Abstract
The basaltic Lost Jim lava flow is the youngest member of the Imuruk Lake distributed volcanic field. The eruption of the well-preserved, inflated pahoehoe lava flow is recorded in the oral history of the indigenous Inupiaq people, and is thought to have occurred in the last few thousand years. The flow is also reported to show evidence of interaction with the underlying permafrost, forming thermokarst pits in the flow as the permafrost melted and formed cavities into which the flow top collapsed (Beget and Kargel, 2008). Our field observations, however, contradict this hypothesis. We propose instead that these are normal inflation pits (lava rise pits) since they preserve inflationary structures around their edges, like rotated surface slabs and flow banding associated with sequential brittle and ductile fracturing, both of which are hallmarks of lava flow inflation. The small diameter of many of the pits (<10 m), compared to flow thickness (at least 5-10 m), also argues against collapse-the relatively high tensile strength of a lava flow would have prevented its collapse into cavities similar in diameter to the flow's thickness. Finally, the pitted surface of the Lost Jim lava flow is similar to satellite views of many other young pahoehoe lava flows scattered across the globe, showing that the Lost Jim flow is simply a typical inflated lava flow, and permafrost played no significant role in its surface morphology.