Other
Scientific paper
May 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003e%26psl.210...17f&link_type=abstract
Earth and Planetary Science Letters, Volume 210, Issue 1-2, p. 17-33.
Other
1
Hydrothermalism, Phreato-Magmatism, Quartz, Compaction, Model
Scientific paper
The few-hundred-meters-thick rhyolitic tuff layers that cover the volcanic island of Milos host intense hydrothermal alteration which completely transforms the tuffs into clays and locally into microcrystalline quartz aggregates. At the top of one of these quartz layers hundreds of 15-20-m-large and 3-4-m-deep craters are found, randomly scattered over a 1-2-km2 area. In another area craters with smaller diameters (1-2 m) and depths (0.5-1 m) are found. Geochemical data reveal that the quartz results from the leaching of the tuff or clay layers by a high-temperature (100-200°C) flow of steam and gas. Because of their micrometer size, the quartz grains have a viscous rheology due to solution transfer creep: the effective viscosity is as low as 1014 Pa s. As a result the steam- or water-saturated porous quartz layers deform by compaction. The compaction length is about 3 and 15 m when the fluid is superheated steam and water, respectively. When the upwelling fluid flow encounters a region with a reduced porosity, compaction generates pocket-like domains of enhanced porosity. These pockets propagate toward the surface by deforming the solid framework of the porous layer. One-dimensional simulations of the porosity waves show that the steam or water concentration in the pockets likely exceeds 70%. The fluid compressibility and viscosity drastically influence the development of the waves. For steam-like (water-like) fluids the size of the pockets is about 15-20 m (3-4 m) and their velocity is about 50-60 m/yr (2-4 m/yr). The fluid overpressure carried by the waves is about 3-4 bar and 0.3-0.5 bar when the fluid is steam-like and water-like, respectively. For steam-like fluids, the 2-4 bar overpressures at the top of the waves are able to re-open healed fissures at a depth of about 5-10 m. Then, the steam catastrophically escapes from the rocks through these fissures and the eruption proceeds as a caldera collapse. This leads to the suggestion that the arrival of steam pockets can trigger phreatic eruptions that form the 15-20-m-large, 3-4-m-deep craters. When the fluid is water-like, the overpressures are too small to generate an eruption. The fluids percolate through the quartz aggregate, leading to the formation of 2-3-m-wide, 0.5-1-m-deep, bowl-like cavities. This model is consistent with field observations at Milos.
Boulègue Jacques
Fontaine Fabrice J.
Rabinowicz Michel
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