The 1975 sub-terminal lavas, mount etna: a case history of the formation of a compound lava field

The 1975 sub-terminal activity was characterised by low effusion rates (0.3–0.5 m³ s⁻¹) and the formation of a compound lava field composed of many thousands of flow units. Several boccas were active simultaneously and effusion rates from individual boccas varied from about 10⁻⁴ to 0.25 m³s⁻¹. The morphology of lava flows was determined by effusion rate (E): aa flows with well-developed channels and levees formed when E > 2 × 10⁻³ m³ s⁻¹, small pahoehoe flows formed when 2 × 10⁻³ m³ s⁻¹ >E > 5 > 10⁻⁴ m³ s⁻¹ and pahoehoe toes formed when E < 5 × 10⁻⁴ m³ s⁻¹. There was very little variation with time in the effusion temperature, composition or phenocryst content of the lava.New boccas were commonly formed at the fronts of mature lava flows which had either ceased to flow or were moving slowly. These secondary boccas developed when fluid lava in the interior of mature aa flows either found a weakness in the flow front or was exposed by avalanching of the moving flow front. The resulting release of fluid lava was accompanied by either partial drainage of the mature flow or by the formation of a lava tube in the parent flow. The temperature of the lava forming the new bocca decreased with increasing distance from the source bocca (0.035°C m⁻¹). It is demonstrated from the rate of temperature decrease and from theoretical considerations that many of the Etna lavas still contained a substantial proportion of uncooled material in their interior as they came to rest. The formation of secondary boccas is postulated to be one reason why direct measurements of effusion rates tend, in general, to overestimate the total effusion rates of sub-terminal Etna lava fields.