MAGMA CHAMBER PROCESSES AND GEOMETRIES

Magma chamber activity is inferred from mapping of historic volcanic eruptions, coupled with geophysical monitoring of seismic activity and shape changes in the volcanic ediface. A clear conceptual gap exists between these temporal datasets and the 2- and 3-D geospatial understanding of fossilized magma systems achieved by mapping in plutonic belts. The current challenge is to find new methods to bridge the philosophical gap and formulate a new integrated understanding of magma system.

Many field and geophysical studies have shown that felsic plutons are predominantly tabular in nature with length/thickness ratios in the region of 10. Internally, many plutons show evidence for having been constructed as a series of tabular sheet-like intrusions. Evidence for processes like crystal accumulation and showing evidence for the mafic replenishment is common. Recent micro-isotope work has shown that the crystal cargoes of many magmas are multiply recycled, with periods of crystallization and resorption in magmas of different composition. This observation is consistent with the frequent input of relatively small magma batches into a compartmentalized sub-volcanic plumbing system. The question remains as to what controls whether this behavior translates into frequent eruptions, or aggregation of magma into large differentiated volumes that might erupt catastrophically at calderas, or solidify into sizeable granitoid plutons.

New mechanical models for flow and seepage in a deforming, shear-dilatant sensitive magma mush have enabled an understanding of the properties of magmas in the 50-20% melt range. Using mush permeability as a proxy, we show that the greatest maximum excess pore pressures develop consistently in rhyolitic (high viscosity) magmas at high rates of shear, implying that during deformation, the mechanical behaviour of basaltic and rhyolitic magmas will differ. For the extreme case of near instantaneous shear arising from earthquakes, flow rates of up to 1 ms-1 are predicted. These high flow rates will result in liquefaction of the mush, extremely rapid segregation of evolved interstitial fluid, and extrusion to form continental rhyolites. Coupling the physics part of the problem with micro- and meso-scale geochemical variations observed in granitic plutons remains an important future goal.