HR: 1645h
AN: V14A-04    [Abstracts]
TI: How do Continental Flood Basalt Lava Flows Attain Their Great Length and Size?
AU: * Self, S
EM: stephen.self@open.ac.uk
AF: The Open University, Department of Earth Sciences, Milton Keynes, Mk7 6AA, United Kingdom
AB: I examine the conditions required to explain the great length of continental flood basalt (CFB) lava flows on Earth. Current estimates suggest that this is in excess of 1000 km in length, with total flow-field volumes approaching 10,000 cubic kilometers. Examples from the ~ 65 Ma old Deccan and ~ 15 Ma Columbia River CFB provinces will be presented. This makes them the largest eruptive units yet reported on Earth. The two simple requirements for a long lava flow are 1) sufficient magma held in a chamber to supply a large volume of lava, and b) an emplacement system that can transport the liquid lava from vent to flow front without the lava freezing. Other environmental conditions of lava emplacement also constrain lengths. For the common lava composition in CFB provinces (tholeiitic basalt) this is a cooling range of only 100 + 50 deg. C. During flow a combination of cooling and degassing promotes groundmass crystallization that is crucial in determining the resulting flow characteristics. An inventory of long lava flows in flood basalt provinces shows them to be exclusively pahoehoe flow fields, or closely related varieties, in type. This is mandated by the fact that `a`a flows are too thermally inefficient, losing ~ 2-5 deg. C per km in the feeder channels and rapidly developing groundmass crystals, to attain lengths much in excess of 100 km on Earth. Only the insulated flow model for pahoehoe flow fields where the dominant lava body is the inflated sheet lobe fits all the quantitative criteria required to explain the great extent of CFB flows. In this model, total effusion rates at the vent(s) are 1000s of cubic meters per sec, lava travels from the vent to the flow front in days to weeks, heat losses are restricted to as little as 0.001-0.1 deg. C per km, and eruption durations are years to centuries. These values dictate that eruptive volumes must be 10s to 1000s of cubic kilometers, a condition met solely by lava flows in CFB provinces. A variety of this lava flow type sometimes found in CFB provinces, informally named rubbly pahoehoe, also meets model criteria for yielding long lava flows.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8429 Lava rheology and morphology
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting