Chapter 4 Dynamics of magma ascent in the volcanic conduit Helge M. Gonnermann and Michael Manga Overview ascent rates, even in low viscosity magmas, melt and exsolved gas remain coupled allowing for This chapter presents the various mechanisms rapid acceleration and hydrodynamic fragmen- and processes that come into play within the tation in hawaiian eruptions. volcanic conduit for a broad range of effu- sive and dry explosive volcanic eruptions. Decompression during magma ascent causes 4.1 Introduction volatiles to exsolve and form bubbles contain- ing a supercritical fluid phase. Viscous magmas, In the broadest sense, volcanic eruptions are such as rhyolite or crystal-rich magmas, do not either effusive or explosive. During explosive allow bubbles to ascend buoyantly and may also eruptions magma fragments and eruption inten- hinder bubble growth. This can lead to signifi- sity is ultimately related to the fragmentation cant gas overpressure and brittle magma frag- mechanism and associated energy expenditure mentation. During fragmentation in vulcanian, (Zimanowski et al. 2003). If the cause of frag- subplinian, and plinian eruptions, gas is released mentation is the interaction of hot magma with explosively into the atmosphere, carrying with external water, the ensuing eruption is called it magma fragments. Alternatively, high vis- phreatomagmatic. Eruptions that do not involve cosity may slow ascent to where permeable external water are called “dry,” in which case outgassing through the vesicular and perhaps the abundance and fate of magmatic volatiles, fractured magma results in lava effusion to prod- predominantly H2O and CO2, as well as magma uce domes and flows. In low viscosity magmas, rheology and eruption rate are the dominant typically basalts, bubbles may ascend buoyantly, controls on eruption style. Eruption styles are allowing efficient magma outgassing and rela- often correlated with magma composition and tively quiescent magma effusion. Alternatively, to some extent this relationship reflects differ- bubbles may coalesce and accumulate to form ences in tectonic setting, which also influences meter-size gas slugs that rupture at the surface magmatic volatile content and magma supply during strombolian eruptions. At fast magma rate. Modeling Volcanic Processes: The Physics and Mathematics of Volcanism, eds. Sarah A. Fagents, Tracy K. P. Gregg, and Rosaly M. C. Lopes. Published by Cambridge University Press. © Cambridge University Press 2012. 9780521895439c04_p55-84.indd 55 8/2/2012 9:36:17 AM 56 HELGE M. GONNERMANN AND MICHAEL MANGA 4.1.1 Getting magma to the surface 4.2 Volatiles Volcanic eruptions represent the episodic or continuous surface discharge of magma from Volatiles play the central role in governing the a storage region. We shall refer to this stor- ascent and eruption of magma, as summarized age region as the magma chamber and to the in Figure 4.1. Dissolved volatiles, in particu- pathway of magma ascent as the conduit. For lar water, have large effects on melt viscosity. magma to erupt, the chamber pressure has to Exsolved volatiles form bubbles of supercritical exceed the sum of frictional and magma-static fluids, which we will also refer to as gas or vapor pressure losses, as well as losses associated bubbles. These bubbles allow for significant with opening of the conduit. The latter may magma compressibility and buoyancy, ultim- be due to tectonic forces and/or excess magma ately making eruption possible (Pyle and Pyle pressure, which in turn are a consequence of 1995, Woods and Cardoso 1997). The decrease in magma replenishment and/or volatile exsolu- pressure associated with magma ascent reduces tion in a chamber within an elastically deform- volatile solubility, leading to bubble nucleation ing wall rock. Magma-static pressure loss is and growth, the latter a consequence of both strongly dependent on magma density, which volatile exsolution and expansion (Sparks 1978). is dependent on volatile exsolution and volume Volatile exsolution also promotes microlite crys- expansion as pressure decreases during ascent. tallization, which in turn affects bubble growth Feedbacks between chamber and conduit and rheology – there is a complex feedback can produce complex and unsteady eruptive between decompression, exsolution, and crys- behavior. tallization (Tait et al., 1989). The occurrence and dynamics of explosive eruptions are mediated 4.1.2 The volcanic conduit by the initial volatile content of the magma, and Eruptions at basaltic volcanoes may begin as the ability of gases to escape from the ascending fissure eruptions, with typical dike widths of magma. the order of one meter (Chapter 3; Wilson and Head 1981). However, magma often erupts at the surface through structures that more closely 4.2.1 Solubility resemble cylindrical conduits. The processes by Volatile solubility is primarily pressure depend- which flow paths become focused into discrete ent, with secondary dependence on tempera- cylindrical vents are not well understood. One ture, melt composition, and volatile speciation possibility is that the temperature-dependent (McMillan 1994, Blank and Brooker 1994, viscosity of magma results in a feedback Zhang et al., 2007). Formulations for volatile between a cooling, viscosity increase and reduc- solubility can be thermodynamic or empirical. tion in flow velocity, so that the flow becomes In either case, calibration is achieved using more focused over time (Wylie-Lister 1995). In measurements of dissolved volatile concentra- addition, thermal erosion may also contribute tions in quenched melts, equilibrated with a to flow focusing. (Bruce Huppert 1989). volatile phase of known composition at fixed The conduit during silicic explosive erup- pressures and temperatures. The principal tions is generally thought to be more or less species we consider here are H2O and CO2. cylindrical, at least at shallow depths. However, Solubility of CO2 in silicate melts is propor- the conduit cross-sectional area can change tional to partial pressure, whereas for H2O dis- with depth and time because of wall-rock ero- sociation into molecular H2O and OH results sion, caused by abrasion as pyroclasts collide in a square root dependence on partial pres- with conduit walls, or shear stress from the sure. An empirical formulation for combined erupting magma, or wall collapse (Wilson et al. solubility of H2O and CO2 in rhyolitic melts at 1980, Macedonio et al. 1994, Kennedy et al. 2005, volcanologically relevant conditions is given Mitchell 2005). by (Liu et al., 2005) 9780521895439c04_p55-84.indd 56 8/2/2012 9:36:17 AM DYNAMICS OF MAGMA ASCENT IN THE VOLCANIC CONDUIT 57 5668 − 55.99 p Figure 4.1 Schematic illustration of conduit processes. w 3/2 . Cpcc= ++()0.4133 pww0.002041 p (a) In effusive eruptions of silicic magma (high viscosity, low T ascent rate) gas may be lost by permeable flow through (4.2) porous and/or fractured magma. (b) During (sub)plinian eruptions bubble walls rupture catastrophically at the Here C is total dissolved H O in wt.%, C is dis- fragmentation surface and the released gas expands rapidly w 2 c solved CO in ppm and T is temperature in as the flow changes from a viscous melt with suspended 2 bubbles to gas with suspended pyroclasts. (c) Extensive loss Kelvin. pw and pc are the partial pressures in of buoyantly rising bubbles occurs during effusive eruptions MPa of H2O and CO2, respectively. This formu- of low-volatile content, low-viscosity magma. (d) Coalescence lation is also approximately applicable to other and accumulation of buoyant bubbles, followed by their melt compositions (Zhang et al., 2007), but more rupture at the surface, produces strombolian explosions in accurate models are available (Dixon 1997, slowly ascending low-viscosity magmas. (e) Bubbles remain Newman and Lowenstern 2002, Papale et al., coupled to the melt in low-viscosity, hawaiian eruptions, 2006). Equilibrium concentrations of dissolved which are characterized by relatively high ascent rates, CO and H O, based on this formulation, are hydrodynamic fragmentation, and sustained lava fountaining. 2 2 shown in Figure 4.2. Notice that almost all CO2 exsolves at >100 MPa for rhyolite and at >25 MPa for basalt, whereas most H2O dissolves at 3/2 Cpww=0.0012439 <100 MPa for rhyolite and at <25 MPa for basalt. 354.94 pp+−9.623 1.5223 p3/2 Consequently, processes in the shallow conduit + ww w (4.1) T are predominantly affected by H2O exsolution. +−p 1.08441×−01−−45pp.362 × 10 c ()ww4.2.2 Diffusivity Volatile diffusivities in silicate melts are best and characterized for H2O. A recent formulation for 9780521895439c04_p55-84.indd 57 8/2/2012 9:36:19 AM 58 HELGE M. GONNERMANN AND MICHAEL MANGA 2 –1 H2O diffusivity (m s ) in rhyolite with H2O con- tents � 2 wt % is (Zhang and Behrens 2000) 10661 p DC= exp −−17.14 − 1.772 m . (4.3) wr w T T A formulation for higher H2O concentrations is provided by Zhang et al. (2007), who also give the following equation for H2O diffusivity in basalt 19110 DC= exp −−8.56 . (4.4) wb w T CO2 diffusivity, Dc, is smaller than water diffu- sivity over a range of conditions (Watson 1994). The most reliable formulation for Dc in silicate melts is based on argon diffusivity, which is essentially identical to CO2 at volcanologically relevant conditions (Behrens and Zhang 2001, Nowak et al., 2004) 17367 + 1.0964 p D = −−18.239 m c T ()855.20+ .2712 pCmw + . T 4.2.3 Pre-eruptive volatile content of magmas Water Basaltic magmas show a wide range of water contents ranging from >0.5 wt.% to 6–8 wt.% (Johnson et al., 1994, Wallace 2005), with non- arc basalts generally being considerably dryer than arc magmas, which have highly variable water contents. Silicic magmas also show a wide range of water contents up to 6 wt.%, and pos- sibly higher (Carmichael 2002). Figure 4.2 Solubility of CO2 and H2O (dashed contours) and equilibrium solubility degassing paths (solid lines) Carbon dioxide for typical rhyolitic and basaltic magmas undergoing CO2 content of magmas is more difficult to con- closed-system dagassing.