While writing the discussion section of the second draft of my paper I started thinking of the possible processes responsible for producing transitional lava flows. Transitional lava flows are one of the focus points of my research, and paper. Before I go any further I will briefly describe what a transitional lava flow is for anyone who does not know.
Transitional lava flows form by mechanical fracturing of a solidified insulating lava crust, rather than the viscous-tearing of a'a flows typically found at Hawaiian eruptions (Figure 1, below). A'a lava flows are formed when they experience changes in rheological properties such as viscosity, crystallinity and yield strength. As lava cools and crystallizes, more force is required to exceed the yield strength of the lava and allow it to continue moving. This is how a'a flows develop, as the lava moves more sluggishly the melt begins tearing itself a part, leaving behind a rough, clinkered surface. Gas within the lava escapes at this point and tears a part the surface. Alternatively, a'a flows form when smooth pahoehoe flows reach a slope. If the lava flow meets a change in slope the following happens: A smooth pahoehoe flow moves across a flat surface and reaches a slope. As the flow moves down the slope the lava is viscously torn by stress, producing an a'a surface. When the flow eventually reaches the base of the slope its reverts back to a smooth pahoehoe lava as it is no longer experiencing any stress imparted by the slope break. Glaze et al. (2014) describes how topographic variability can disrupt the surface of a lava by causing melt in the flow interior to circulate at the slope break and induce stress on the base of the lava crust.
If you would like more background knowledge on lava flows, here are two videos that you might find useful:
https://www.youtube.com/watch?v=KP1ueULz_B4
https://www.youtube.com/watch?v=vJ94FmGgy24
I was going to discuss in my paper how topographic variability could be responsible for the formation of the transitional lava flow at COTM, however it is more likely that when a lava flow meets a slope, it becomes an a'a flow. Glaze et al. (2014), Glaze and Baloga (2007), and Hamilton et al. (2013) discuss how sudden changes in slope can disrupt the surface of a lava flow. Figure 2 (below) illustrates the mechanics of how the surface could be disrupted at the slope break. This is why I wrote this blog, to talk to my lab friends and colleagues about what processes could produce transitional lava flows.
I want to focus on two particular topics: (1) changes in effusion rate or (2) emplacement over older rough lava flows. Disruption of the surface can occur from these processes. The discussion is whether they could be responsible for the transitional lava flow at COTM. There is no quantitative results for either topic, this is all based off field, petrographic, and geochemical data.
Effusion Rate
Effusion rates in volcanism describes the change in volume of magma erupted onto the surface during a set time period. Some eruptions have low effusion rates, small or large amounts of magma or lava emplaced on the surface during a time span of approximately hundred thousands to millions of years (e.g. the Columbia River Basalts). Large effusion rates can emplace large quantities of lava within short time periods (e.g. Holuhraun eruption).
In relation to transitional lava flows, a sudden increase in lava supply could fracture and brecciate the solidified crust. The process is illustrated in figure 3, showing step by step how effusion rate can mechanically disrupt the lava flow.
The first step involves the new supply of lava exerting pressure on the crust, causing the lava flow to inflate. The inflation of the lava roughens the surface and becomes hummocky and bumpy. Eventually the durability of the crust is exceeded and begins to fracture and fragment, creating rubble-sized slabs. Continued fracturing breaks down the crust fragments into centimetre to decimeter-sized rubble, producing a rubbly pahoehoe lava flow.
Emplaced over Rough Lava Flows
Eruption history at Craters of the Moon began around 15 ka when the first lava flows were emplaced. Since then, polygenetic eruptions have produced over 50 lava flow, most flowing on top of older lava flows. The transitional lava flow studied here is one of the youngest in the lava field, dated to be approximately 2000 years old. Kuntz et al. (1982) summarized the eruption history of Craters of the Moon, showing that the transitional lava flow was emplaced on top of an older lava flow. Due to its rough surface, no geophysical surveying can be conducted to map out the subsurface beneath the lava flow. We can only assume that if the lava flow moved over a rough lava flow, it maybe responsible for mechanically fracturing the crust.
Reference
Glaze, L. S. & Baloga, S. M. (2007). Topographic variability on Mars: Implications for lava flow modeling. Journal of Geophysical Research E: Planets 112, 1–9.
Glaze, L. S., Baloga, S. M., Fagents, S. A. & Wright, R. (2014). The influence of slope break on lava flow surface disruption. Journal of Geophysical Research: Solid Earth 119, 8728–8747.
Hamilton, C. W., Glaze, L. S., James, M. R. & Baloga, S. M. (2013). Topographic and stochastic influences on pāhoehoe lava lobe emplacement. Bulletin of Volcanology 75, 1–16.
Kuntz, M. a, Champion, D. E., Spiker, E. C., Lefebvrelsd, R. H. & Mcbroomes, L. a (1982). The Great Rift and the Evolution of the Craters of the Moon Lava Field , Idaho. Cenezoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, 423–437.