Excerpt from the Proceedings of the COMSOL Conference 2009 Boston Coupled Models of Lithospheric Flexure and Magma Chamber Pressurization at Large Volcanoes on Venus Gerald A. Galgana1*, Patrick J. McGovern1, and Eric B. Grosfils2 1Lunar and Planetary Institute, USRA; 2Geology Department, Pomona College *Corresponding author, postal address: 3600 Bay Area Blvd., Houston, TX; email: [email protected] Abstract: We present an implementation of the Structural Mechanics module of COMSOL 1. Introduction to Venus Volcano Morphology Multiphysics to model the state of stress associated with the emplacement of large volcanic edifices on The topography of the planet Venus is hidden the surface of a planet (in this paper, the planet beneath its thick and opaque atmosphere. Venus). These finite element models capture two Nonetheless, the Magellan spacecraft, equipped essential physical processes: (1) Elastic flexure of with Synthetic Aperture Radar (SAR), has imaged the lithosphere (the mechanically strong outer layer the surface of Venus at high resolution to reveal of a planet) beneath the edifice load, and (2) over 150 large volcanic edifices with lava flow Pressurization of a magma-filled chamber that diameters greater than 100 km [1, 2]. Identified on serves as the supply source for the lava flows that the basis of conical to domical topographic highs of build the edifice. We demonstrate that the several km and numerous digitate lava flows that COMSOL model adequately represents the form broad, flat aprons surrounding the edifices, response of an elastic plate to loading by these volcanoes are analogous to (but generally benchmarking against an analytic axisymmetric somewhat broader than) the gently sloped Hawaiian flexure solution. We present results and preliminary volcanoes on Earth. interpretations of model stress states in terms of locations of predicted chamber wall failure and For instance Sif Mons, a 2 km high by 200 km orientations of magma bodies (intrusions) radius volcano on Venus, is characterized by gentle emanating from the failed chamber. one to three degree slopes around the main edifice. These slopes evolve into lower slants of less than 1 Key words: Geomechanics, volcano-tectonics, degree on its outer flow apron [3]. crustal deformation, volcanoes, planetary science. Figure 1. Perspective view of Maat Mons, the tallest volcano on Venus (height ~ 9 km above mean planetary radius). Synthetic Aperture Radar (SAR) backscatter image draped over DEM from altimeter. Vertical exaggeration is 25 x. Original data are from NASA’s Magellan mission. The main summit is dotted with calderae, radial pit d4 w chains, and fractures. Similarly, Maat Mons (Figure D 4 = −()ρm − ρ w gw [1] 1) features a significantly higher summit caldera, dx with the same broad, gentle slopes characterized by radial sheet and digitate lava flows. Both volcanoes, where w is the deflection, x is the distance from the and many others like them, clearly add significant loading point, D is the flexural rigidity, g is the 2 loads to the pre-edifice surface. gravitational constant (g = 8.87 m/s for Venus), ρm and ρw are the density of the elastic lithosphere and the overlying mass. The amount of deflection 2. Theory of Volcanic Deformation and Elastic throughout the section is described as a function of Flexure distance from the point of loading: Complementing field and analogue investigations, V α 3 −x volcanic deformation on Earth has been explored in w() x = o eα []cos( xα )+ sin( x α ) [2] the past using analytical models. Inflating magma 8D sources are commonly approximated as point, ellipsoidal, and crack-like sources of deformation, where Vo is the load, and α is the flexural relating surface displacement to magma chamber parameter. The flexural parameter (α) and the pressure or volume change [i.e., 4, 5, and 6]. flexural rigidity (D) are described by the following equations: In addition, numerical finite element models 4/1 provide the opportunity to explore further details, ⎡ 4D ⎤ for instance by adding the effects of layering at α = ⎢ ⎥ [3] various scales, varying reservoir geometry, or ⎣ g()ρm ρ w ⎦ introducing the delayed pressure response caused by viscoelastic aureoles around terrestrial magma chambers [e.g.,7 – 13]. Use of numerical techniques is constantly evolving; as an example, elaborate 3 ETe [4] models introduced recently include coupling of D = 2 physical parameters such as thermal effects on 12(1−υ ) strain [e.g., 14]. Both analytical and numerical approaches have Where Te is the thickness of the elastic lithosphere, been used to explore the effects of an overlying E is Young’s Modulus, and υ is the Poisson’s ratio. volcanic edifice load, and several authors have analyzed the resulting stress patterns around More complete analytic models of plate flexure, the magma reservoirs and the implications for reservoir “thick-plate” formulations, account for shear and stability [e.g., 15, 16]. Recent models of magma vertical normal stresses within the plate. We will chamber stability, which incorporate gravitational use such a solution, the axisymmetric formulation loading and lithostatic pre-stresses, have refined of Comer [20], to benchmark our finite element these efforts [e.g., 17], but until now interactions solution (see below). between magma chambers and flexural stress states have not been examined. In a flexing lithosphere beneath the load, the upper part experiences horizontal compression while the The loading effect of a volcanic edifice on the lower part experiences horizontal extension: a lithosphere can be considered as a flexing beam or “dipole” state of stress. The central part is a low- plate problem in mechanics. The analytical flexure stress/strain region called the neutral plane. The equation [18, 19], using a “thin-plate” approxi- maximum deflection occurs at the point near the mation that simplifies the mathematics, shows the load, which steadily decreases towards the hinge relationship of a point load with the deflection of an line (termed the forebulge, which is a topographic elastic lithosphere (or alternatively, any loaded high), a distance from the point of loading. The beam or plate): distant section of the lithosphere experiences negligible deformation. 3. Method here as the onset of tensile (or positive) stress values. We implement a series of numerical experiments with the COMSOL Multiphysics Structural Using these parameters, COMSOL solves for Mechanics Module [21], in order to evaluate how a stresses in and deformation of the lithosphere, flexural stress state changes the characteristic including stresses along the magma chamber wall. response of a pressurized, inflating magma From the results, we examine the stress distribution chamber. We consider variations in both the of the lithosphere, and determine the relationship of lithospheric stress state (modulated by elastic flexure and reservoir pressurization. lithosphere thickness Te and volcanic load magnitude) and magma chamber stresses (controlled by chamber radius, magma density and overpressure). The depth at which the chamber is embedded in the lithosphere is a critical parameter, as it affects the characteristic stress state (horizontal extension or compression) encountered by the chamber. We use the COMSOL Structural Mechanics Module to build a two-dimensional axisymmetric elastic model of the Venusian lithosphere, loaded with a large, conical volcanic edifice of radius 200 km and height 5 km (Figure 2). This volcano is situated at the center of a model domain extending Figure 2. A close-up of a portion of the finite element out to 900 km. We adopt two elastic lithosphere mesh near the symmetry axis, for the thin lithosphere thicknesses (i.e., Te = of 20 and 40 km), model with a three-kilometer deep magma chamber. This representing the best estimates of elastic thickness model comprises about 17,100 triangular elements. The of the Venusian lithosphere. This is consistent with spatial resolution of individual elements increases toward values derived from topographic profiles and the curved magma chamber. topography/gravitational admittance models for Venus [22, 23]. The material parameters of the 4. Benchmark lithosphere and the volcano correspond to those adopted by McGovern and Solomon [2]. To demonstrate the ability of COMSOL Multiphysics to model flexure, we benchmark a The effects of gravitational loading are added to the simplified model (lacking a structural edifice and model through body forces, including “lithostatic” magma chamber) to the analytic thick-plate pre-stress. The response of the asthenosphere (or axisymmetric flexure solution of Comer [20]. the mechanically weak, viscously deforming zone beneath the lithosphere) is modeled as a buoyant In the COMSOL benchmark model, the load of the fluid with “Winkler”-type restoring forces applied edifice is represented as a sum of point loads to the base of the model. A constant force applied to the surface of the plate, rather than as a counteracting the vertical pre-stress is also applied stress-bearing mechanical extension of the model at the base. The volcano edifice in each model acts domain as in the full coupled models. This ensures as a distributed load on the surface, and bends the an “apples-to-apples” comparison with the analytic lithosphere. Vertical roller conditions are applied models, in which the edifice load is also not stress- at the outer boundary of the model to constrain bearing. horizontal motions at that end. In the benchmark model, the load is equivalent to A pressurized