HR: 14:10h
AN: V13E-03    [Abstracts]
TI: Solitary Waves, Magma Migration and Dome Building Eruptions at Mt. St. Helens, Washington
AU: * Ryan, M P
EM: mryan@usgs.gov
AF: M.P. Ryan, 926A National Center, U.S. Geological Survey, Reston, VA 20192, United States
AU: Stanley, B
EM: bstanley52@gmail.com
AU: Plasker, M
AB: Solitary waves have first-order attributes that include shape and volume conserving packets of fluid that migrate with characteristic wavelengths, amplitudes, wave numbers, and pulse durations. To ascend through dike-like, magma-filled fractures or sub-circular conduits, the solitary wave pulse duration is directly proportional to the conduit wall region viscosity and inversely proportional to the density contrast that drives the flow. Solitary waves are produced by the collapse of conduit wall rocks following the passage of a magma batch. The 1980-current eruptions at Mt. St. Helens display a variable time-series in their erupted volumes, as well as lava dome \(or spine\) heights / volumes and vent flow rates. Inter-eruption repose periods, however, have often shown broad regularity over extended periods. The rhythmic ‘beat' of eruptive episodes within a long-lived series and their roughly regular repose periods arises directly from the solitary wave migration mechanism. Composite domes are suggested to be the products of solitary wave incremental additions of dacite, as in the 1980-1983 composite dome resulting from at least 9 such solitary wave-controlled additions. The 18 May 1980 dacite cryptodome may now be interpreted as a composite of several solitary wave-based intrusions leading to the climatic eruption volume. Domes may be either solitary or composite but are built up of one or several batches of evolving magma that ascend individually from the 8 to 1 km depth storage reservoir as solitary waves. Analytical calculations of wave speed, wave length, batch volume, parcel shapes and repose periods reveal the dependence on material properties appropriate for Mt. St. Helens intrusions and dome-building eruptions. Predicted solitary wave volumes and flow rates are in good agreement with observed values for dacitic dome and spine-building eruptions from 1980-1986 and from 2004-2007. Conduit dimensions are inferred to vary over the range R=2 to R=20 m. Magma viscosities minimally span the inferred range 7E6 to 2E8 Pa s, appropriate for dacites of Mt. St. Helens composition, crystallinity, and water content at P and observed &\ computed T. Higher viscosities may prevail in the cooled carapace of the dome / spine. Second-order effects that modify pathways and compositions include heat loss to conduit wall rocks, and progressive crystallization episodes along conduit walls. Shallow vesiculation is a secondary effect superposed on a fundamental theme dominated by solitary wave-based magma transport.
DE: 3641 Extrusive structures and rocks
DE: 5104 Fracture and flow
DE: 8419 Volcano monitoring (7280)
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration and fragmentation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting