Heat transfer in active volcanoes: models of crater lake systems

Stevenson, David Stacey (1992). Heat transfer in active volcanoes: models of crater lake systems. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000e036


Heat transfer in active volcanoes was investigated in this thesis. A general model of a crater lake system was developed that takes inputs of lake temperature, volume, chemical content, and meteorological conditions, and outputs the mass, energy, and chemical flows to and from the lake. The model was applied to lakes at Poás (Costa Rica) and Ruapehu (New Zealand), yielding volcanic power outputs of ~102-103 MW, and heat fluxes of ~102-104 W m-2. Heat is added to the lakes by hot brine and steam, derived from lake seepage and magmatic gas.

The heat source is magma crystallising, cooling, and degassing. Background heat inputs are maintained by hydrothermal infiltration of magma, releasing latent and specific heat. Infiltration of the conductive boundary layer surrounding magma was modelled. The permeability created by contractive cooling was equated with the permeability required for two-phase convection to transport heat away from the boundary. Infiltration rates of ~1-100 m a-1 (metres/year), into conductive layers ~30-0.3 m thick, creating permeabilities of ~10-10-10-14m2, will provide the required heat flux. Cracking temperatures of magma depend upon infiltration rate, ranging from hydrothermal system temperatures at slow rates, to magma temperatures at the fastest rates. Predicted maximum rates are ~300 m a-1 for near-surface magma, and ~800 m a-1 for magma at ~1-3 km depth.

Measured SO2 fluxes at Poás, and calculated influxes of HC1 to both lakes imply that degassed magma volumes (~0.004-0.08 km3 a-1) are much larger than likely intrusions. A new model was developed of small, vesiculating intrusions that circulate magma due to the density increase associated with gas loss. Dense, degassed magma descends, whilst buoyant, volatile-rich magma rises from a deep source. Pipe-like intrusions of radius ~5 m, tapping magma volumes >-0.05 km3, can produce the gas fluxes needed. Intrusions of this type probably occurred in 1980/81 and 1986 at Pods, and in 1968,1971,1975,1977,1981 and 1985 at Ruapehu, and were followed by intermittent eruptions and degassing. This degassing mechanism probably occurs at many volcanoes where high gas fluxes are observed, but no evidence exists for large, shallow intrusions. A model of compressible fluid flow in a rough fumarole conduit, with conductive heat loss to the surroundings, allows fumarole temperatures to be used to estimate the depth of their magma source. This also indicates shallow magma was emplaced at Poás in 1980/81 and 1986.

In summary, heat transfer is achieved by a combination of intermittent gas release from minor shallow intrusions, together with infiltration of deeper magma. Infiltration is one mechanism for providing fractures allowing the release of gas from shallow intrusions, and circulation probably ceases due to freezing caused by infiltration.

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