A FIELD STUDY OF TUMULI MOUNDS AT INITIAL POINT VOLCANO, WESTERN SNAKE RIVER PLAIN, IDAHO

Initial Point, near Kuna, Idaho marks the center of a 14 km wide basaltic Pleistocene shield volcano in the Western Snake River Plain. Elliptical basalt mounds ranging in height from 0.5 to 10 meters are scattered outward from the Initial Point vent in a pahoehoe flow field that extends to the west and southwest for several kilometers. Local geology affected flow direction with surrounding buttes acting as topographic controls.

The mounds consist of jointed and weathered blocks with an axial crack and a vesiculated upper surface. Geomorphic study of the mounds identifies them as pahoehoe tumuli, similar to those seen on shield volcanoes in Hawaii and Iceland. The random distribution of the tumuli long axes supports their classification as flow-lobe tumuli, but the close proximity to the source vent is unusual for tumuli of this type. On going study is investigating internal pressures and flow regime during tumuli emplacement to more accurately classify the tumuli mounds.