The creation and influence of bifurcations and confluences in Hawaiian lava flows on conditions of flow emplacement
Abstract
Hawaiian lava channels are characterized by numerous bifurcations and confluences that have important implications for flow behavior. The ubiquity of anastomosing flows, and their detailed observation over time, makes Hawai`i an ideal place to investigate the formation of these features and their effect on simple models of lava flow emplacement. Using a combination of high-resolution LiDAR data from the Kilauea December 1974 and Mauna Loa 1984 flows, orthoimagery of the Mauna Loa 1859 flow, and historical and InSAR mapping of the current eruption of Kilauea (1983-present), we quantify the geometry of distributary, anastomosing, and simple channel networks and compare these to flow advance rates and lengths. We use a pre-eruptive DEM of the Mauna Loa 1984 flow created from aerial photographs to investigate the relationship between underlying topography and channel morphology. In the Mauna Loa 1984 flow, the slope of the pre-eruptive surface correlates with the number of parallel channels. Slopes >4° generate up to thirteen parallel channels in contrast to slopes of <4° that produce fewer than eight parallel channels. In the 1983-1986 lava flows erupted from Pu`u `O`o, average effusion rate correlates with the number of bifurcations, each producing a new parallel channel. Flows with a volume flux <60 m3/s only have one bifurcation at most in the entire flow, while flows with a volume flux >60 m3/s contain up to four bifurcations. These data show that the splitting and merging of individual flows is a product of both the underlying ground surface and eruption rate. Important properties of the pre-eruptive topography include both the slope and the scale of surface roughness. We suggest that a crucial control is the height of the flow front in comparison to the scale of local topography and roughness. Greater slopes may create more active channels because the reduced flow thickness allows interaction with local obstacles of a greater size range. Conversely, higher viscosities could reduce the number of active channels by increasing the flow thickness. The effusion rate also influences the degree of flow branching, possibly by generating overflows and widening the flow. Branched channels can also rejoin at confluences, which occur on the leeward sides of obstacles and where the flow is confined against large-scale features, including fault scarps and older flow margins. We expect the maintenance of parallel channels past an obstacle that splits the flow to be a function of the slope and flux, which drives the flow downhill and governs the formation of levees. Our data reveal that by controlling the effective lava flux, bifurcations slow flow advance and restrict flow length. We postulate that flow branching may therefore restrict most Mauna Loa flow lengths to ~25 km, despite a wide range of effusion rates. In contrast, both confluences and the shut off of an active branch accelerate the flow. The complexity of Hawaiian flows has largely been ignored in predictive models of flow emplacement in Hawaii, but the flow geometries must be incorporated to improve syn-eruptive prediction of lava flow behavior.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V41A2484D
- Keywords:
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- 8414 VOLCANOLOGY / Eruption mechanisms and flow emplacement;
- 8425 VOLCANOLOGY / Effusive volcanism;
- 8429 VOLCANOLOGY / Lava rheology and morphology;
- 8485 VOLCANOLOGY / Remote sensing of volcanoes