Geological and physical observations and constraints are applied to the development of a model of the ascent and emplacement of basaltic magma on the earth and moon. Mathematical models of the nature and motion of gas/liquid mixtures are developed and show that gas exsolution from terrestrial and lunar magmas commonly only occurs at shallow depths (less than 2 km); thus the ascent of bubble‐free magma at depth can be treated separately from the complex motions caused by gas exsolution near the surface. Magma ascent is related to dike or conduit width; a lower limit to width is determined by the presence of a finite magma yield strength or by excessive magma cooling effects related to magma viscosity. For terrestrial basalts with negligible yield strengths and viscosities greater than 102 Pa s, widths in the range 0.2–0.6 m are needed to allow eruptions from between depths of 0.5–20 km. Fissure widths of about 4 m would be needed to account for the output rates estimated for the Columbia River flood basalt eruptions. As the magma nears the surface, bubble coalescence will tend to occur, leading to intermittent explosive strombolian‐style activity. For commonly occurring lunar and terrestrial basalts the magma rise speed must be greater than 0.5–1 m/s if strombolian activity is to be avoided and relatively steady fire fountaining is to take place. Terrestrial fire fountain heights are dictated by the vertical velocity of the magma/gas dispersion emerging through the vent, increasing with increasing magma gas content and mass eruption rate, and decreasing with increasing magma viscosity. Terrestrial fire fountain heights up to 500 m imply the release of up to 0.4 wt % water from the magma, corresponding to initial water contents up to 0.6 wt %. The presence of extremely long lava flows and sinuous rules on the moon has often been cited as evidence for very high extrusion rates and thus a basic difference between terrestrial and lunar magmas and crustal environments. However, the differences between terrestrial and lunar magma rheologies and crustal environments do not lead to gross differences between the effusion rates expected on the two planetary bodies, for similar‐sized conduits or fissures. Thus the presence of these features implies only that tectonic and other forces associated with the onset of some lunar eruptions were such as to allow wide fissures or conduits to form. The surface widths of elongate fissure vents need be no wider than 10 m to allow mass eruption rates up to 10 times larger than those proposed for terrestrial flood basalt eruptions; 25‐m widths would allow rates 100 times larger. It therefore appears unlikely that source vents on the moon with widths greater than a few tens of meters represent the true size of the unmodified vent. The main volatile released from lunar magmas was probably carbon monoxide, released in amounts proportionally less than terrestrial magmas by more than an order of magnitude. However, decompression to the near‐zero ambient lunar atmospheric pressure causes much g...

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Plinian air-fall deposits and ignimbrites are the principal products of explosive eruptions of hgh viscosity magma. In this paper, the flow of gas/ pyroclast dispersions and high viscosity magma through various magma chamber/conduit/vent geometries is considered. It is argued that after the first few minutes of an eruption magma fragmentation occurs at a shallow depth within the conduit system. Gas pressures at the fragmentation level are related to exsolved gas contents by consideration of the exsolution mechanism.The sizes of blocks found near vents imply that gas velocities of 200 to 600 m s-' commonly occur. These velocities are greater than the effective speed of sound in an erupting mixture (90-200ms-') and the transition from subsonic to supersonic flow is identified as occurring at the depth at which the conduit has its minimum diameter. The range of values of this minimum diameter (-5 to -100 m) is estimated from observed and theoretically deduced mass-eruption rates.The energy and continuity equations are solved, taking account of friction effects, for numerous geometries during the evolution, by wall erosion, of a conduit. Conduit erosion ceases, near the surface, when an exit pressure of one atmosphere is reached. Eruption velocities are found to depend strongly on exsolved magma gas content and weakly on radius of conduit and friction effects. Assuming water as the main volatile phase, velocities of 400-600 m s-l for plinian events imply magma water contents of 3-6 per cent by weight, Three scenarios are presented of eruptions in which: (1) conduit radius increases but gas content remains constant; (2) conduit radius increases and gas content decreases with time; and (3) conduit radius remains fured and gas content decreases. These models demonstrate that the reverse grading 118 L. Wilson, R. S. J. Sparks and G. P. L. Walker commonly observed in plinian air-fall deposits is primarily a consequence of conduit erosion, which always results in increasing eruption intensity and eruption column height with time. The models also show that a decrease in gas content as deeper levels in a magma chamber are tapped or an increasing vent radius as conduit walls are eroded leads to the prediction of a progression from air-fall activity through ignimbrite formation to cessation of eruption and caldera collapse.

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 kilomete...

[1] 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 Tharsisradial 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 À3 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 c...

Volcanic activity is a fundamental mechanism of heat transfer from planetary interiors, and the characteristics, distribution, and morphology of volcanic deposits provide significant insight into (1) the relation of volcanism and tectonism, (2) eruption style, and (3) the chemistry and volatile content of the eruption products. Eruption styles and processes on the planets are known to be strongly influenced by such factors as gravity, temperature, and atmospheric characteristics. We model the ascent and eruption of magma on Venus in the current Venus environment, taking into account the influence of the extreme surface temperatures (650–750 K) and pressures (4–10 MPa) on these processes. These conditions produce a thermal gradient difference such that the temperature is higher at a given depth on Venus than on Earth, and a pressure distribution difference leading to much smaller ratios of subsurface to surface pressure on Venus than on Earth. Among the more significant consequences of this for volcanic style on Venus is that there will be less cooling of magma in the final stages of ascent and that once the magma reaches the surface, convective heat losses will be much more important than in the subaerial terrestrial environment because of the high atmospheric gas density. In general, however, our treatment suggests that there is no reason to expect large systematic differences between lava flow morphologies on Venus and Earth. On the other hand, conditions on Venus will tend to inhibit the subsurface exsolution of volatiles, and pyroclastic eruptions involving continuous magma disruption by gas bubble growth may not occur at all unless the exsolved magma volatile content exceeds several weight percent. However, Strombolian activity, in which bubble coalescence can cause sufficient concentration of gas to produce intermittent explosions in low‐viscosity magmas ascending slowly toward the surface, can occur at much lower volatile contents. If pyroclastic eruptions do occur, pyroclastic fragment velocities and clast cooling will be less than on Earth, and the higher atmospheric pressure and temperature will cause convective cloud rise heights to be considerably lower, and pyroclasts to be much less widely dispersed, than on Earth. For example, eruption cloud heights of 50 km (suggested as a means of raising sulfur dioxide into the upper atmosphere (Esposito, 1984)) could only be reached if exsolved magma volatile contents exceeded 4 wt %, regardless of gas species. On the basis of our analysis, a series of predictions can be made concerning the expected characteristics of volcanic deposits and landforms on Venus. Comparison of these predictions with recent observations from Pioneer Venus, Arecibo, and Venera data support the view that regional pyroclastic deposits are very rare, that magma volatile contents do not commonly exceed about 4 wt %, and that the atmospheric pressure has been about the same as the present value over a time period equivalent to the average age of the northern areas of the northern hemisphe...

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