length. This is considered fortuitous. The evidence suggests that the main 1981 flow stopped because the lava supply ceased and was thus volume controlled. The 1983 flow-field had a more complex history of branching, but in this case it appears that, for the longest individual flow, cooling played an important role in controlling the maximum extent of the flows.

Venus Volcanism: Initial Analysis from Magellan Data

Magellan images confirm that volcanism is widespread and has been fimdamentally important in the formation and evolution of the crust of Venus. High-resolution imaging data reveal evidence for intrusion (dike formation and cryptodomes) and extrusion (a wide range of lava flows). Also observed are thousands of small shield volcanoes, larger edifices up to several hundred kilometers in diameter, massive outpourings of lavas, and local pyroclastic deposits. Although most features are consistent with basaltic compositions, a number of large pancake-like domes are morphologically similar to rhyolite-dacite domes on Earth. Flows and sinuous channels with lengths of many hundreds of kilometers suggest that extremely high effusion rates or very fluid magmas (perhaps komatiites) may be present. Volcanism is evident in various tectonic settings (coronae, linear extensional and compressional zones, mountain belts, upland rises, highland plateaus, and tesserae). Volcanic resurfacing rates appear to be low (less than 2 Km3/yr) but the significance of dike formation and intrusions, and the mode of crustal formation and loss remain to be established.

Impact craters on venus: initial analysis from magellan.

Magellan radar images of 15 percent of the planet show 135 craters of probable impact origin. Craters more than 15 km across tend to contain central peaks, multiple central peaks, and peak rings. Many craters smaller than 15 km exhibit multiple floors or appear in clusters; these phenomena are attributed to atmospheric breakup of incoming meteoroids. Additionally, the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across. Ejecta is found at greater distances than that predicted by simple ballistic emplacement, and the distal ends of some ejecta deposits are lobate. These characteristics may represent surface flows of material initially entrained in the atmosphere. Many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid. Craters are absent from several large areas such as a 5 million square kilometer region around Sappho Patera, where the most likely explanation for the dearth of craters is volcanic resurfacing. There is apparently a spectrum of surface ages on Venus ranging approximately from 0 to 800 million years, and therefore Venus must be a geologically active planet.

Volcanic geology of Furnas Volcano, São Miguel, Azores

Abstract Furnas is the easternmost of the three active central volcanoes on the island of Sao Miguel in the Azores. Unlike the other two central volcanoes, Sete Cidades and Fogo, Furnas does not have a well-developed edifice, but consists of a steep-sided caldera complex 8×5 km across. It is built on the outer flanks of the Povoacao/Nordeste lava complex that forms the eastern end of Sao Miguel. Constructive flanks to the volcano exist on the southern side where they form the coastal cliffs, and to the west. The caldera margins tend to reflect the regional/local tectonic pattern which has also controlled the distribution of vents within the caldera and areas of thermal springs. Activity at Furnas has been essentially explosive, erupting materials of trachytic composition. Products associated with the volcano include plinian and sub-plinian pumice deposits, ignimbrites and surge deposits, phreatomagmatic ashes, block and ash deposits and dome materials. Most of the activity has occurred from vents within the caldera, or on the caldera margin, although strombolian eruptions with aa flows of ankaramite and hawaiite have occurred outside the caldera. The eruptive history consists of at least two major caldera collapses, followed by caldera infilling. Based on 14 C dates, it appears that the youngest major collapse occurred about 12,000–10,000 years BP. New 14 C dates for a densely welded ignimbrite suggest that a potential caldera-forming eruption occurred at about 30,000 years BP. Recent eruptions (

Small volcanic edifices and volcanism in the plains of Venus

The most widespread terrain type on Venus consists of volcanic lowland plains. Several styles of volcanism are represented in the plains. The most extensive volcanic units consist of flood lavas, the largest of which have volumes of the order of thousands of cubic kilometers. As with terrestrial flood lavas, they are inferred to have erupted at high effusion rates. They show a range of radar backscatter characteristics indicating different surface textures and ages. Small edifices on the plains occur mainly in clusters associated with fracture belts. The majority are shield volcanoes that may be up to a few tens of kilometers across but are generally 10 km or less in diameter. Volcanic cones have the same size range. Volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 km3. These are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava. For many domes, the flanks were unstable during and afte eruption and suffered gravity sliding that produced steep, scalloped outer margins. Because of the high atmospheric pressures on Venus, explosive activity is less likely to occur than on Earth. However, n a few plains areas there is evidence of pyroclastic deposits surrounding craters, indicating that volatile contents in some of the magmas may be high in comparison to Earth. The clusters of small volcanic edifices are considered to be analogous to plains volcanism, similar to that of the Snake River Plain of Idaho. There may also be analogues with terrestrial volcanic clusters associated with mid-oceanic ridges.

Preliminary results from the Viking orbiter imaging experiment

During its first 30 orbits around Mars, the Viking orbiter took approximately 1000 photographic frames of the surface of Mars with resolutions that ranged from 100 meters to a little more than 1 kilometer. Most were of potential landing sites in Chryse Planitia and Cydonia and near Capri Chasma. Contiguous high-resolution coverage in these areas has led to an increased understanding of surface processes, particularly cratering, fluvial, and mass-wasting phenomena. Most of the surfaces examined appear relatively old, channel features abound, and a variety of features suggestive of permafrost have been identified. The ejecta patterns around large craters imply that fluid flow of ejecta occurred after ballistic deposition. Variable features in the photographed area appear to have changed little since observed 5 years ago from Mariner 9. A variety of atmospheric phenomena were observed, including diffuse morning hazes, both stationary and moving discrete white clouds, and wave clouds covering extensive areas.

Erosion by flowing lava: Field evidence

Erosion of substrate materials by melting or mechanical means has been suggested in active lava flows on Earth and other planets. Although there are many references to lava erosion on Earth, unambiguous evidence is rare; geological relationships commonly cited as evidence of downcutting by lava can be explained without recourse to erosion. In order to assess possible erosion by flowing lava we carried out field studies of tube-fed basalt flows, sheet flows of the Columbia River Basalt Province (CRB), and Precambrian komatiites. Unequivocal evidence for thermal erosion (melted dacite substrate) was found at the Cave Basalt lava tube, Mount St. Helens, for which fluid dynamic analysis indicates laminar flow, although erosion was enhanced in areas of locally steep slopes, possibly as a result of localized turbulence. Other lava tubes in our study display strong, but inconclusive, evidence for erosion. Komatiite flows display good evidence for erosion of their substrate, possibly in a turbulent regime, but assessment of the extent of erosion is hampered by limited and disrupted exposures. No evidence for thermal erosion was found in the CRB. Our findings suggest that an erosional origin for planetary sinuous rilles and canali would be favored by high Reynolds number flows (high mass flux, low-viscosity lava, steep slopes) and substrates having a lower melting temperature than the lava or low mechanical strength (e.g., regolith).

An historic subplinian/phreatomagmatic eruption: the 1630 AD eruption of Furnas volcano, Sa˜o Miguel, Azores

The 1630 AD eruption on the island of Sa˜o Miguel in the Azores took place from a vent in the southern part of the 7 × 5 km caldera of Furnas volcano. Precursory seismic activity occurred at least 8 hours before the eruption began and was felt over 30 km away. This seismic activity caused extensive damage destroying almost all buildings within a 10 km radius and probably triggered landslides on the southern coast. The explosive activity lasted ~ 3 days and ashfall occurred as far as 550 km away. Published models yield a volume of 0.65 km3 (DRE) for the explosive products. Throughout the course of the eruption more than six discrete airfall lapilli layers, each of subplinian magnitude, were generated by magmatic explosive activity. Dispersal directions initially to the west and finally northeast of the vent indicate a change in wind direction during the eruption. Isopleth maps suggest column heights of up to 14 km and wind speeds varying between 20°) at least one lapilli layer (L2) shows pinch and swell thickness variations, and rounded pumice clasts suggesting instant remobilisation as grain flows. Ash-rich layers with abundant accretionary lapilli and vesicular textures are interbedded with the lapilli layers and represent the deposits formed by phreatomagmatic phases that punctuated the purely magmatic activity. The ash-rich layers show lateral thickness variations, as well as cross-bedding and sand-wave structures suggesting that low-concentration, turbulent flows (surges) deposited material on topographic highs. These pyroclastic surges were probably responsible for the 80 people reported burned to death 4 km southwest of the vent. High-particle-concentration, non-turbulent pyroclastic flows were channelled down steep valleys to the southern coast contemporaneously with the low-concentration surges. The massive flow deposits (~ 2 m thick) pass laterally into thin, stratified, accretionary lapilli-rich ashes (~ 20 cm thick) over 100 m horizontally. Lateral transition between thick massive and thin stratified facies occurs on a flat surface unconfined by topography indicating that the flows had an effective yield strength. Effusive activity followed the explosive activity building a trachytic lava dome with a volume of ~20 × 106 m3 (0.02 km3 DRE) within the confines of the tuff/pumice cone formed during the explosive phase. Historic records suggest that dome building occurred over a period of at least two months. Calculated durations for eruptive phases and the fluctuation in eruptive style suggest that the eruption was pulsatory which may have been controlled by variable magma supply to the surface.

PULSED INFLATION OF PAHOEHOE LAVA FLOWS : IMPLICATIONS FOR FLOOD BASALT EMPLACEMENT

Abstract Dilated fractures in Hawaiian pahoehoe lava flows contain three zones that show the kinematics of inflation. The upper columnar zone forms through thermal contraction prior to inflation, the middle planar zone reflects inflation-induced tension, and the lower banded zone contains evidence of brittle and ductile deformation. The formation of the lower banded zone requires varying strain rates during fracture propagation and is best explained by a model where small pulses of lava inject beneath the cooled flow crust through a network of preferred pathways. We demonstrate via simple models of pipe flow that this inflation mechanism is incapable of producing areally extensive continental flood basalts on Earth, although it may explain related features on large Martian volcanoes.