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

On the mechanisms of heat loss beneath continents and oceans

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1981.Microfiche copy available in Archives and Science.Bibliography: leaves 200-215.by Claude Jaupart.Ph.D

The feeder system of the Toba supervolcano from the slab to the shallow reservoir

The Toba Caldera has been the site of several large explosive eruptions in the recent geological past, including the world’s largest Pleistocene eruption 74,000 years ago. The major cause of this particular behaviour may be the subduction of the fluid-rich Investigator Fracture Zone directly beneath the continental crust of Sumatra and possible tear of the slab. Here we show a new seismic tomography model, which clearly reveals a complex multilevel plumbing system beneath Toba. Large amounts of volatiles originate in the subducting slab at a depth of ∼150 km, migrate upward and cause active melting in the mantle wedge. The volatile-rich basic magmas accumulate at the base of the crust in a ∼50,000 km3 reservoir. The overheated volatiles continue ascending through the crust and cause melting of the upper crust rocks. This leads to the formation of a shallow crustal reservoir that is directly responsible for the supereruptions

Radiogenic heat production, thermal regime and evolution of continental crust

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Archean thermal regime and stabilization of the cratons

International audienceArchean provinces are presently characterized by low heat flow, with an average of 41 mW m-2, less than the global continental average (56 mW m-2) . The range of regionally averaged heat flow values in Archean Provinces (18-54 mW m-2) is narrower than in younger terranes. At the end of the Archean, when crustal heat production was double the present, surface heat flow varied over a range (≈45-90 mW m-2) as wide as that presently observed in Paleozoic Provinces. For the present-day vertical distribution of radio-elements, high heat production during the Archean is insufficient to account for elevated lower crustal temperatures. High temperature-low pressure metamorphism conditions require additional heat input, for example by emplacement of large volumes of basaltic melts, crustal thickening or larger concentrations of radio-elements in the lower crust. In the Archean, with a crust thicker than 40 km or with the radio-elements uniformly distributed throughout a 40 km thick crust, the lower crust was near melting and, with an effective viscosity ≈1019Pa s, it could not sustain the stress due to crustal thickening. Long-term crustal stability requires the enrichment of the upper crust in radio-elements through melt extraction from the lower crust. After root emplacement, thermal conditions in cratons remained far from equilibrium for 1-2 Gy. Depending on the mechanism of root formation, temperature at 150 km might only be 150 K higher than present, implying that the lithospheric mantle remained sufficiently cold and strong to preserve Archean features. In thick continental lithosphere, temperatures in the crust and deep in the continental root are effectively decoupled for a long time

Les éruptions volcaniques « explosives » : des grandes aux petites échelles

Les éruptions volcaniques riches en gaz, que l’on qualifie d’« explosives », émettent de grandes quantités de gaz et de fragments de magma dans l’atmosphère. Elles peuvent prendre deux formes très différentes : une colonne légère pouvant s’élever jusqu’à plusieurs dizaines de kilomètres d’altitude, ou bien une coulée dense s’écoulant rapidement sur les pentes du volcan. Ces deux formes correspondent à de faibles différences de la quantité de gaz volcaniques dans l’éruption, qui sont elles-mêmes dues à de faibles différences de la taille des fragments de magma. Ces fragments, une fois trempés, deviennent des pierres ponces et des cendres que l’on ramasse facilement sur le sol. Ils contiennent des informations précieuses sur les phénomènes complexes qui se produisent sous terre avant l’éruption à la surface terrestre

Secular cooling and thermal structure of continental lithosphere

International audienceTemperatures in thick continental lithosphere do not adjust rapidly to secular changes of mantle temperature and in-situ radioactive decay. In the past, enhanced heat production may have led to geotherms that turn over above the base of the lithosphere, such that the lower lithosphere was hotter than, and was losing heat to, the underlying convecting mantle. Lithosphere with a turning geotherm would be unstable, undergoing delamination due to convective shear stresses imparted by the underlying mantle and in-situ partial melting if the lithospheric mantle contains small amounts of water and carbon. Both processes act to stabilize continental roots through reductions of lithospheric thickness and in-situ heat production, and hence may be responsible for the present-day characteristics of those roots that have survived until today. According to these arguments, a stable thermal structure requires that the average heat production rate in the lithospheric mantle does not exceed a critical value which depends on lithosphere thickness. The threshold value of heat production is 0.025 μW m− 3 in lithosphere that is thicker than 300 km. Cratonic roots that grow by underplating of oceanic lithosphere in a subduction environment undergo an initial heating phase which may lead to partial melting and to the formation of near-solidus mantle melts without any external heating event involved