HR: 1340h
AN: V53B-1575    [Abstracts]
TI: Origins and Implications of Zigzag Rift atterns on the surface of lava lakes
AU: * Karlstrom, L
EM: lkarlstr@gladstone.uoregon.edu
AF: Dept Physics, Univ Oregon, Eugene, OR 97403 United States
AU: Manga, M
EM: manga@eps.berkeley.edu
AF: Dept Earth and Planetary Science, Univ California, Berkeley, Berkeley, CA 94720-4767 United States
AB: The distinctive rift patterns observed on newly formed lava lakes are very likely a product of interaction between heat transfer (cooling of lava) and the ability of the solid crust to deform in response to applied stresses. Ragnarsson et al. ( Phys Rev Lett. 1996) observed similar features in analog wax experiments. The experimental setup consisted of a layer of liquid wax heated from below and cooled from above to create a solid crust, which is then pulled apart to form a rift filled with liquid wax. Of particular interest for lava lakes is a regime in which "zigzag" shaped rifts form. We performed a similar series of analog wax experiments designed to characterize the symmetric zigzag rift patterns associated with the cooling and deformation of a newly formed crust layer. The experimental setup is similar to that in Manga and Sinton ( JGR 2004). We vary the wind speed, which controls the cooling rate, and pulling speed. The rifts are characterized by two quantities: the angle between rifts and a line perpendicular to the pulling direction Φ, and the amplitude of the zigzags A. We find experimentally the relationship between Φ, A and cooling rate. We also develop a model to explain the observed relationships: Φ is determined by a balance of pulling and solidification speeds, the latter being governed by wind speed (the primary control on cooling rate); the amplitude A is limited by the thickness of the solid wax crust. The theoretical model is based on a local energy balance in which the conductive heat flux through solidifying crust is balanced by convective and radiative heat loss to the overlying air. Model predictions agree well with experimental results. We can scale this model to basaltic lava lakes. Observed angles are consistent with model predictions. The critical thickness of the solid crust, limiting the amplitude of zigzags (and rift propagation) is a few centimeters, consistent with other estimates (Harris et al., JVGR 2005). The model enables us to calculate crust spreading speed, or crust thickness, given the measurable Φ and A. This makes it potentially useful in other applications where these quantities are important.
DE: 8429 Lava rheology and morphology
DE: 8439 Physics and chemistry of magma bodies
DE: 8485 Remote sensing of volcanoes
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005