Computer Science – Sound
Scientific paper
Jan 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996eiaf.conf...40w&link_type=abstract
Workshop on Evolution of Igneous Asteroids: Focus on Vesta and the HED Meterorites, p. 40
Computer Science
Sound
Asteroids, Basalt, Lava, Magma, Meteoritic Composition, Surface Temperature, Vesta Asteroid, Volcanology, Volcanoes, Cooling, Ejection, Rock Intrusions, Fluid Flow, Fracture Strength, Gas Expansion, Pressure Gradients, Elastic Waves
Scientific paper
Partial melting in asteroid mantles produces basaltic melts that percolate along grain boundaries and collect into veins. Veins of sufficient size, overpressured by volume expansion caused by melting, grow by cracking the rocks at their tips and scavenge melt from intersecting smaller veins. Buoyancy and excess pressure combined allows them to migrate as isolated dikes, cracking open at the upper end while pinching shut at the lower end. The dike sizes depend on the physical and rheological properties of the magmas, the gravitational acceleration, and the country rock elastic properties, especially the apparent fracture toughness, Kf. Small amounts of gas accumulating in dike tips greatly increase Kf over laboratory values. Calculations of sizes and rise speeds of dikes in a Vesta-sized asteroid for a wide range of conditions show that if Kf is too small the magma will cool excessively before the dike can reach the surface. Since eucritic meteorites do appear to be the products of surface eruptions we infer a lower limit for the Kf value controlling the feeder dikes. As a dike opens to the surface of an asteroid with no atmosphere, an expansion wave propagates down into the magma at a large fraction of the local speed of sound in the fluid and initiates decompression as magmatic gas expands and accelerates magma upward. Expansion of the gas provides the work done against gravity and against wall friction and the magma kinetic energy. Eventually gas bubbles expand to the point where the magma disrupts into a spray of liquid droplets entrained in the gas flow. Above the magma disruption level the spray of gas and droplets accelerates under a pressure gradient maximizing the mass flux, achieved when the pressure at the vent, Pv is such that the mean speed of the erupting mixture, uv, is equal to the local speed of sound, i.e., the flow is choked. Above the vent the gas expands adiabatically and accelerates upward and sideways, becoming supersonic. Within a distance of a few times the dike width the velocity approaches a limit uinfinity at which all the internal energy of the gas is converted to kinetic energy. The gas-droplet spray spreads out to a maximum angle from the vertical determined by the changing local Mach number. Magma droplets cool by radiation after ejection from a vent, but only if they can "see" their surroundings. If the number density of droplets in a fountain is large, only those within a critical distance L of the outer edge can lose heat, the rest reaching the ground at magmatic temperatures to coalesce into a lava pond feeding lava flows.
Keil Klaus
Wilson Leslie
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