Lionel Wilson

Lunar mare volcanism - Stratigraphy, eruption conditions, and the evolution of secondary crusts

Abstract Lunar mare basalt deposits cover 17% of the lunar surface, occur preferentially in topographic lows on the nearside, and have a total volume estimated at 1 × 107 km3. Analysis of returned samples and photogeologic and remote sensing studies shows that mare volcanism began prior to the end of heavy bombardment (the period of cryptomare formation), in pre-Nectarian times, and continued until the Copernican Period, a total duration approaching 3.5–4 Ga. Stratigraphic analyses show that the flux was not constant, but peaked in early lunar history, during the Imbrian Period (which spans the period 3.85-3.2 Ga). Average volcanic output rate during this peak period was about 10 −2 km 3 a , very low relative to the present global terrestrial volcanic output rate and comparable to the present local output rates for individual volcanoes such as Vesuvius and Kilauea, Hawaii. Volcanic landforms indicate that many eruptions were of high volume and long duration. Some eruptions associated with sinuous rules may have lasted of the order of a year and emplaced 103 km3 of lava, representing the equivalent in one year of about 70,000 years at the average flux. Shallow magma chambers were uncommon. The nearsidefarside mare deposit asymmetry can be readily explained by differences in crustal thickness. Magma ascending from the mantle or from a buoyancy trap at the base of the crust should preferentially extrude to the surface on the nearside, but should generally stall and cool in dikes in the farside crust, extruding only in the deepest basins. The occurrence of farside maria within craters whose diameter is generally near to or less than the thickness of the crust may be accounted for by the difference between local and regional compensation. Dikes that establish pathways to the surface on the nearside should have very high volumes, comparable to the volumes associated with many observed flows and sinuous rilles. An abundance of dikes should exist in the lower crust of the Moon, many more than those feeding surface eruptions (the upper limit is 37–50% of the crust by volume). The presence and abundance of such dike swarms have important implications for the interpretation of the average composition of the lunar crust and the composition of basin and crater ejecta. The interplay between thermal contraction and differentiation leads to net cooling and ultimate contraction of the outer portions of the Moon, resulting in a regional horizontal compressive stress acting on the lunar crust. In addition, overall cooling and deepening of sources require the production of ever larger volumes of magma in order to reach the surface. With time, stresses became high enough so that few dikes could open to the surface, causing eruptive activity to be severely diminished in the Eratosthenian, and perhaps to cease in the Copernican Period. Lower stress levels are required to terminate eruptive activity on the lunar farside, consistent with the Imbrian age of the farside maria and the nearside location of the youngest maria. Lunar mare deposits provide an example of the transition from primary crusts to secondary crusts and illustrate the significance of several factors in the evolution of secondary crusts, such as crustal density, variations in crustal thickness, presence of impact basins, state and magnitude of stress in the lithosphere, and general thermal evolution. These factors are also responsible for the extremely low volcanic flux, even during periods of peak extrusion.

Relationships between pressure, volatile content and ejecta velocity in three types of volcanic explosion

Abstract Consideration of the energy equation for a flowing compressible fluid shows that the so-called modified Bernoulli equation, commonly used to relate ejects velocity to pre-explosion pressure in vulcanian-style volcanic explosions, is inadequate in almost all circumstances because of its neglect of the detailed role of volatiles in explosive eruptions. The physical differences between three common types of explosive volcanic activity, typified by plinian, strombolian and vulcanian events, are reviewed and simple mathematical models are proposed for them. The models relate velocities of ejects to initial pressures at the start of an explosive phase and to mass fractions of volatiles (generally taken to be water) in the explosion products. When fitted to observed ejects velocities (or velocities deduced from the dispersal of debris) up to 500 m/s the models predict pressures up to 300 bars — almost always much lower than those deduced in earlier treatments.

Giant radiating dyke swarms on Earth and Venus

Abstract Concentrations of dykes of basic composition emplaced in the same igneous episode or along similar trends are known as mafic dyke swarms and they occur in a wide variety of environments and over a wide range of scales on Earth. Recent radar mapping of Venus has revealed families of linear features interpreted to be the surface expression of near-surface dyke swarms. The lack of significant erosion on Venus provides a view of the surface manifestation of dyke swarm emplacement, one which complements the terrestrial perspective of erosion to deeper levels. The goal of this review is to synthesize the information available on both planets in order to use the complementary and synergistic record of mafic dyke swarm emplacement to build toward a better understanding of this important phenomenon in planetary history. We focus on the formation and evolution of giant dyke swarms which cover tens to hundreds of thousands of square kilometres on both Earth and Venus. Mafic dyke swarms on Earth occur in a wide range of modes and are observed in environments ranging from volcanic edifices (e.g., Hawaii), to central complexes (e.g., Spanish Peaks Complex, USA; Ramon Swarm, Israel), spreading centres and ophiolite complexes, compressional plate boundaries in back-arc settings (Columbia River Basalts, USA) and in continent-continent collisions. One of the most impressive modes of occurrence is that linked to the formation and evolution of mantle plumes. Terrestrial examples include a giant radiating swarm covering 100° of azimuth (the Mackenzie swarm, Canada), a 360° giant radiating swarm (the Central Atlantic reconstructed swarm), deformed giant radiating swarms (the Matachewan swarm, Canada), rift-arm associated swarms (e.g., Grenville swarm, Canada; Yakutsk swarm, Siberia), and one consisting of widely separated dykes (e.g., the Abitibi swarm, Canada). We summarize the geometric, chemical and isotopic characteristics of terrestrial dyke swarms, including their size and geometry, ages, presence and absence of subswarms, and the relation between swarms of different ages. We also summarize the characteristics of individual dykes, examining dyke length and continuity, en echelon offsets, dyke bifurcation, dyke height, width and depth, dyke intrusion and cooling history, and evidence for flow directions. On Venus at least 163 large radiating lineament systems (radius generally > 100 km) composed of graben, fissure and fracture elements have been identified. On the basis of their structure, plan view geometry and volcanic associations, the radial elements of more than 70% of these are interpreted to have formed primarily through subsurface dyke swarm emplacement, with the remainder forming through uplift or some combination of these two mechanisms. These systems are essentially uneroded and provide a view of the surface characteristics of giant radial swarms prior to the erosion which commonly occurs on Earth. The individual graben, fissures and fractures of which the systems are composed are typically less than several kilometres in width and cluster near the centre, with fissures grading smoothly into fractures at greater distances to define the overall radial pattern. While the largest systems, like those on Earth, are thousands of kilometres in radius, the population average is about 325 km, and they generally do not extend to equal lengths in all directions. In their distal regions, however, the elements in 72% of the systems continue along a purely radial trend, while distal elements in the remaining 28% curve gradually into unidirectional, sub-parallel geometries, generally interpreted to be related to regional stress patterns. The radial systems have a strong association with volcanism; all but seven display some form of volcanic signature. A review of models of the emplacement of lateral dykes from magma chambers under constant (buffered) driving pressure conditions and declining (unbuffered) driving pressure conditions indicates that the two pressure scenarios lead to distinctly different styles of dyke emplacement. Emplacement of lateral dykes in the constant driving pressure (buffered) case, however, can produce dykes which have sizes and widths which are very large and independent of chamber size. On Earth, the characteristics of giant mafic dyke swarms such as the Mackenzie dyke swarm in Canada strongly suggest that they were emplaced in buffered conditions. On Earth, giant radiating dyke swarms are usually preserved as fan-shaped fragments which have been dismembered and distorted by subsequent plate tectonic rifting events. The abundant intact giant radiating swarms on Venus provide criteria by which fragmented terrestrial swarms can be reconstructed.

Mars: Review and analysis of volcanic eruption theory and relationships to observed landforms

We present a theoretical treatment of the ascent, emplacement, and eruption of magma on Mars. Because of the lower gravity, fluid convective motions and crystal settling processes driven by positive and negative buoyancy forces, as well as overall diapiric ascent rates, will be slower on Mars than on Earth, permitting larger diapirs to ascend to shallower depths. This factor also favors a systematic increase in dike widths on Mars by a factor of 2 and, consequently, higher effusion rates by a factor of 5. As a result of the differences in lithospheric bulk density profile, which in turn depend on differences in both gravity and surface atmospheric pressure, magma reservoirs are expected to be deeper on Mars than on Earth, by a factor of about 4. The combination of the lower Martian gravity and lower atmospheric pressure ensures that both nucleation and disruption of magma occur at systematically greater depths than on Earth. Although lava flow heat loss processes are such that no major differences between Mars and Earth are to be expected in terms of flow cooling rates and surface textures, the lower gravity causes cooling-limited flows to be longer and dikes and vents to be wider and characterized by higher effusion rates. Taken together, these factors imply that we might expect compositionally similar cooling-limited lava flows to be about 6 times longer on Mars than on Earth. For example, a Laki type flow would have a typical length of 200-350 km on Mars; this would permit the construction of very large volcanoes of the order of 500-700 km in diameter. For strombolian eruptions on Mars the main difference is that while the large particles will remain near the vent, the finer material will be more broadly dispersed and the finest material will be carried up into a convecting cloud over the vent. This means that there would be a tendency for broader deposits of fine tephra surrounding spatter cones on Mars than on Earth. On Mars, strombolian eruption deposits should consist of cones that are slightly broader and lower relative to those on Earth, with a surrounding deposit of finer material. Martian hawaiian cones should have diameters that are about a factor of 2 larger and heights that are, correspondingly, about a factor of 4 smaller than on Earth; central craters in these edifices should also be broader on Earth by a factor of up to at least 5. Grain sizes in Martian hawaiian edifices should be at least 1 order of magnitude finer than in terrestrial equivalents because of the enhanced magma fragmentation on Mars. Differences in the atmospheric pressure and temperature structure cause Martian plinian eruption clouds to rise about 5 times higher than terrestrial clouds for the same eruption rate. Essentially the same relative shapes of eruption clouds are expected on Mars as on Earth, and so the cloud-height/deposit-width relationship should also be similar. This implies that Martian fall deposits may be recognized as areas of mantled topography with widths in the range of several tens to a few hundred kilometers. A consequence of the lower atmospheric pressure is that Martian plinian deposits of any magma composition will be systematically finer grained than those on Earth by a factor of about 100, almost entirely subcentimeter in size. Basaltic plinian eruptions, rare on Earth, should be relatively common on Mars. The production of large-scale plinian deposits may not signal the presence of more silicic compositions, but rather may be linked to the enhanced fragmentation of basaltic magma in the Martian environment or to the interaction of basaltic magma with groundwater. The occurrence of steep-sided domes, potentially formed by viscous, more silicic magma, may be largely precluded by enhanced magma fragmentation. Pyroclastic flow formation is clearly inherently more likely to occur on Mars than on Earth, since eruption cloud instability occurs at a lower mass eruption rate for a given magma volatile content. For a given initial magma volatile content, eruption speeds are a factor of at least 1.5 higher on Mars, and so the fountains feeding pyroclastic flows will be more than twice as high as on Earth. Pyroclastic flow travel distances may be a factor of about 3 greater, leading to values up to at least a few hundred kilometers. Martian environmental conditions thus operate to modulate the various eruption styles and the morphology and morphometry of resulting landforms, providing new insight into several volcanological problems.

Tharsis‐radial graben systems as the surface manifestation of plume‐related dike intrusion complexes: Models and implications

Several zones of graben (Memnonia, Sirenum, Icaria, Thaumasia, and Claritas Fossae) extend radially away from the Tharsis rise in the southern hemisphere of Mars for distances of up to 3000–4000 km. These graben systems are commonly interpreted to be related to regional tectonic deformation of the Tharsis rise associated with either upwelling or loading. We explore the possibility that these giant Tharsis-radial graben systems could be the surface manifestation of mantle plume-related dike intrusion complexes. Emplacement of dikes causes near-surface stresses that can produce linear graben, and lateral dike emplacement related to plumes on Earth can produce dike swarms with lengths of many hundreds to several thousands of kilometers. We develop a Mars dike emplacement model and explore its implications. We find that the properties (outcrop patterns, widths, and depths) of the extensive Tharsis-radial graben systems are consistent with an origin through near-surface deformation associated with lateral propagation of magma-filled cracks (dikes) from plumes beneath Tharsis, particularly beneath Arsia Mons and Syria Planum. Such dikes are predicted to extend through the crust and into the upper mantle and can have widths of up to several hundred meters. Analyses of summit caldera complexes on Martian volcanoes imply that the magma supply from the mantle into shallower reservoirs is episodic on Mars, and we interpret the graben systems to be large swarms of laterally emplaced giant dikes resulting from the tapping of melt from episodically rising mantle plumes in a buffered magma supply situation. The magmatic interpretation of the Tharsis-radial graben potentially removes one of the conundrums of Tharsis tectonics in which it appeared necessary to require two distinct modes of support for Tharsis in order to explain the presence of radial graben on both the elevated flanks (attributed to isostatic stresses) and outside the rise (more consistent with flexure): dikes capable of forming the observed graben can be emplaced under a wide range of stress fields, including zero stress. The fact that almost no eruptive features are associated with the graben further restricts the ranges of magma density to values between ∼3100 and 3200 kg m−3 and crustal stress to tensions less than ∼30 MPa. Eruptions from giant dikes would be more likely to occur in regions where the crust was thinner, such as the northern lowlands, providing a potential mechanism for emplacement of recently documented Early Hesperian volcanic plains (Hr) there. Dike-related graben systems represent efficient mechanisms of lateral heat transfer in the crust and near-surface environments. Lateral dike intrusions could penetrate the cryosphere and cause melting and release of groundwater, as in the Mangala Valles area, and could also drive hydrothermal circulation systems. The geometries of such dike systems will create barriers which are likely to influence regional to global groundwater flow patterns, which may help to explain the abundance of outflow channel sources in eastern Tharsis. Improved knowledge of the Martian crust and mantle density structure will help to refine this analysis and to provide estimates of the magma densities for dikes underlying specific graben.

The influence of shape on the atmospheric settling velocity of volcanic ash particles

Abstract Experimental measurements of terminal fall velocities at sea level are reported for pumices, glass shards, and feldspar crystals with mean diameters between 30 and500 μm. The velocities depend significantly on particle shape and rotation mode as well as density and size. Six tumbling modes were observed, of which two are predominant. The measurements have been converted to drag coefficients and Reynolds numbers so that they can be used to compute terminal velocities at any height in the atmosphere. It was found that if the drag coefficient and Reynolds number are defined empirically in terms of the arithmetic mean particle diameter, the effects of shape and rotation can be fully accounted for by defining a shape parameter, F, for each particle. In terms of the lengths of the longest, intermediate and shortest principal axes of the particle, denoted a, b, and c, respectively, F =(b + c)/2a. A simple formula for the drag coefficient, Ca, as a function of Reynolds number, Ra, and shape parameter is: Download : Download full-size image A more fundamental analysis allows the measurements made here to be compared with theoretical curves and experimental data on simple particle shapes from wind tunnel studies.

The pyroclastic deposits of the 1875 eruption of Askja, Iceland

The 1875 explosive eruption of Askja, Iceland was part of a series of regional volcanic and tectonic events which took place in the northern rift zone in 1874 and 1875. These events were marked by regional seismicity, graben formation and a basaltic fissure eruption at Sveinagja, and the plinian eruption of Askja on 28-29 March. Crustal rifting caused basaltic magma to be mixed with rhyolitic magma, triggering the plinian eruption. A caldera, Oskjuvatn, was formed in Askja measuring 3 x 4 km and 267 m deep. Six distinguishable pyroclastic layers can be recognized. The main eruption began with a small sub-plinian pumice eruption forming layer B. The next phase produced a fine-grained, poorly sorted pumice and ash deposit with well developed stratification (layer C), which contains base surge beds near source and is interpreted as phreatomagmatic in origin. The main plinian phase of the eruption lasted 6 h and formed a coarse-grained, poorly bedded pumice-fall deposit (layer D) which contains 75% of the total ejecta. Late-stage explosions formed a layer of lithic clasts (layer E). Isopach and grain-size isopleth maps show that the vents migrated from south to north along a line 1.5 km long in the area now occupied by Oskjuvatn. The intensity and column height of the eruption increased with time as shown by reverse grading and an increasing dispersal index in successive layers. Most of the ejecta is composed of white rhyolitic pumice and ash. Lithics consist of rhyolitic obsidian, partially fused trondhjeimite, and basalt fragments: layer D contains 2.1 mass % lithics. All layers contain abundant grey pumice clasts consisting of intimate mixtures of dark brown basaltic and brown rhyolitic glasses. The mass percentage of mixed pumice in layer D is 4.7, of which 40 % is basaltic glass. These mixed pumice clasts are concentrated at distances of 30-80 km in layer D by aeolian sorting. A grey, crystal-rich, andesitic pumice occurs as inclusions in the white pumice. Layer D shows a systematic decrease in median grain diameter, but no change in cr^ with distance from source. Layer C shows no change in median grain diameter, but a decrease in with distance from source. Phreatomagmatic deposits such as layer C can be readily distinguished from plinian deposits on a Md

Tarawera 1886, New Zealand — A basaltic plinian fissure eruption

against cr^ diagram, on a against a* (skewness) diagram and on the F against D plot of Walker (1973). The downwind, coarse-tail grading in layer C is attributed to fall-out of fine ash as clumps and aggregates. The total grain-size distributions of both layers D and C show bimodality. In layer D a minor mode in the ash size classes reflects secondary processes of fragmentation by collisions in the vent and column, whereas the major mode is due to disruption of magma by expanding gases. In layer C the fine mode is dominant and represents extensive fragmentation by explosive interaction with water. Field and grain-size studies of layer D show that impact breakage is of major importance near source.

Basaltic pyroclastic eruptions: Influence of gas-release patterns and volume fluxes on fountain structure, and the formation of cinder cones, spatter cones, rootless flows, lava ponds and lava flows

Abstract New Zealands biggest and most destructive volcanic eruption of historical times was that of Tarawera in 1886. The resulting scoria fall has a dispersal very similar in extent to that of the Vesuvius A.D. 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type. A new estimate of the volume (2 km 3 ) is significantly greater than previous estimates. The basalt came mainly from a 7-km length of fissure, and emission and exit velocity were fairly uniform along at least 4 km of it, this is one of the few documented examples of a plinian eruption from a fissure vent. Primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min −1 . A model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1.5–3%. Violent phreatic explosions occurred in the southwestern extension of the fissure across the Rotomahana geothermal field, and it is thought that some of the water responsible for the power of the plinian eruption came from this source, though its amount was not sufficient to turn the eruption into a phreatoplinian one.

Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits

Abstract In basaltic pyroclastic eruptions, two variables — magma gas content and magma volume flux — determine the detailed dynamic structure of the fountain and the size distribution of clasts within it. The fountain structure and clast size distribution in turn determine the nature of the resulting deposits, whether these be stationary pyroclastic constructs or active lava flows. Although the physical relationships between gas content and clast size are not fully understood, empirical data are available for basaltic eruptions. Fountain dynamic structure is determined by the velocity profile at any given pressure level and the maximum spread angle of the fountain from the vertical. These two parameters completely determine the paths of pyroclasts in the fountain and their ultimate resting places. The combination of the pyroclast size and the spatial distribution determines the clast number density and thus the opacity of the fountain and the ability of the pyroclasts to cool in their local fountain environment. For a given set of conditions, two factors thus become important in determining the structure and morphology of pyroclastic deposits: local temperature and accumulation rate . For example, in typical basaltic pyroclastic eruptions, the majority of pyroclasts remain inside the optically thick central part of the fountain, undergo minimal cooling, and return to the surface to coalesce and contribute to a lava pond or lava flow. In the optically thinner outer parts of the fountain, clasts undergo relatively more cooling and return to the surface to contribute to the building of the pyroclastic cone (if the accumulation rate is low) or to form rootless flows (if the accumulation rate is high and minimal further cooling occurs). The relationships between these various parameters are investigated for Hawaiian-style eruptions in general and applied qualitatively to the interpretation of post-eruption deposits.