The May 18, 1980 eruptive column at Mount St. Helens fluctuated in height through the day, but the eruption subsided by late afternoon. By early May 19, the eruption had stopped. By that time the ash cloud had spread to the central United States. Two days later, even though the ash cloud had become more diffuse, fine ash was detected by systems used to monitor air pollution in several cities of the northeastern United States. Some of the ash drifted around the globe within about 2 weeks. -- From: Tilling et.al., 1990, The Eruptions of Mount St. Helens: Past, Present, and Future: USGS General Interest Publication.