The Propagation and Morphology of Lava Flows

Ross C. Kerr 1 , Ross W. Griffiths 1 , Aaron W. Lyman 1 , Katharine V. Cashman 2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Department of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, USA

Using a powerful combination of laboratory experiments and theoretical analyses, we have investigated the propagation and morphology of basaltic and silicic lava flows. In one set of experiments, molten polyethylene glycol wax was released at a constant flow rate under cold water on a sloping plane (Figure 11). Initially, the wax spreads both down and across the slope at the same rate in an early-time viscous regime, before undergoing a transition to a long-time viscous regime where downslope flow is faster than lateral flow. Eventually, the lateral flow is stopped by the yield strength of the growing surface crust, and the flow then travels downslope in a channel of constant width. Using scaling analysis, we have derived expressions for the final channel width in both the early-time and long-time flow regimes, as a function of the flow rate, the slope, the density difference driving the flow, the lava viscosity, the thermal diffusivity, and the strength of the surface crust. We have also found a dimensionless flow morphology parameter that controls whether the subsequent channel flow occurs with a ‘mobile crust' morphology (involving lots of exposed melt and consequent high heat loss) or in a ‘tube' morphology (involving a complete solid roof and relatively small heat loss from the melt). The theory has been successfully applied to understand the formation of a basaltic sheet flow lobe in Hawaii, for which we estimated the crust strength of 60kPa. Our results provide important insight into conditions that promote lava tube formation, which is in turn responsible for greatly increasing the length of individual flows. 

In a second set of experiments, we rapidly released a fixed volume of molten polyethylene glycol wax , which propagated as a two-dimensional flow down a sloping channel under cold water. We found that four dynamical flow regimes can arise: an inertial slumping regime, a horizontal viscous regime, a sloping viscous regime, and a surface crust regime that finally stops the flow. With this dynamical understanding, we analyzed the flow of blocky andesite lava from the 1988-1990 eruption of Lonquimay Volcano, Chile. We found that the surface crust regime, for a crust strength of 2 MPa, is able to predict the entire propagation of the lava flow (figure 12). This discovery is very exciting, because it offers the prospect of being able to predict the evolution of future blocky lava flows, without the almost impossible task of predicting the rheology of the interior lava as a function of position and time. We hope that our work will encourage volcanologists to very carefully monitor both the erupted volume and flow length as a function of time in future flows, as such early detailed observations are crucial to allow dynamical models to be used to understand or predict the propagation of lava flows.

Figure 11. The formation of channelized lava flows, modeled using molten polyethylene glycol wax flowing down a wide uniform slope under cold water. The flow shown is in the mobile crust morphology. Molten wax is transparent, solid wax is white, and the base of the tank is black with a 20 cm grid.

Figure 12. Flow length as a function of time (diamonds) for the 1988-1990 andesite lava flow of Lonquimay Volcano , Chile . The triangles show the predicted propagation in the crust strength regime, for a surface crust yield strength of 2 MPa.

 

References: Kerr, R. C., Griffiths, R. W. and Cashman, K. V. (2006) The formation of channelized lava flows on an unconfined slope. Journal of Geophysical Research 111 , B10206, doi:10.1029/2005JB004225.
Kerr, R. C. and Lyman, A. W. (2007) The importance of surface crust strength during the flow of the 1988-1990 andesite lava of Lonquimay Volcano, Chile. Journal of Geophysical Research , in press.
Lyman, A. W. and Kerr, R. C. (2006) The effect of surface solidification on the emplacement of lava flows on a slope. Journal of Geophysical Research 111 , B05206, doi:10.1029/2005JB004133.