DEVELOPMENT OF LAVA TUBES IN THE LIGHT OF OBSERVATIONS AT MAUNA-ULU, KILAUEA VOLCANO, HAWAII

During the 1969-1974 Mauna Ulu eruption on Kilauea's upper east rift z one, lava tubes were observed to develop by four principal processes: (1) flat, rooted crusts grew across streams within confined channels; (2) overflows and spatter accreted to levees to build arched roofs acr oss streams; (3) plates of solidified crust floating downstream coales ced to form a roof; and (4) pahoehoe lobes progressively extended, fed by networks of distributaries beneath a solidified crust. Still anoth er tube-forming process operated when pahoehoe entered the ocean; larg e waves would abruptly chill a crust across the entire surface of a mo lten stream crossing through the surf zone. These littoral lava tubes formed abruptly, in contrast to subaerial tubes, which formed graduall y. All tube-forming processes were favored by low to moderate volume-r ates of flow for sustained periods of time. Tubes thereby became ubiqu itous within the pahoehoe flows and distributed a very large proportio n of the lava that was produced during this prolonged eruption. Tubes transport lava efficiently. Once formed, the roofs of tubes insulate t he active streams within, allowing the lava to retain its fluidity for a longer time than if exposed directly to ambient air temperature. Th us the flows can travel greater distances and spread over wider areas. Even though supply rates during most of 1970-1974 were moderate, rang ing from 1 to 5 m3/s, large tube systems conducted lava as far as the coast, 12-13 km distant, where they fed extensive pahoehoe fields on t he coastal flats. Some flows entered the sea to build lava deltas and add new land to the island. The largest and most efficient tubes devel oped during periods of sustained extrusion, when new lava was being su pplied at nearly constant rates. Tubes can play a major role in buildi ng volcanic edifices with gentle slopes because they can deliver a sub stantial fraction of lava erupted at low to moderate rates to sites fa r down the flank of a volcano. We conclude, therefore, that the tenden cy of active pahoehoe flows to form lava tubes is a significant factor in producing the common shield morphology of basaltic volcanoes.