A period of explosive volcanism rocked the western United States approximately 25-40 million years ago (Armstrong and Ward, 1991), producing hot ash flows and volcanic deposits that covered parts of the region. This event, termed the Mid-Tertiary Ignimbrite Flare-up, erupted a truly voluminous amount of igneous material, perhaps over 120,000 cubic miles of rock (Johnson, 1991)!! This volume is almost unimaginable so to get a better handle on comprehending the amount of ash erupted erupted in this event, consider that Colorado has a land area of about 104,000 square miles (U.S. Census Bureau, 2002). If the ash from this explosive volcanic event was deposited across Colorado only, a blanket of rock over a mile thick would cover the state!! The image to the right is a pyroclastic flow off of Mt. St. Helens in 1980 (CVO, 2002a). About seven-tenths of a cubic mile of material was erupted from Mt. St. Helens (CVO, 2002b). Incredibly, the Mid-Tertiary Ignimbrite Flare-up was almost 200,000 times as voluminous as Mt. St. Helens. To learn more about the flare-up, please follow the links below. If you are looking for a description of the event with minimal scientific jargon, click on the Summary link. If you are interested in more detailed information, then click on the Investigation link. For complete references, please select the References link.
SUMMARY | INVESTIGATION | REFERENCES
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Pyroclastic flow off Mt. St. Helens
(CVO, 2002a). Photo by P. Lipman.