Ascent and eruption of basaltic magma on the Earth and Moon

Abstract

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...

Geological evolution of the Coombs–Allan Hills area, Ferrar large igneous province, Antarctica: Debris avalanches, mafic pyroclastic density currents, phreatocauldrons

“…• Modelling of basalt flow in the Earth's crust following Wilson & Head (1981) indicates that a vertical cylindrical conduit 10 m in radius can sustain a magma flow of 3.4 x 10 7 kg/s (Table 3) • Mapped plugs at Coombs Hills ≤200 m across: even if full width of a plug not filled with flowing magma at any given time, mass eruption rates sufficient to feed typical flood lavas are readily achievable (mean rate for Roza ∼1.1 x 10 7 kg/s; peak rate for Laki ∼1.2 x 10 7 kg/s; see Self, 1993 andSelf et al, 1997) Clastic dikes (Pocknall et al, 1994) and in Olympus Range area, to the S (Isaac et al, 1996). Sills are also present in two arms of Allan Hills (Grapes et al, 1974).…”

Deep submarine pyroclastic eruptions: theory and predicted landforms and deposits

“…For any chosen magma viscosity and total volatile content there is a critical magma rise speed below which there will be extensive bubble coalescence and strombolian activity will be unavoidable. A computer program to simulate coalescence numerically was developed by Wilson and Head (1981) and Par¢tt and Wilson (1995). We have used this program to ¢nd the maximum magma rise speeds, u s , allowing strombolian activity when the volatile involved is CO 2 and the eruptive vent is at least 1500 m below sea level.…”

“…Wilson and Head (1981) show that the work done against friction after the magma fragments is very small, and so we only need to evaluate the work done against gravity. If we assume that the pressure gradient in the erupting magma is close to the lithostatic gradient in the surrounding host rocks (arguments supporting this assumption are given by Wilson and Head, 1981), and take the host rock density to be b h = 2700 kg m 33 (to allow for a small amount of vesicularity in the ocean £oor crust), then the vertical distance between the magma fragmentation depth and the sea-£oor vent is d f where (P f 3P v ) = (b h g d f ) and the potential energy change per unit mass of mag-…”

“…These rise speeds have implications for the widths of the dikes through which the eruptions are occurring. A magma with a given viscosity R will have a rise speed u s which is a function of the dike width W s and the pressure gradient dP/dz driving the motion (Wilson and Head, 1981) such that in laminar £ow (which can be shown to be the relevant condition for all of the cases above by retrospectively calculating the Reynolds number of the motion):…”

Controls on the explosivity of scoria cone eruptions: Magma segregation at conduit junctions

“…Vergnoille and Jaupart, 1989; Wilson and Head, 1981). To a first approximation, the 150 phase distribution is controlled by the relative momentum fluxes of the two phases, with 151 the higher momentum phase concentrating in the vertical conduit.…”