IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

0P2_3H-O8Room A5

Date/Time: July 20 16:30-16:45

Emplacement of continental flood basalt lava flow fields
Stephen Self1 , Charlotte Vye-Brown2 , Richard J Brown3
1US-NRC, NMSS Mail Stop EBB 2B2, Washington DC 20555, USA, 2 British Geological Survey,
Murchison House, West Mains Road, Edinburgh, EH9 3LA, UK, 3 Department of Earth Science, Durham
University, Science Labs, Durham, DH1 3LE, UK
E-mail: steve.self1815@gmail.com

We examine the conditions required to explain the great length (up to 1000 km) and the large volume (perhaps up
to 10,000 cu km) of continental flood basalt lava flows. Two simple requirements for a long lava flow are 1: sufficientmagma held in a chamber to supply a large volume of lava, and 2: an emplacement system that can transport theliquid lava from vent to flow front without the lava freezing. For the common lava composition in many provinces,including the Columbia River basalt (CRB) flood basalts, this is a cooling range of only approximately 150 plus
or minus 50 C. During flow-field emplacement, a combination of cooling and degassing promotes groundmass
crystallization that is crucial in determining the resulting flow characteristics. All large lava flows in flood basalt
provinces are exclusively pahoehoe, or closely related varieties. This is mandated by the fact that aa lava flows are
too thermally inefficient, cooling by approximately 2 to 5 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 flood basalt flows: effusion rates are on the order of 1000s cu m 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 to 0.1 C per km, anderuption durations are years to centuries. These values dictate that eruptive volumes must be 100s to 1000s of cukm, a condition met by only by lava flow eruptions in flood basalt provinces. Such enormous mean output rates
from fissure vents (100s-1000s cu m per s) would feed numerous inflating sheet lobes (each containing 0.05-0.5
cu km of lava), into which most of the lava output is emplaced. However, each lobe need only have local injection
rates of a few to 10s of cu m per second in order to thicken within plausible time-frames (months to a few years).
Significantly higher mean output rates (equating to much shorter eruption duration rates) do not provide sufficient
time to explain the growth of inflated lobes with the observed crustal thicknesses and show that, compared withalmost all historic basaltic volcanism, rates at which typical CFB lavas formed were at times spectacularly high,especially for certain parameters. New results explaining the details of internal lava transport within simple flow
fields from the CRB suggest that there is only time to emplace the observed number of sheet lobes if several areforming at once. Vent systems for flood basalts are poorly understood, and maps of vents on the 180-km-long CRB
Roza fissure system will help us understand the nature of venting and eruption columns, important for assessmentof the environmental impact of flood basalt volcanism.

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