Hélène Massol

The generation of gas overpressure in volcanic eruptions

Abstract Observations of natural eruption products show that different parts of a single magma batch may experience different degassing histories during ascent in a volcanic conduit. In non-explosive eruptions, lava issuing from a volcanic vent may contain overpressured gas bubbles. These important features of volcanic eruptions cannot be accounted for by existing flow models, which rely on simplifying hypotheses for the relationship between pressures in the gas phase and in the bulk flow. Volcanic flows involve highly compressible material which undergoes large viscosity variations as degassing proceeds. We show that these properties may lead to large gas overpressures in erupting lava. The magnitude of this overpressure depends on the initial volatile content of magma and is largest for relatively volatile-poor magmas, due to the extreme viscosity variations at water contents less than 1 wt%. We develop a simple analytical model to illustrate the main features of compressible viscous flows: (1) at any level, gas pressure is larger near the conduit axis than at the walls, (2) gas overpressure is an increasing function of mass discharge rate.

Ascent and decompression of viscous vesicular magma in a volcanic conduit

During eruption, lava domes and flows may become unstable and generate dangerous explosions. Fossil lava-filled eruption conduits and ancient lava flows are often characterized by complex internal variations of gas content. These observations indicate a need for accurate predictions of the distribution of gas content and bubble pressure in an eruption conduit. Bubbly magma behaves as a compressible viscous liquid involving three different pressures: those of the gas and magma phases, and that of the exterior. To solve for these three different pressures, one must account for expansion in all directions and hence for both horizontal and vertical velocity components. We present a new two-dimensional finite element numerical code to solve for the flow of bubbly magma. Even with small dissolved water concentrations, gas overpressures may reach values larger than 1 MPa at a volcanic vent. For constant viscosity the magnitude of gas overpressure does not depend on magma viscosity and increases with the conduit radius and magma chamber pressure. In the conduit and at the vent, there are large horizontal variations of gas pressure and hence of exsolved water content. Such variations depend on decompression rate and are sensitive to the “exit” boundary conditions for the flow. For zero horizontal shear stress at the vent, relevant to lava flows spreading horizontally at the surface, the largest gas overpressures, and hence the smallest exsolved gas contents, are achieved at the conduit walls. For zero horizontal velocity at the vent, corresponding to a plug-like eruption through a preexisting lava dome or to spine growth, gas overpressures are largest at the center of the vent. The magnitude of gas overpressure is sensitive to changes of magma viscosity induced by degassing and to shallow expansion conditions in conduits with depth-dependent radii.

Conditions for detection of ground deformation induced by conduit flow and evolution

[1]xa0At mature andesitic volcanoes, magma can reach the surface through the same path for several eruptions thus forming a volcanic conduit. Because of degassing, cooling, and crystallization, magma viscosity increase in the upper part of the conduit may induce the formation of a viscous plug. We conducted numerical simulations to quantify the deformation field caused by this plug emplacement and evolution. Stress continuity between Newtonian magma flow and elastic crust is considered. Plug emplacement causes a ground inflation correlated to a decrease of the magma discharge rate. A parametric study shows that surface displacements depend on three dimensionless numbers: the conduit aspect ratio (radius/length), the length ratio between the plug and the conduit, and the viscosity contrast between the plug and the magma column. Larger displacements are obtained for high-viscosity plugs emplaced in large aspect ratio conduits. We find that only tiltmeters or GPS located close to the vent (a few hundred meters) might record the plug emplacement. At immediate proximity of the vent, plug emplacement might even dominate the deformation signal over dome growth or magma reservoir pressurization effects. For given plug thicknesses and viscosity profiles, our model explains well the amplitude of tilt variations (from 1 to 25 μrad) measured at Montserrat and Mt. St. Helens. We also demonstrate that at Montserrat, even if most of the tilt signal is due to shear stress induced by magma flow, pressurization beneath the plug accounts for 20% of the signal.

The relative influence of H2O and CO2 on the primitive surface conditions and evolution of rocky planets

How the volatile content influences the primordial surface conditions of terrestrial planets and, thus, their future geodynamic evolution is an important question to answer. We simulate the secular convective cooling of a 1-D magma ocean (MO) in interaction with its outgassed atmosphere. The heat transfer in the atmosphere is computed either using the grey approximation or using a k-correlated method. We vary the initial CO2 and H2O contents (respectively from 0.1 × 10−2 to 14 × 10−2xa0wtxa0% and from 0.03 to 1.4 times the Earth Ocean current mass) and the solar distance—from 0.63 to 1.30xa0AU. A first rapid cooling stage, where efficient MO cooling and degassing take place, producing the atmosphere, is followed by a second quasi steady state where the heat flux balance is dominated by the solar flux. The end of the rapid cooling stage (ERCS) is reached when the mantle heat flux becomes negligible compared to the absorbed solar flux. The resulting surface conditions at ERCS, including water oceans formation, strongly depend both on the initial volatile content and solar distance D. For D > DC, the “critical distance,” the volatile content controls water condensation and a new scaling law is derived for the water condensation limit. Although todays Venus is located beyond DC due to its high albedo, its high CO2/H2O ratio prevents any water ocean formation. Depending on the formation time of its cloud cover and resulting albedo, only 0.3 Earth ocean mass might be sufficient to form a water ocean on early Venus.

Thermal radiation of magma ocean planets using a 1‐D radiative‐convective model of H2O‐CO2 atmospheres

This paper presents an updated version of the simple 1D radiative-convective H2O-CO2 atmospheric model from Marcq [2012] and used by Lebrun et al. [2013] in their coupled interior-atmosphere model. This updated version includes a correction of a major miscalculation of the outgoing longwave radiation (OLR), and extends the validity of the model (P-coordinate system, possible inclusion of N2, improved numerical stability). It confirms the qualitative findings of Marcq [2012], namely (1) the existence of a blanketing effect in any H2O-dominated atmosphere: the outgoing longwave radiation (OLR) reaches an asymptotic value, also known as Nakajimas limit and first evidenced by Nakajima et al. [1992], around 280W/m2 neglecting clouds, significantly higher than our former estimate from Marcq[2012]. (2) The blanketing effect breaks down for a given threshold temperature Tϵ, with a fast increase of OLR with increasing surface temperature beyond this threshold, making extrasolar planets in such an early stage of their evolution easily detectable near 4μm provided they orbit a red dwarf. Tϵ</suBincreases strongly with H2O surface pressure, but increasing CO2 pressure leads to a slight decrease of Tϵ. (3) clouds act both by lowering Nakajimas limit by up to 40%, and by extending the blanketing effect, raising the threshold temperature Tϵ by about 10%.