Channel Development and Evolution on the 1942 Mauna Loa Lava Flow

Detailed mapping of the 1942 Mauna Loa flow allows quantification of structures such as branches, pressure ridges, incipient channels, and well-developed channels. In the (distal) zone of dispersed flow [Lipman and Banks, 1987, USGS Prof. Pap. 1350], shear planes accommodated velocity differences. Pressure ridges are common on lava that was flowing between pairs of shear planes, termed here an incipient channel. A few hundred meters upflow, incipient channels are more common whereas pressure ridges are less so. Numerous lens-shaped islands of lava occur between adjacent incipient channels. Some shear planes are asymmetric in that they have no opposite mate. A well-developed channel becomes distinct ~5 km upflow from the distal end. It is sinuous in map view but contains few very sharp bends. It is comprised of portions of many incipient channels connected end-to-end and utilizes the portions that produce a relatively straight overall path. Those portions of incipient channels diverging at sharp angles from the average flow direction did not become part of, and are now truncated by, the trace of the main channel. Obviously, the direction in which a lava flow develops is controlled by gravity. Small variations in the path of a developing lava channel result from the interplay between gravity pulling the lava downslope and lava pushing from upslope. A channel will be relatively straight if these two forces are of approximately equal strength and direction. If they differ by more than ~30° and/or the upslope push is considerably stronger, the downslope pulling direction is commonly abandoned. If the directions differ but the forces are of equal strength, the flow branches. These controls on flow direction are important considerations for numerical models of lava flow emplacement and their use for determining inundation hazards.