Brilha J.B., Sequeira Braga M.A., Proust D., Dudoignon P.

Apresentado no International Volcanological Congress, Puerto Vallarta, México, Jan/97

Abstract
The lava extrusion on submarine environment is followed by a high cooling rate until the attainment of the thermal equilibrium between the lava and the surroundings. The cooling rate is witnessed by volcanic glass formation for rates above 500°C/hour and by the crystalline habit for lower rates. That crystalline habit reflects the physical-chemical conditions of the medium during the nucleation and crystal growth phases.

The occurrence of olivine, pyroxene and plagioclase with lantern-like and "swallow-tail" crystalline habit is frequent in submarine lavas. This morphology is typical of cooling rate about 450°C/hour.

The studied basaltic rock outcrop consists on lava which has solidified inside the vertical conduit of the ancient volcanic system (55±18Ma) originating a well crystallized tephrite as a volcanic plug. This alkali basalt contains olivine crystals with the same crystalline habit as those described for submarine rocks, revealing high cooling rates in a lava that solidified without reach the Earth surface.

Mathematical modeling based on the theories of thermal transfer for this particular example did not give cooling rates compatible with these crystalline habits. So, the high cooling rate can only be explained by a sudden and fugacious magma ascent episode (mean velocity of 5m/s) inside the vertical conduit of the volcanic system. This ascent was contemporaneous with olivine crystallization between the thermal interval 1200-1100°C. The ephemeral feature of this episode is justified by the absence of these habits on the remaining minerals of the rock.