Geology 108 Key to Homework #3

Mt. Pinatubo, 1991 eruption

2. Main impacts include tephra fall deposits which led to local building collapse (probably assisted by seismicity and strong winds), and lahars which inundated many settlements along local drainages. The latter process remains quite active to date as storm runoff erodes the poorly consolidated volcanic deposits and reworks them. About 300 people were killed, mainly by collapsing roofs. Crops and forest were buried and much agricultural land was rendered unuseable. Drainages were choked with debris from the eruption and this led to flooding by rain runoff as new channels were excavated. Economic devastation and social disruption from those lahars were severe (buried towns and agricultural land, destroyed homes and bridges, and frequently cut roadways). Main relief efforts centered on relocating and rebuilding some communities. A significant population (~30,000) remains displaced from their pre-1991 homes. The situation remains uncertain for the near future; there will likely be repeated disruptions until vegetation restabilizes the land surfaces and drainages are reestablished.

3. About 4000 square kilometers was impacted with significant tephra and other deposits (>5 cm thickness, with maxima approaching 0.5 m). A circle of this area has a radius of 35.7 km (21.4 mi, comparable to the area of 'Greater Houston'). Tephra actually covered a more extensive region albeit with lesser accumulations; fallout was recorded at distances up to 1200 km to the southwest (prevailing wind direction). Total volume of airfall material is ~2-4 cubic km. Pyroclastic flow deposits added another 5-7 cubic km. These are based on preliminary reconnaissance surveys; web site http://volcano.und.edu/vwdocs/Gases/pinatubo.html quotes a volume of 5 cubic km.

4. Assuming that Pinatubo produced a total of ~5 cubic km of combined tephra and pyroclastic flow material we can compare with Cascades eruptions as follows: 1980 Mount St. Helens (~1 km³); 7000 ka Mazama climactic eruption (~40 km³). So Pinatubo was comparable to some moderate Cascades events, but considerably larger than the 1980 MSH eruptions which caused considerable economic and social disruption.

5. There are 14 major tephra sets at MSH (each containing multiple closely related layers). Diviving this number into 50,000 years, we get a rough recurence interval for eruptive episodes of about 3500 years. With more detailed records preserved or identified for more recent MSH activity, it is apparent that either the actual recurrence intervals should be shorter or the periodicity is non-linear and becoming more frequent with time.

6. Impact of a repeat event near population centers would cause severe disruption of infrastructure, loss of property and forest, possibly some fatalities, and severe economic effects.

7. The main action was to evacuate most of the people within the impact zone of the volcano. This probably saved tens of thousands of lives. There was little that could be done to save property, especially close to the volcano or along nearby drainages. Now, almost ten years later the area is far from returning to normal conditions.