LAVA TUBE, LAVA RISE, FLOW LOBE, HOLLOW TUMULUS AND OTHER CAVES OF THE 1919 "POSTAL RIFT" LAVA FLOW, KILAUEA CALDERA, HI: A SUBCRUSTAL FOLLOWUP OF GEORGE WALKER'S 1991 PAPER

In 1991, George PL Walker discussed "tumuli, 'lava rises', 'lava rise pits' and 'lava inflation clefts'" in a small section of the 1919 "Postal Rift" lava flow in Kilauea Caldera, HI. In that paper he described and depicted one hollow tumulus and noted the presence of a subcrustal grotto in a lava rise. In the course of a 12-year intermittent survey of caves throughout this flow, the Hawaii Speleological Survey subsequently expanded this study area to include the entire 1919 flow. We identified and mapped 1) a second hollow tumulus with a sizeable "melthole" connection to the one depicted by Walker, 2) a complex circumferential cave in the perimeter ridge of one lava rise he depicted plus a segmented circumferential cave in the perimeter ridge of another, and 3) a penetrable crevice cave which drained a lava pond in the second lava rise. In addition we inventoried other hollow tumuli and other lava rise caves just outside Walker's study area, and nearly 200 other caves of several types elsewhere in this flow. The internal morphology of the lava rise caves differs significantly from that of the hollow tumuli, and substantiates Walker's differentiation of lava rises from tumuli. Also, nearly all of the 200+ caves in this flow are drained structures originally formed by subcrustal injection of lava rather than as classical lava tube conduit caves. This provides additional insight into the emplacement of pahoehoe flow fields.