EMPLACEMENT AND INFLATION OF PAHOEHOE SHEET FLOWS - OBSERVATIONS AND MEASUREMENTS OF ACTIVE LAVA FLOWS ON KILAUEA VOLCANO, HAWAII

Inflated pahoehoe sheet flows have a distinctive horizontal upper surf ace, which can be several hundred meters across, and are bounded by st eep monoclinal uplifts. The inflated sheet flows we studied ranged fro m 1 to 5 m in thickness, but initially propagated as thin sheets of fl uid pahoehoe lava, generally 20-30 cm thick. Individual lobes originat ed at outbreaks from the inflated front of a prior sheet-flow lobe and initially moved rapidly away from their source. Velocities slowed gre atly within hours due to radial spreading and to depletion of lava sto red within the source flow. As the outward flow velocity decreases, co oling promotes rapid crustal growth. At first, the crust behaves plast ically as pahoehoe toes form. After the crust attains a thickness of 2 -5 cm, it behaves more rigidly and develops enough strength to retain incoming lava, thus increasing the hydrostatic head at the flow front. The increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow, resulting in uniform uplift of the entir e sheet-flow lobe. Initial uplift rates are rapid (flows thicken to 1 m in 1-2 hours), but rates decline sharply as crustal thickness increa ses, and as outbreaks occur from the margins of the inflating lobe. On e flow reached a final thickness of nearly 4 m after 350 hr. Inflation data define power-law curves, whereas crustal cooling follows square root of time relationships; the combination of data can be used to con struct simple models of inflated sheet flows. As the flow advances, pr eferred pathways develop in the older portions of the liquid-cored flo w; these pathways can evolve into lava-tube systems within a few weeks . Formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1-2 orders of magnitude slower. If advance of the sheet flo w is terminated, the tube remains filled with lava that crystallizes i n situ rather than draining to form the cave-like lava tubes commonly associated with pahoehoe flows. Inflated sheet flows from Kilauea and Mauna Loa are morphologically similar to some thick Icelandic and subm arine sheet flows, suggesting a similar mechanism of emplacement. The planar, sheet-like geometry of flood-basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly inst antaneous emplacement as literal floods of lava.