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. And now you know. Join us again every week for a new CoreFact. For other CoreFacts, or for CoreCast, our in-depth science podcast, go to usgs.gov/podcasts. If you’d like to have a question featured on our show, give us an e-mail at firstname.lastname@example.org or a phone call at 703-648-5600. Remember, long distance fees do apply. CoreFacts is a product of the U.S. Geological Survey, Department of the Interior.