A mathematical model for 2D heat transfer dynamics in fluid systems with localized sink of magmatic fluid into local fractured zones above the top of crystallizing intrusions

A model describing two-dimensional (2D) dynamics of heat transfer in the fluid systems with a localized sink of a magmatic fluid into local fractured zones above the roof of crystallizing crustal intrusions is suggested. Numerical modeling of the migration of the phase boundaries in 2D intrusive chambers under retrograde boiling of magma with relatively high initial water content in the melt shows that, depending on the character of heat dissipation from a magmatic fluid into the host rock, two types of fluid magmatic systems can arise. (1) At high heat losses, the zoning of fluidogenic ore formation is determined by the changes in temperature of the rocks within the contact aureole of the intrusive bodies. These temperature variations are controlled by the migration of the phase boundaries in the cooling melt towards the center of the magmatic bodies from their contacts. (2) In the case of a localized sink of the magmatic fluid in different parts of the top of the intrusive chambers, a specific characteristic scenario of cooling of the magmatic bodies is probably implemented. In 2D systems with a heat transfer coefficient ?? _k < 5 × 10⁴ W/m² K, an area with quasi-stationary phase boundaries develops close to the region of fluid drainage through the fractured zone in the intrusion. Therefore, as the phase boundaries contract to the sink zone of a fluid, specific thermal tubes arise, whose characteristics depend on the width of the fluid-conductive zone and the heat losses into the side rocks. (3) The time required for the intrusion to solidify varies depending on the particular position of the fluid conductor above the top of the magmatic body.     

Incipient magma chamber formation as a result of repetitive intrusions

An analytical solution for periodic magma intrusions in conduits was developed to study the onset of shallow magma chamber formation. The solution is based on determining when a repetitive series of intrusions can cause the wall rock of a conduit to reach its melt temperature. The results show that magma chamber formation in conduits is a strong function of the volume rate of intrusion and that magma chamber formation is likely when the intrusion rate exceeds 10^?3 km³/ yr. which agrees with observations by other investigators. Once this critical value of intrusion rate is reached, magma chambers are likely to begin forming after only a few intrusive pulses (less than ten). Results for both cylindrical conduits and dikes show cylindrical conduits are more favourable for the formation of shallow magma chambers.     

Intrusion of ultramafic magmatic bodies into the continental crust: Numerical simulation

Intrusions of ultramafic bodies into the lower density continental crust are documented for a large variety of tectonic settings spanning continental shields, rift systems, collision orogens and magmatic arcs. The intriguing point is that these intrusive bodies have a density higher by 300-500 kg m⁻³ than host rocks. Resolving this paradox requires an understanding of the emplacement mechanism. We have employed finite differences and marker-in-cell techniques to carry out a 2D modeling study of intrusion of partly crystallized ultramafic magma from sublithospheric depth to the crust through a pre-existing magmatic channel. By systematically varying the model parameters we document variations in intrusion dynamics and geometry that range from funnel- and finger-shaped bodies (pipes, dikes) to deep seated balloon-shaped intrusions and flattened shallow magmatic sills. Emplacement of ultramafic bodies in the crust lasts from a few kyr to several hundreds kyr depending mainly on the viscosity of the intruding, partly crystallized magma. The positive buoyancy of the sublithospheric magma compared to the overriding, colder mantle lithosphere drives intrusion while the crustal rheology controls the final location and the shape of the ultramafic body. Relatively cold elasto-plastic crust (T_Moho = 400 °C) promotes a strong upward propagation of magma due to the significant decrease of plastic strength of the crust with decreasing confining pressure. Emplacement in this case is controlled by crustal faulting and subsequent block displacements. Warmer crust (T_Moho = 600 °C) triggers lateral spreading of magma above the Moho, with emplacement being accommodated by coeval viscous deformation of the lower crust and fault tectonics in the upper crust. Strong effects of magma emplacement on surface topography are also documented. Emplacement of high-density, ultramafic magma into low-density rocks is a stable mechanism for a wide range of model parameters that match geological settings in which partially molten mafic-ultramafic rocks are generated below the lithosphere. We expect this process to be particularly active beneath subduction-related magmatic arcs where huge volumes of partially molten rocks produced from hydrous cold plume activity accumulate below the overriding lithosphere.     

Evidence of an early alteration process driven by magmatic fluid in Mururoa volcano

Evidence for a deuteric alteration process induced by a magmatic fluid has been found in the feeder zone of the Mururoa volcano (French Polynesia). Within the dikes, where basaltic glass does not show any evidence of pervasive alteration, vesicles are filled with dioctahedral smectites and calcite, while olivine phenocrysts are replaced by dioctahedral smectites, ankerite and calcite.The ¹³C signature of carbonates, the carbon and H₂O content of the whole rocks and their impoverishment in deuterium are compatible with the presence of magmatic CO₂ during the crystallization of intruding lavas and exclude contamination by seawater. Mass balance calculations on selected thin sections photographs of partly filled up vesicles and replaced olivine crystals, constrain, assuming a closed system interaction, the chemical composition of the initial fluid and the respective amounts of the initial solid phases involved in the alteration process. Thermodynamic modelings using the EQ3/6 software package correctly predict the mineralogic, chemical and isotopic exchanges accompanying alteration, thus validating the closed system assumption. The model which allows prediction of the influence of CO₂ on the alteration products, shows that, above a 0.25 CO₂ mole fraction in the initial fluid, the alteration is entirely controlled by the chemical composition of the initial solid phases. The presence of CO₂ implies the precipitation of dioctahedral smectites and carbonates instead of the magnesian smectites commonly observed in CO₂-free systems.The Mururoa feeder zone shows alteration features typical of a closed system interaction between the basaltic rock and a magmatic fluid in which seawater did not take part.     

Numerical Analysis of Thermal Remediation in 3D Field‐Scale Fractured Geologic Media

Thermal methods are promising for remediating fractured geologic media contaminated with volatile organic compounds, and the success of this process depends on the coupled heat transfer, multiphase flow, and thermodynamics. This study analyzed field‐scale removal of trichloroethylene (TCE) and heat transfer behavior in boiling fractured geologic media using the multiple interacting continua method. This method can resolve local gradients in the matrix and is less computationally demanding than alternative methods like discrete fracture‐matrix models. A 2D axisymmetric model was used to simulate a single element of symmetry in a repeated pattern of extraction wells inside a large heated zone and evaluate effects of parameter sensitivity on contaminant recovery. The results showed that the removal of TCE increased with matrix permeability, and the removal rate was more sensitive to matrix permeability than any other parameter. Increasing fracture density promoted TCE removal, especially when the matrix permeability was low (e.g., <10^?17 m²). A 3D model was used to simulate an entire treatment zone and the surrounding groundwater in fractured material, with the interaction between them being considered. Boiling was initiated in the center of the upper part of the heated region and expanded toward the boundaries. This boiling process resulted in a large increase in the TCE removal rate and spread of TCE to the vadose zone and the peripheries of the heated zone. The incorporation of extraction wells helped control the contaminant from migrating to far regions. After 22 d, more than 99.3% of TCE mass was recovered in the simulation.     

Effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies

What controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood. I propose that for volcanoes at divergent plate boundaries, these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them. This stress concentration is mainly a function of the applied tensile stress, associated with spreading, and the aspect (height/width) ratios of the chambers. High spreading rates and/or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies. It is found that for chambers at the same depth in a volcanic zone, the one of the highest aspect ratio will normally intrude magma most frequently. Also, if the chambers are of equal aspect ratios, the one at the greatest depth will intrude magma most frequently. Because the extrusion frequency of a volcano is a fraction of its intrusion frequency, the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano. These results are applied to the divergent plate boundary in Iceland, where many of the volcanoes appear to be fed by “double chambers”, that is, shallow chambers fed by deep-seated chambers. It is found that, except when the aspect ratio of the deep-seated chamber is much less than that of the shallow chamber, the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep-seated chamber. It is concluded that whereas the short-term (i.e., ≤10³ yrs) extrusion frequencies of volcanoes at the plate boundary in Iceland may be similar to the dike intrusion frequencies of the source chambers, the long-term (i.e., ≥10⁴ yrs) extrusion frequencies may be about ten times lower than the intrusion frequencies.     

Oxygen isotopic evidence for meteoric water interaction with the Captains Bay pluton,Aleutian Islands

Low¹⁸O/¹⁶O values have been found in rocks from the Captains Bay pluton and surrounding country rocks on Unalaska Island in the Aleutian Islands. The pluton is surrounded by a metamorphic aureole in which the country rocks have been domed, faulted and chaotically fractured. Samples with lower¹⁸O/¹⁶O values (δ¹⁸O = ?4.1 to +2.7‰, SMOW) commonly have interstitial micrographic intergrowths of quartz and turbid potassium feldspar. These facts suggest extensive interaction of circulating meteoric waters with the pluton. Unalaska Island must have been subaerially exposed at or shortly after the time of intrusion of the pluton. The pervasive low-grade metamorphism of the “early series” volcanic rocks of all of the Aleutian Islands might be a result of the interaction of intrusive bodies with meteoric waters rather than the consequence of ridge subduction.     

Short intense bursts in magmatic activity in the south of Siberian Platform (Angara-Taseeva depression): the paleomagnetic evidence

Based on the paleomagnetic study of intrusive and explosive Permian-Triassic traps in the Angara River basin, Siberian Platform, it is established that the formation of the traps was marked by three short and highly intense bursts in magmatic activity, which resulted in the intrusion of three large dolerite sills (Tolstomysovsky, Padunsky and Tulunsky) and the deposition of the tuffs of the Kapaevsky Formation. These magmatic bursts occurred against the long-lived less intense background magmatism, which caused the formation of small intrusive bodies and tuff sequences. The geochronological data and correlation of the Angara traps to the effusive trap sequences in the north of the Siberian Platform (Norilsk and Maymecha-Kotuy regions) indicate that intrusion of the Tolstomysovsky sill and eruption of its comagmatic tuffs of the Kapaevsky Formation occurred in the Early Triassic. The obtained paleomagnetic data contradict the existing idea that the Padunsky and Tulunsky sills are coeval. Moreover, these data show that the magmatic bodies of different ages were mistakenly referred to the same sill.     

Distribution of uranium in intrusive bodies due to combined migration and diffusion

Uranium is enriched in the border zones of magmatic bodies and the enrichment is believed to be caused by the migration of hydrous solutions which carry that element along intergranular paths towards the contact zone with the wall rock. We propose that the contact zone is a geochemical barrier at which the uranium, present in the solution, would be deposited if it were not for diffusion away from the increased concentration at the margins.The two particle flows, the one caused by migration and the other caused by diffusion, can be described by a differential equation, whose solution is the concentration of uranium as a function of time, diffusion coefficient and velocity of migration.The distribution of uranium in two intrusive bodies, the Mont Blanc granite (Swiss Alps) and a pluton in the Dshetui-Oguz massif (U.S.S.R.), gives the following parameters: duration of process 0.3–1 m.y., diffusion coefficient 4 × 10^?4 to 5 × 10^?4 cm²/s, and velocity of migration 0.1–0.3 cm/year.The combined process of migration and diffusion is assumed to be an important mechanism for controlling the distribution of uranium throughout the earth's crust and for its change in geological time.     

Subsurface Temperature Changes Due to the Crustal Magmatic Activity – Numerical Simulation

Geothermal aspects of the hypothesis, relating the earthquake swarms in the West Bohemia/Vogtland seismoactive region to magmatic activity, are addressed. A simple 1-D geothermal model of the crust was used to assess the upper limit of the subsurface heating caused by magma intrusion at the assumed focal depth of 9 km. We simulated the process by solving the transient heat conduction equation numerically, considering the heat of magma crystallization to be gradually released in the temperature interval 1100°C to 900°C. The temperature field prior to the intrusion was in steady-state with a surface temperature of 10°C and heat flow of 80 mWm ^–2 , the temperature at the 9 km depth was 270°C. The results suggest that the temperature and heat flow in the uppermost 1 km of the crust begin to grow 100 ka after the intrusion emplacement only, and that the amplitudes of the changes for the realistic lateral extent (a few kilometres) of the intrusion are very small. It was also found that the rate of magma solidification depends strongly on the thickness of the intrusion. It takes about 100 years for a 50 m thick sill to cool down from 1100°C to 600°C, which value represents the lower limit of the solidus temperature. The same cooling takes only 60 days if the sill is 2 m thick. If the nature of the strongly reflected boundaries, interpreted from the January 1997 Nový Kostel seismograms, is connected with the fresh emplacement of magma, the calculated cooling rates have a predictive potential for the temporal changes of the waveforms.     

The extraction of heat from magmas based on heat transfer mechanisms

Analytical heat transfer calculations are used to relate geological surface evidence to conditions that should exist in magma chambers for the purpose of improving estimates of possible commercial heat extraction rates. These calculations indicate that an upward-melting magma system necessarily is approximately equidimensional and that injected magmas with very high aspect (L/D) ratios are likely formed by a forced intrusion process which involves little if any melting or natural convection. Calculations along with surface heat flow measurements suggest that steady-state heat extraction rates for emplaced heat exchangers in currently suspected shallow magma chambers will probably be below 10 kW m⁻², a value that is low by engineering standards.     

2-D Crustal Velocity Structure Along Hirapur-Mandla Profile in Central India: An Update

The 2-D crustal velocity model along the Hirapur-Mandla DSS profile across the Narmada-Son lineament in central India (Murty et al., 1998) has been updated based on the analysis of some short and discontinuous seismic wide-angle reflection phases. Three layers, with seismic velocities of 6.5–6.7, 6.35–6.40 and 6.8 km s^–1, and upper boundaries located approximately at 8, 17 and 22 km depth respectively, have been identified between the basement (velocity 5.9 km s^–1) and the uppermost mantle (velocity 7.8 km s^–1). The layer with 6.5–6.7 km s^–1 velocity is thin and is less than 2-km deep between the Narmada north (at Katangi) and south (at Jabalpur) faults. The upper crust shows a horst feature between these faults, which indicates that the Narmada zone acts as a ridge between two pockets of mafic intrusion in the upper crust. The Moho boundary, at 40–44 km depth and the intra-crustal layers exhibit an upwarp suggesting that the Narmada faults have deep origins, involving deep-seated tectonics. A smaller intrusive thickness between the Narmada faults, as compared to those beyond these faults, suggests that the intrusive activities on the two sides are independent. This further suggests that the two Narmada faults may have been active at different geological times. The seismic model is constrained by 2-D gravity modeling. The gravity highs on either side of the Narmada zone are due to the effect of the high velocity/high density mafic intrusion at upper crustal level.     

Numerical simulation of the detachment dynamics in North China Basin

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New geological insights and structural control on fluid circulation in La Fossa cone (Vulcano,Aeolian Islands,Italy)

Electric resistivity tomography (ERT), self-potential (SP), soil CO₂ flux, and temperature are used to study the inner structure of La Fossa cone (Vulcano, Aeolian Islands). Nine profiles were performed across the cone with a measurement spacing of 20 m. The crater rims of La Fossa cone are underlined by sharp horizontal resistivity contrasts. SP, CO₂ flux, and temperature anomalies underline these boundaries which we interpret as structural limits associated to preferential circulation of fluids. The Pietre Cotte crater and Gran Cratere crater enclose the main hydrothermal system, identified at the centre of the edifice on the base of low electrical resistivity values (<20 Ω m) and strong CO₂ degassing, SP, and temperature anomalies. In the periphery, the hydrothermal activity is also visible along structural boundaries such as the Punte Nere, Forgia Vecchia, and Palizzi crater rims and at the base of the cone, on the southern side of the edifice, along a fault attributed to the NW main tectonic trend of the island. Inside the Punte Nere crater, the ERT sections show an electrical resistive body that we interpret as an intrusion or a dome. This magmatic body is reconstructed in 3D using the available ERT profiles. Its shape and position, with respect to the Pietre Cotte crater fault, allows replacing this structure in the chronology of the development of the volcano. It corresponds to a late phase of activity of the Punte Nere edifice. Considering the position of the SP, soil CO₂ flux, and temperature maxima and the repartition of conductive zones related to hydrothermal circulation with respect to the main structural features, La Fossa cone could be considered as a relevant example of the strong influence of pre-existing structures on hydrothermal fluid circulation at the scale of a volcanic edifice.     

基于孔隙网络模型的非牛顿流体驱替模式研究

本文提出了一种考虑了流体可压缩性与非牛顿流体剪切流变性的非稳态孔隙网络模型,用以研究非混相驱替中的毛管指进、黏性指进和两者之间的过渡区这三种不同的驱替模式.通过在孔隙网络模型中饱和非牛顿流体,研究不同注入毛管数和不同黏度比情况下的注水驱替模式.结果表明,非稳态孔隙网络模型可以模拟再现三种不同的流体驱替模式,并提出特征前缘流量用于区分三种驱替模式.毛管指进的入口压力的相对波动较大且手指的无序生长会抑制前缘的移动速度,因此特征前缘流量变化较小.黏性指进会抑制指进横向生长和向入口方向的流动,入口压力在突破时快速降低.在过渡区,驱替流体倾向于占据由更大半径孔喉组成的少量路径,并以更大的特征流量向出口处快速突破,导致更细的手指与较低的驱替效率.本文还对比了被驱替相为牛顿流体时的情况.结果表明,剪切流变性会减弱被驱替相黏性阻力的影响,从而使得过渡区的范围更宽.本研究有助于更好地研究复杂流体间黏性力与毛管力对驱替模式的影响,对于提高油气采收率和二氧化碳地质封存效率具有现实意义.

     

Refroidissement d’un système de dykes et d’une chambre magmatique sous-jacente

In volcanic areas occurring in zones of extension, basaltic magma rises up to the surface, through fissures, to form dykes which in depth are connected with one or several magmatic chambers located in the crust or the upper mantle. Starting form this geological situation, we propose models of increasing complexity to study cooling by heat conduction of a system composed of parallel dykes and underlying magmatic chamber. This work has been carried out 1) by numerical methods which take into account the variation with temperature of the thermic parameters and 2) by using an analytical solution of the Fourier equation (an initial fictive temperature is then calculated). The thermic individuality of the dykes disappears quickly and the dyke system may be replaced by a single « intrusion » which cools slowly from a temperature = =2/β θ₀ much lower than θ₀ (θ₀=initial temperature; 1/β=injection density). The temperature gradient due to the dyke system has been estimated for different time intervals after the intrusion. For the calculation of the thermal effect of a magmatic chamber, we have taken into account its size, depth and age. Numerical application for appropriate geological cases have been carried out and allow one to estimate the respective effects of dykes and magma chamber.     

Badrinath-Gangotri plutons (Garhwal, India): petrological and geochemical evidence for fractionation processes in a high Himalayan leucogranite

The Gangotri leucogranite is the western end of the Badrinath granite, one of the largest bodies of the High Himalayan Leucogranite belt (HHL). It is a typical fine grained tourmaline + muscovite ± biotite leucogranite. The petrography shows a lack of restitic phases. The inferred crystallization sequence is characterized by the early appearance of plagioclase, quartz and biotite and by the late crystallization of the K-feldspar. This suggests that, in spite of being of near minimum melt composition, the granite probably had long crystallization or melting interval, in agreement with previous experimental studies. Tourmaline and muscovite have a mainly magmatic origin. Even though the major element composition is homogeneous, there are several geochemical trends (when CaO decreases there is an increase in Na₂O, Rb, Sn, U, B, F and a decrease in K₂O, Fe₂O₃, TiO₂, Sr, Ba, Zr, REE, Th) which are best explained by a fractionation process with early crystallizing phases. Experimental solubility models for zircon and monazite in felsic melt support a magmatic origin for these two accessory phases as well.Rb/Sr isotope data show this granite to have, like other HHL, heterogeneous isotopic values for Sr (initial ⁸⁷Sr/⁸⁶Sr ratios, calculated at 20 Ma, range between 0.765 and 0.785). Therefore no mixing (i.e. no convection) occurred between the different batches of magma. In contrast ¹⁸O data show little variation (13.04% ± 0.25), implying a source with homogeneous ¹⁸O values. Differences in timing between fluid infiltration and the onset of melting, related to differences in temperature of the source, could explain why source homogenization occurred for the Gangotri and not for the Manaslu granite.The use of experimental results for solubility and the position of the accessory minerals during melting, predict a low viscosity for the melt during its extraction. This in turn explains the lack of restitic phases (major and accessory) in the granite as well as some field features (lensoid shape, pronounced magmatic layering). Based on the petrographic and isotopic studies, it is suggested that the mechanism of ascent was not diapiric but rather that the melt ascended along several fractures and the level of emplacement was partialy controlled by the density contrast between the melt and host rocks.     

南海礼乐盆地新生代构造热演化特征及其影响因素

为深入认识新生代礼乐盆地的热体制特征,利用耦合岩石圈变形、热演化和沉积过程的热力学数值模型,重建了8条骨干剖面的构造热演化史,并对主要构造单元的热体制进行了分析.结果表明：张裂阶段,热流总体上随时间增加,张裂结束时,海底热流一般介于70~80 mW·m^-2,基底浅埋区热流高于邻近凹陷区内热流;裂后阶段,非礁体发育区热流逐渐降低,现今海底热流一般介于65~70 mW·m^-2,局部区域热流因岩体侵位而有所增高,礁体发育区受到礁体与周围海水热交换的影响,海底热流降低或为负值,而基底热流可以达到70 mW·m^-2左右.进一步分析表明,礼乐盆地新生代热体制主要是在古近纪岩石圈强烈减薄基础上,叠加了晚期岩浆侵位、基底起伏、沉积过程以及海底地形等局部因素影响的结果,礁体发育区热体制还受到礁体与周围海水热交换的影响;盆地凹陷中心区生油门限深度一般介于2000~2500 mbsf,门限温度介于90~110℃;礁体发育区生油门限深度明显大于邻近的北1凹陷沉积中心区.

     

Evolution of double diffusion convection in a felsic magma chamber

The onset of double diffusion convection (DDC) is modeled in a two-dimensional case in respect to magma chambers. The viscosity model for the melt takes into account the effects of temperature and concentration of the dissolved component (H₂O). The upper boundary of the convecting magma chamber is assumed to be anhydrous and at constant temperature, whereas the lower boundary is treated as being hydrous permeable with a temperature greater than that within the upper boundary. The case of positive compositional and thermal buoyancy of melt is studied assuming a H₂O diffusion coefficient small in comparison with thermal diffusivity. The DDC has been modeled using a system of equations solved by the finite difference method on a square grid. The convective pattern evolution has been studied for fixed boundary conditions as well as for cooling and degassing. Due to the higher viscosity in the upper zone, the upper boundary layer is thicker than the lower one. The variation of water concentration in this zone of the convective cell can be significant. In nature, the high gradient of water concentration can be responsible for the observed variations of water content in minerals crystallized from a granite melt (e.g., biotite). Because of a high Lewis number (= 100), temperature variations in the magma chamber decay much faster than the water concentration. In this case the intensive convection can continue at a constant temperature due to the non-zero water content in the chamber. In principle, the effect can be applied to the formation of magmatic bodies. If the cooling and degassing system reaches a uniform temperature distribution prior to the crystallization temperature, water content throughout the body may still remain variable.     

Structural controls on the 1998 volcanic unrest at Iwate volcano: Relationship between a shallow, electrically resistive body and the possible ascent route of magmatic fluid

Magnetotelluric (MT) measurements were conducted at Iwate volcano, across the entirety of the mountain, in 1997, 1999, 2003, 2006, and 2007. The survey line was 18 km in length and oriented E–W, comprising 38 measurements sites. Following 2D inversion, we obtained the resistivity structure to a depth of 4 km. The surface resistive layer (~ several hundreds of meters thick) is underlain by extensive highly conductive zones. Based on drilling data, the bottom of the highly conductive zone is interpreted to represent the 200 °C isotherm, below which (i.e., at higher temperatures) conductive clay minerals (smectite) are rare. The high conductivity is therefore mainly attributed to the presence of hydrothermally altered clay. The focus of this study is a resistive body beneath the Onigajo (West-Iwate) caldera at depths of 0.5–3 km. This body appears to have impeded magmatic fluid ascent during the 1998 volcanic unrest, as inferred from geodetic data. Both tectonic and low-frequency earthquakes are sparsely distributed throughout this resistive body. We interpret this resistive body as a zone of old, solidified intrusive magma with temperatures in excess of 200 °C. Given that a similar relationship between a resistive body and subsurface volcanic activity has been suggested for Asama volcano, structural controls on subsurface magmatic fluid movement may be a common phenomenon at shallow levels beneath volcanoes.