The Natural and Political Caldera-lake crisis of Mount Pinatubo, Philippines, 2001

In 1991 Mount Pinatubo's eruptions produced a caldera with a 5.4 km2 catchment that ever since has gathered a lake with a surface that has risen roughly 10 m every rainy season. The rim is lowest at 960+ masl in the northwest, at the Maraunot notch, named after the stream into which overtopping lake water would drain, thence along the Balin Baquero and Bucao to the town of Botolan and the South China Sea 40 km downstream. In December 2000, with only 10 m of remaining notch freeboard, Philippine government geologists and American colleagues were aware of the potential for catastrophic breaching, because the surface 20 m of rock beneath the rim is highly erodible breccia. A breakout of as much as 60 x 106 m3 is considered possible. Easily eroded eruption debris is abundant in the path of the flood, which could "bulk up" into worst-case lahars with a volume of 3 x 108 m3. Government engineers discussed lowering the lake with siphons or a tunnel, or scraping down and strengthening the notch, but did nothing. Only in August, three months into a very wet monsoon season, when only about 5 m of freeboard remained, did the government inform the 46,000 Botolan inhabitants of the danger. It did so only after Oxfam GB, a humanitarian organization, issued a report written by private geological consultants familiar with Pinatubo and its lahars. The crisis, still evolving, unfortunately is pitting government attitudes and policies -- strict control of information and decisions regarding hazards -- against those of academic science, and of some NGOs concerned with community development and empowerment. In August, the government abruptly abandoned its initial denials of a serious threat, and decided to build a canal with which to induce a breach at a propitious time. Poorly paid and supervised aborigine labor has inadvertently reduced the freeboard to only 2 m, and the government is announcing that it will evacuate Botolan and induce the breach as early as September 5. Spontaneous breaching during rainstorms remains very possible.