Preliminary Investigation of the Crater Lake Breach at Mt. Pinatubo, Philippines

Abstract

The climactic eruption of Mt. Pinatubo in the Philippines on June 15, 1991, formed a summit crater lake with a diameter of about 2, 000 meters and a depth of 600 meters. A radial drainage, which originated from the Crater Lake, deeply eroded the young pyroclastic deposits, thereby providing advantageous conditions in facilitating slope failures and lahars. Steep valley wall failures and subsequent lahars have posed a considerable threat to the downstream. On July 10, 2002, during an intense storm event, a huge lahar along the Bucao River occurred, which was presumably from the summit Crater Lake breach at the Maraunot Notch; this provoked wide public attention. Consequently, aerial and site investigation was immediately carried out to understand the Crater Lake breach mechanism and the reason for the resultant lahars. This paper summarizes partly the observations and gives the conclusions about the breakout of the Crater Lake and its influence on the lahars around Mt. Pinatubo, geological explanation of the causes of the Crater Lake breach and suggestion on mitigation methods for further breakout of the Crater Lake.