HR: 13:40h
AN: V13D-01 INVITED     [Abstracts]
TI: Acoustic Oscillations in Volcanoes
AU: Garces, M
EM: milton@isla.hawaii.edu
AF: Infrasound Laboratory, University of Hawaii, Manoa, 73-4460 Queen Kaahumanu Hwy., #119, Kailua-Kona, HI 96740-2638 United States
AU: * Marchetti, E
EM: marchetti@geo.unifi.it
AF: Dipartimento di Scienze della Terra, Universit… di Firenze, via G. La Pira 4, Firenze, I-50121 Italy
AU: Ripepe, M
EM: mripepe@geo.unifi.it
AF: Dipartimento di Scienze della Terra, Universit… di Firenze, via G. La Pira 4, Firenze, I-50121 Italy
AB: The intensity of infrasonic waves produced by volcanic activity ranges from very low amplitude pressure signals (mPa) to violent shock waves produced during explosive eruptions (MPa). Recorded waveforms vary from simple single pulses to complicated, long lasting signals where echoes and/or multiple pulses may be present. Whether echoes occur, are sustained, and are recorded depends on the elasticity of the surrounding walls, the attenuation of the fluid, the depth of the source, and the relative position of the sensor. A shallow explosion would release most of its energy to the atmosphere. In this case, echoes would be primarily associated with reflections from crater walls or nearby mountains. A deep explosion in a vesiculated magma column may not be multiply reflected (and thus maintain resonance) in a conduit if it has to propagate through a heavily attenuating magma-gas mixture. Yet highly vesiculated foams, with their low sound speeds and their sensitive dependence of gas exsolution and viscosity on ambient pressure, are extremely unstable under any fluid flow conditions. Due to the decrease in density and sound speed with increased vesiculation, an acoustic pulse arriving from some depth in a moving magma column would encounter an increase in Mach number as it approaches a highly vesiculated region. When this pulse reaches the foam, the pressure perturbation and its associated streaming may induce rapid exsolution and trigger a fragmentation-enhanced explosive eruption that could lower the fragmentation void fraction threshold and enhance jet flow. Lowering of the fragmentation threshold may permit conduit reverberation. Cavitation may occur when a fluid is excessively tensed. Flow acceleration through a constriction (choked flow), or the passage of an intense sound pulse can induce cavitation and produce a bubble oscillation. The precondition of existing bubbles for cavitation lend vesiculated foams particularly vulnerable to collapse. Sound from periodic turbulent vortices induced by surface discontinuities or shear (Aeolian tones, edge tones, vortex sheets) may occur at depth in the melt or at the ground-air interface. Avalanches, landslides, and pyroclastic flows would also generate acoustically active turbulent structures, as well as a sound from impact and explosive gas release. Jet noise can be produced by fumaroles, lava tubes, and eruptions. Jet flow resonance, known as screech, may occur within a supersonic jet and be observable during vigorous eruptions. Vigorous lava fountaining events radiate discrete infrasonic pulses which may be indicative of oscillations in the pressure driving the fluid flow. Infrasound from the oscillation of a lava tube or lava lake may be produced by the movement of the magma. Sound from lava falls, as seen through skylights in Pu'u O'o, may be enhanced by ringing of the air in a lava tube. As in the ocean, standing waves in a molten lava lake may generate sound efficiently if they slam into walls or if they entrain periodic flow into confined regions. As in a furnace, pressure and thermal oscillations may be induced in a lava tube when the gas in the tube is overburned, leading to a low pressure with gas overdrawing, followed by a fiery pressure increase during subsequent overburning.
DE: 8414 Eruption mechanisms
DE: 8419 Eruption monitoring (7280)
DE: 8439 Physics and chemistry of magma bodies
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting