STEADY-STATE OPERATION OF STROMBOLI VOLCANO, ITALY - CONSTRAINTS ON THE FEEDING SYSTEM

Stromboli volcano has been in continuous eruption for several thousand years without major changes in the geometry and feeding system. The thermal structure of its upper part is therefore expected to be close to steady state. In order to mantaim explosive activity, magma must release both gas and heat. It is shown that the thermal and gas budgets of the volcano lead to consistent conclusions. The thermal budget of the volcano is studied by means of a finite-element numerical model under the assumption of conduction heat transfer. It is found that the heat loss through the walls of an eruption conduit is weakly sensitive to the dimensions of underlying magma reservoirs and depends mostly on the radius and length of the conduit. In steady state, this heat loss must be balanced by the cooling of magma which flows through the system. For the magma flux of about 1 kg s-1 corresponding to normal Strombolian activity, this requires that the conduits are a few meters wide and not deeper than a few hundred meters. This implies the existence of a magma chamber at shallow depth within the volcanic edifice. This conclusion is shown to be consistent with considerations on the thermal effects of degassing. In a Strombolian explosion, the mass ratio of gas to lava is very large, commonly exceeding two, which implies that the thermal evolution of the erupting mixture is dominated by that of the gas phase. The large energy loss due to decompression of the gas phase leads to decreased eruption temperatures. The fact that lava is molten upon eruption implies that the mixture does not rise from more than about 200 m depth. To sustain the magmatic and volcanic activity of Stromboli, a mass flux of magma of a few hundred kilograms per second must be supplied to the upper parts of the edifice. This represents either the rate of magma production from the mantle source feeding the volcano or the rate of magma overturn in the interior of a large chamber.