P-TYPE AND S-TYPE PAHOEHOE - A STUDY OF VESICLE DISTRIBUTION PATTERNS IN HAWAIIAN LAVA FLOWS

Two main types of Hawaiian pahoehoe lava, P-type (pipe vesicle-bearing) and S-type (spongy), are characterized. Their origin is inferred from features of the vesicle sizes and abundances, particularly in their chilled margins which record, frozen in, the vesicle population at the time and place of solidification. S-type margins are distinctly more vesicular than P-type. Porosity and vesicle size and range in S-type margins are highest at the vent. Vesicle size reaches a minimum in medial areas, and then increases while porosity decreases onto the flatter ground of the coastal terrace. The vesicles are inferred to be part of the complement of bubbles that was present at the vent, modified by loss and coalescence during travel in lava tubes. The minimum porosity occurs on the coastal terrace and is the result of degassing during a longer residence time in lava tubes there. The systematically lower porosity of P-type margins than S-type is attributed to a still longer residence, allowing more time for bubble coalescence and loss. P-type pahoehoe is common only on flatter ground, such as the coastal terrace. It is found where tumuli and lava rises are widespread and where extensive networks of lava-expansion clefts occur through which lava moving below the surface crust can lose gas without significant cooling. Additionally, P-type pahoehoe shows clear evidence that a strong inward movement and growth of bubbles took place from the outermost selvage into an inner selvage concentration zone. A mechanism that might cause this inward movement is surface tension, operating against deforming forces in the steep viscosity gradient of flow margins, causing down-gradient bubble translation. Inward movement does not usually occur in the upper crust of thick (> 2 m) flow-units because of the absence of strong deforming forces in the shallow velocity gradient there. Many of the larger bubbles that entered the inner selvage rose buoyantly to form pipe vesicles. P-type pahoehoe is characterized by sampled profiles across 20 flow-units. Continued gas loss through surface clefts caused a general overall reduction in vesicle content within each flow-unit to less than that in the upper crust, and the pronounced depletion of vesicles from its lower half is attributed to scavenging by ascending pipe bubbles. In a small proportion of flow-units, overall gas loss was small and a median gas blister fed by pipe bubbles developed instead.