EXTENT AND EMPLACEMENT OF CONTINENTAL FLOOD BASALT LAVA FLOW FIELDS

Appreciation of the immense size and extent of the Fish Canyon Tuff, recognized as the world’s largest explosive eruption unit (M 9.1, or ~ 1.2 x 10¹⁶ kg of magma), has been elucidated by Peter Lipman and colleagues over many years of mapping. As Pete can attest, it takes the most amount of work to define the true extent of the biggest units. This applies to both ash-flow tuff (ignimbrite) and flood basalt super-eruption units. Work that was begun in the Columbia River flood basalts (CRB) by Steve Reidel and Peter Hooper and others, and applied elsewhere (such as the Deccan, DVP) show that some continental flood basalt (CFB) eruption units may attain M 9 or above. This talk demonstrates the size and extent of CFB eruption units using examples from the DVP and CRB - the latter being the only CFB province where extents of individual eruptive units has been defined by many years of mapping, compositional, and paleomagnetic studies. We examine conditions required to explain the great length (1000 km) and large volume (~ 10,000 km³) of some continental flood basalt lava flows. Two simple requirements for long lava flows are 1) a sufficiently large magma reservoir to supply a large volume of lava, and b) an emplacement system that can transport liquid lava from vent to flow front without the lava freezing. For the common lava composition in many provinces, including the CRB and DVP, this is a cooling range of only ~ 150 + 50 ^oC. All long CRB flows are exclusively pahoehoe flow fields, or closely related varieties. This is mandated because aa flows are too thermally inefficient, losing ~ 2 to 5 ^oC per km in the feeder channels while undergoing groundmass crystallization, to attain great lengths (usually < 100 km). Only the insulated flow model for pahoehoe flow fields, where the dominant lava body is the inflated sheet lobe, fits all quantitative criteria required to explain the extent of CFB flows: mean output rates are on the order of 1000s m³/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 ^oC per km, and eruption durations are years to centuries. These values dictate that eruptive volumes must be 100s to 1000s of km³, a condition met only by lava flow eruptions in CFB provinces.

[Statements herein are those of the author and do not reflect the view or regulatory position of US NRC.]