HR: 08:00h
AN: V31D-01 INVITED     [Abstracts]
TI: Basaltic Lava Channels
AU: * Cashman, K V
EM: cashman@uoregon.edu
AF: University of Oregon, Department of Geological Sce, Eugene, OR 97403 United States
AU: Griffiths, R W
EM: Ross.Griffiths@anu.edu.au
AF: Australian National University, Research School of Earth Sci, Canberra, ACT 0200 Australia
AU: Kerr, R C
EM: Ross.Kerr@anu.edu.au
AF: Australian National University, Research School of Earth Sci, Canberra, ACT 0200 Australia
AB: In Hawaii, the mode of lava transport - through open channels or through insulating lava tubes - determines the thermal, rheological, and emplacement history of a lava flow. Most Hawaiian lavas are erupted at near-liquidus temperatures and are therefore crystal-poor; lava transport through open channels allows rapid cooling and consequent rapid increases in lava crystallinity. Solidified aa flows resulting from channelized flow are typically fine-grained throughout their thickness, indicating cooling of the entire flow thickness during transport. In contrast, transport of lava through insulating tubes permits flow over long distances with little cooling. Flows emerging from such tubes typically have pahoehoe flow surfaces with glassy crusts. Groundmass textures that coarsen from the flow rind to the interior reflect rates of post-emplacement, rather than syn-emplacement, cooling. To distinguish eruption conditions that result in lava channels from those that allow formation of lava tubes, we have performed a series of laboratory experiments involving injection of PEG 600 (a wax with a Newtonian rheology and freezing temperature of 19§C) into cold water through both uniform and non-uniform sloping channels. In uniform channels, tube formation can be distinguished from open channel flow using a dimensionless parameter based on a solidification time scale, an advection time scale, and a Rayleigh number that describes convection by heat loss from crust-free shear zones. Theoretical analysis predicts that in the open channel regime, the width of the crust (dc) will vary with the channel width (W) as dc = W$^{5/3}$. Crustal coverage of non-uniform channels in both laboratory experiments and field examples from Kilauea Volcano, Hawaii, is consistent with this prediction. However, experiments in non-uniform channels illustrate additional controls on the surface coverage of lava channels. Most important is crustal extension resulting from flow acceleration through constrictions or channel bends that exposes more core lava to cooling than simply that of the shear zones. Thus the channel geometry plays a major role in the thermal history of a flow. As lava flows rarely flow through pre-existing channels of prescribed geometry, we have performed an additional set of analog laboratory experiments to determine the relationship between flow rate, slope, and channel formation in solidifying flows. All flows develop stable uniform channels within solidified levees except when the flow rate is sufficiently low to permit flow front solidification, inflation, and tube formation. On constant slopes, increasing flow rates result in increases in both the rate of flow advance rate and the channel width, and a decrease in levee width. At constant flow rates, both channel width and levee width decrease with increasing slope while flow advance rate increases. Limited data on the geometry of basaltic lava channels indicate that experimental data are consistent with field observations, however, both additional field data and scaling relationships are required to fully utilize the laboratory experiments to predict channel development in basaltic lava flows.
DE: 8414 Eruption mechanisms
DE: 8419 Eruption monitoring (7280)
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting