HR: 15:25h
AN: V43D-06    [Abstracts]
TI: Magma Chamber Evolution, Caldera Formation, and Eruption of Very Large Volume, Crystal-rich Ignimbrites
AU: * Christiansen, E H
EM: eric_christiansen@byu.edu
AF: Brigham Young University, Department of Geology, Provo, UT 84602 United States
AB: If very large volume (>1000 km3) crystal-rich (>40%) magmas exist but are rheologically dead, why do they erupt to form dacitic ignimbrites? Does water-saturation of the progressively cooling and evolving interstitial melt cause chamber expansion cracking the roof and driving eruption? Many crystal-rich dacitic magmas lack evidence that they were water-saturated before eruption. Why don't very large volume magmas erupt at some earlier stage when they are less chemically evolved and entirely liquid? If several thousand cubic kilometers of magma were emplaced at a shallow level in a molten, near liquidus state, what would prevent the roof from collapsing to initiate an eruption of crystal-poor magma? Alternatively, a very large crystal-rich dacitic magma chamber may be the product of protracted growth of a shallow magma system that was never near its liquidus. Rather, magma may have accumulated sequentially at a rate and temperature that precluded complete solidification or complete melting. Over hundreds of thousands years, sequential accumulation of small batches of magma could occur at a shallow level. Initially the magma cools rapidly against cold wall rock and is largely crystalline. If the flux of magma continues and is sufficient, a crystal-rich magma chamber grows and grows. The magma becomes less and less crystalline as temperature increases and the fraction of melt grows (Hirt and Hatton 2004). Repeated episodes of mixing, melting, and crystallizing may mark the magma system's protracted history. Eventually, as the magma body becomes larger and larger and weaker and weaker (as a consequence of increasing proportion of melt), a critical melt fraction is reached where the roof of the chamber is no longer supported. Most experimentalists find a significant drop in magma strength between 40% and 60% crystals. Foundering of the roof into the underlying chamber could then trigger a voluminous eruption of crystal-rich magma as volatile-saturation of the interstitial melt followed de-compression. Thus, caldera collapse and eruption may occur as a consequence of the growth of a large volume of weak (but crystal-rich) magma.
DE: 8178 Tectonics and magmatism
DE: 8428 Explosive volcanism
DE: 8439 Physics and chemistry of magma bodies
DE: 8440 Calderas
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005