Yellowstone National Park is home to more than 10,000 hot springs and geysers, including the famous Old Faithful. Such abundant geothermal activity offers evidence of an underlying hot spot--an uprising plume of melted rock from the earth's mantle that can melt portions of the crust in large magma chambers. Now scientists at the University of Wisconsin-Madison have mapped the historical volcanic activity in the region over the past two-million years and suggest that a major eruption might happen in the distant future. Their results appear in this month's issue of Earth and Planetary Letters and in next month's Journal of Petrology.

The exotic landscape of Yellowstone that draws tourists from around the globe features many large craters, or calderas, formed by volcanic explosions or by the collapse of volcanic cones. The periodicity of the last three major eruptions--two-million years ago, 1.3-million years ago and 600,000 years-ago--follows a pattern in which the next catastrophic eruption would occur within 100,000 years. To study this pattern, the scientists examined rocks that spanned all three eruptive events. They concluded that the volcanic cycle is waning, but that the possibility for a massive eruption in the near geologic future (between 100,000 and one-million years from now) still exists.

"These magmas usually erupt in a very catastrophic way," co-author Ilya Bindeman says. "By comparison, the eruption of Mount St. Helens sent about two cubic kilometers of ash into the atmosphere. These catastrophic types of eruptions send thousands of cubic kilometers skyward."

The new research also suggests that the volcanic activity at Yellowstone does not involve new magma erupting from deep within the earth. By studying isotopes of oxygen present in samples of zircon and quartz from the park, the scientists propose that surface rocks get completely remelted and are recycled in subsequent eruptions.

"The unique thing about Yellowstone is that the volcanic rocks that erupted following the collapse of the big calderas contain up to 50 percent oxygen which was ultimately derived from rain waters," Bindeman adds. "The zircon and quartz tell us that rocks near the surface were altered by heated snow and rainwater. These rocks were then remelted to become magmas."