Christine Burrill November 19, 2012 AA Flow Dynamics, Mauna Loa 1984 by Peter W. Lipman & Norman G. Banks
The paper on ‘a’a flow dynamics written by Lipman and Banks is one of the first comprehensive studies regarding detailed observations of a long and complex ‘a’a lava flow. This paper was also one of the first to establish connections between lava flow morphology and changes in lava properties, specifically gas and crystal content. The flow that was studied was generated by the 1984 eruption along the northeast rift zone of Mauna Loa volcano, Hawaii (fig. 1). The major flow reached 27 km from the source vent after four ...view middle of the document...
However, these pahoehoe overflows quickly transformed to ‘a’a.
Transitional or slabby ‘a’a was recognized when the flow contained solid and semisolid slabs and blocks that disrupted flow streamlines in the channel. Observations of this transition at flow 1 were between 3 and 5 km from the vent. Scoriaceous lava was observed at 5 to 12 km from the vent as lava became more viscous and dense, resulting in a frothy red-brown oxidized scoria with a thin glassy surface (Lipman and Banks, 1987). Farther downstream, 12-15 km from the vent, fluid lava was mostly obscured by clinkery ‘a’a that was carried along on top of the flow. Clinkery ‘a’a was distinctive from scoriaceous ‘a’a by its angular surfaces, darker color, lesser degree of oxidation and vesicularity (Lipman and Banks, 1987). Near the flow front, about 20 km from the vent, the lava moved as a mass of gray rubble called blocky ‘a’a with a highly viscous to partly solidified core. As the eruption progressed and the flow progressively stagnated upstream, these distinctive areas of ‘a’a were blurred and mixed, especially at the flow front where the different types of ‘a’a were carried and deposited.
‘A’A Flowage Zones, Levees and Channel Evolution
The major flow (flow 1) was divided into four zones by the authors based on systematic changes that were observed in the structure of the lava channel: a stable channel, transitional channel, dispersive flow and a flow toe or flow front (fig. 2). These zones became established relatively quickly after the eruption began and later changed in length and character as eruption rate decreased and lava properties changed. The major flow changed from a simple, narrow channel to a complex and more widespread flow.
Characteristics of the flow toe were found to vary primarily with the slope of the terrain and distance from the vent. Close to the vent on steep slopes channels were well established and lava moved quickly to the flow toe. The flow toe advanced rapidly and contained scoriaceous and slabby ‘a’a with a near flat surface. These quickly moving flow fronts were also characterized by relatively low heights of 1-3 m compared to slow moving flow fronts traveling on gentle slopes further from the vent that allowed material to pile up more vertically at the flow front. Denser blockier lava and fine granular material from the grinding of the lava blocks against each other were major components of the slower flows (Lipman and Banks, 1987).
The dispersive flow zone immediately behind the flow toe had a quickly moving central flow of blocky ‘a’a with little to no incandescent fluid lava. Closer to the vent more incandescent fluid lava was concentrated in a central channel. The dispersive zone was very limited in extent until the eruption began to wane and later extended back up to 10 km from the flow toe (Lipman and Banks, 1987).
The transitional zone was distinguished by a distinct channel and clinkery ‘a’a. The channel walls consisted of blocky ‘a’a and...