CHANNEL OVERFLOWS OF THE POHUE BAY FLOW, MAUNA-LOA, HAWAII - EXAMPLESOF THE CONTRAST BETWEEN SURFACE AND INTERIOR LAVA

A number of overflows from a large lava channel and tube system on the southwest rift zone of Mauna Loa were studied. Initial overflows were very low viscosity gas-rich pahoehoe evidenced by flow-unit aspect ra tios and vesicle sizes and contents. Calculated volumetric flow-rates in the channel range between 80 and 890 m(3)/s, and those of the overf lows between 35 and 110 m(3)/s. After traveling tens to hundreds of me ters the tops of these sheet-like overflows were disrupted into a surf ace composed of clinker and pahoehoe fragments. After these 'a'($) ove r bar a overflows came to rest, lava from the interiors was able to br eak out on to the surface as pahoehoe. The surface structure of a lava flow records the interaction between the differential shear rate (usu ally correlated with the volumetric flowrate) and viscosity-induced re sistance to flow. Howev er, the interior of a flow, being better insul ated, may react differently or record a later set of emplacement condi tions. Clefts of toothpaste lava occurring within fields of clinker on proximal-type 'a'($) over bar a flows also record different shear rat es during different times of flow emplacement. The interplay between v iscosity and shear rate determines the final morphological lava type, and although no specific portion of lava ever makes a transition from 'a'($) over bar a back to pahoehoe, parts of a flow can appear to do s o.