MORPHOLOGY OF THE 1984 OPEN-CHANNEL LAVA FLOW AT KRAFLA VOLCANO, NORTHERN ICELAND

An open-channel lava flow of olivine tholeiite basalt, 9 km long and 1 -2 km wide, formed in a volcanic eruption that took place in the Krafl a volcano, Iceland, on the 4-18 September 1984. The eruption started w ith emplacement of a pahoehoe sheet which was fed by a 8.5-km-long fis sure. After two days of eruption, lava effusion from the fissure cease d but one crater at the northern end of the fissure continued to relea se lava for another twelve days. That crater supplied an open-channel flow that moved toward the north along the rift valley. The lava was e mplaced on a slope of 1 degrees. The final lava flow is composed of fi ve flow facies: (1) the initial pahoehoe sheet; (2) proximal slab paho ehoe and aa; (3) shelly-type overflows from the channel; (4) distal ru bbly aa lava; and (5) secondary outbreaks of toothpaste lava and cauli flower aa. The main lava channel within the flow is 6.4 km long. The m ean width of this channel is 189 m (103 m S.D.). An initial lava chann el that forms in a Bingham plastic substance is fairly constant in wid th. This channel, however, varies in width especially in the proximal part indicating channel erosion. Large drifted blocks of channel walls are found throughout the flow front area and on the top of overflow l evees. This suggests that the channel erosion was mainly mechanical. T he lava flow has a mean height of 6 m above its surroundings, measured at the flow margins. However, a study of the pre-flow topography indi cates that the lava filled a considerable topographic depression. Comb ined surface and pre-flow profiles give an average lava-flow thickness of 11 m; the thickness of the initial sheet-flow is estimated as 2 m. The volume of the lava flow calculated from these figures is 0.11 km( 3). The mean effusion rate was 91 m(3)/s. When lava flow models are us ed to deduce the rheological properties of this type of lava flow, the following points must be considered: (1) when a lava flow is emplaced along tectonic lineaments, its depth and volume may be significantly larger than what the surface exposure suggests; (2) lava channels may become severely eroded during channel flow even if a lava flow was for med in a relatively shea time; (3) the levee dimensions, and hence lav a flow dimensions, may be significantly altered by extensive overflows .