-- Edward W. Wolfe and Thomas C. Pierson, 1995, Volcanic-Hazard Zonation for Mount St. Helens, Washington, 1995: USGS Open-File Report 95-497
| Mount St. Helens remains a potentially active and dangerous volcano, even though it is now (1995) quiescent. In the last 515 years, it is known to have produced 4 major explosive eruptions (each with at least 1 cubic kilometer of eruption deposits) and dozens of lesser eruptions. Two of the major eruptions were separated by only 2 years. One of those, in 1480 A.D., was about 5 times larger than the May 18, 1980 eruption, and even larger eruptions are known to have occurred during Mount St. Helens' brief but very active 50,000-year lifetime. Following the most recent major eruption, on May 18, 1980, there were 5 smaller explosive eruptions over a period of 5 months. Thereafter, a series of 16 dome-building eruptions through October 1986 constructed the new, 270-meter- (-880- feet) high, lava dome in the crater formed by the May 18, 1980 eruption. Future eruptions are certain. Although we do not know when the next one will occur, it should be planned for. This report delineates areas that are likely to be at risk (hazard zones) during another major eruption. It updates previous assessments, taking into account both recent experience at erupting volcanoes and topographic, hydrologic, and geologic changes initiated at Mount St. Helens by the 1980 eruptions. These changes include (1) beheading of the summit, forming a truncated cone with a deep crater open to the north, (2) a large and growing volume of snow and ice in the crater, (3) the existence of a large, potentially unstable, debris-dammed lake (Castle Lake) in a tributary to the North Fork Toutle River, and (4) large volumes of erodible sediment in most of the river valleys draining the volcano. -- Wolfe and Pierson, 1995 |
![Graphic [13K,GIF]: Eruptive history of Mount St. Helens shown on a logarithmic time scale](https://webharvest.gov/peth04/20041025184726/http://vulcan.wr.usgs.gov/Imgs/Gif/MSH/OFR95-497/figure1.gif){kind=link}
![Graphic [6K,GIF]: Volume of ice and snow in the Mount St. Helens crater](https://webharvest.gov/peth04/20041025184726/http://vulcan.wr.usgs.gov/Imgs/Gif/MSH/OFR95-497/figure4.gif){kind=link}