July 15, 2002 -- The Yellowstone hotspot, which powers Yellowstone National Park's geysers and hot springs, produced 142 huge volcanic eruptions during the last 16.5 million years - far more than the 100 previously known blasts, University of Utah geologists found.

The cataclysmic explosions - known as "caldera eruptions" - typically generated 250 to 600 times as much volcanic ash as the 1980 eruption of Mount St. Helens in Washington state, and some were up to 2,500 times larger, covering as much as half the continental United States with inches to feet of volcanic ash.

While geologists Michael Perkins and Barbara Nash identified many more of these catastrophic eruptions than had been known previously, they also showed the rate of such eruptions has slowed: about 32 giant eruptions per million years before 15.2 million years ago, slowing to 10 to 20 huge eruptions per million years between 15.2 million and 8.5 million years ago, and then only 2.5 cataclysmic blasts per million years during the past 8.5 million years.

Caldera eruptions have that name because they create giant craters known as calderas that measure tens of miles wide. They are the most devastating but most rare type of eruption.

The Yellowstone hotspot - which many scientists believe is a plume-like zone of hot and molten rising from at least 125 miles beneath Earth's surface - produced its three most recent caldera eruptions at or near the present site of Yellowstone National Park 2 million, 1.3 million and 642,000 years ago. The other, earlier eruptions happened as western North America drifted southwest over the hotspot during the past 16.5 million years, creating a chain of volcanic fields or centers extending from the Oregon-Nevada-Idaho border northeast across southern Idaho toward Yellowstone's present location in Wyoming.

Researchers previously identified about 100 caldera eruptions, including the three most recent ones at Yellowstone. But in a study of distinct volcanic ash layers deposited by each eruption, Perkins and Nash showed there were at least 142 catastrophic caldera eruptions during the past 16.5 million years, and at least four more in the preceding 500,000 years.

"The most active source of volcanism in the continental United States - the Yellowstone hotspot - was much more active and produced much greater volumes of volcanic material in the last 16 million years than we had thought as it passed from Oregon across Idaho to its present site at Yellowstone National Park," Nash says.

The study was published in the March 2002 issue of Geological Society of America Bulletin.

Only the hotspot's three most recent caldera eruptions originated from the hotspot's current location beneath Yellowstone National Park. The 45-by-30-mile-wide Yellowstone caldera, formed by the caldera explosion 642,000 years ago, is the "new kid on the block," Nash says. To understand the Yellowstone hotspot's long-term behavior, Perkins and Nash looked for clues left by the older eruptions from now-inactive calderas.

Nash, a volcanologist, compares the hotspot phenomenon with moving hand over a candle. "If you move your hand slowly over the flame in one direction, the flame will leave a burn track that extends in the opposite direction along your hand."

As the hotspot location moved, new calderas were born, matured, and died. The 142 eruptions were clustered in six or seven volcanic fields or "centers" extending from the Oregon-Idaho-Nevada border northeast to Yellowstone. There were multiple eruptions from one or more calderas at each field.

Detecting the locations of these ancient cataclysmic eruptions is difficult because as Earth's crust drifted over the hotspot, smaller "post-caldera" eruptions covered or destroyed the older volcanic centers. However, each huge caldera eruption left behind widespread ash deposits, which researchers have found from the Pacific seafloor off the California coast to the high plains of Nebraska and south to the Gulf of Mexico.

Perkins and Nash spent a decade finding these ash fall tuffs - rock beds formed as volcanic ash settled to the ground and cooled - studying their chemistry and determining a chronology of the eruptions that produced them. The age and chemical composition link each tuff to a specific eruption and to one of the six or seven volcanic centers along the hotspot track.

Scientists date volcanic ashes using the radioactive decay of potassium-40 to argon-40 in a potassium mineral called sanidine. Because argon is a gas, researchers assume that no argon was present in the mineral when it was erupted. Argon accumulates in the mineral's crystal lattice as potassium decays. Thus the amount of argon reveals the age of the ash and the date of the eruption that produced it. The age of undated ashes can be estimated from the ages of ash layers above and below them.

The hotspot's history is marked not only by a slowdown in how often big eruptions occur, but by distinct changes in the compositions and temperatures of the magma, or molten rock, that erupted. Based on such changes, Perkins and Nash classify the hotspot's activity into three stages of volcanic activity or "magmatism" in the past 16.5 million years. Nash also hypothesizes about an earlier period based on ash fall tuffs from four big eruptions not included in the count of 142 because of their different chemical composition. She believes there may have been even more eruptions from this earlier stage some 17 million years ago.

Magma temperatures from millions of years ago can be deduced using mineral geothermometers. Some minerals have chemical compositions that vary depending on the temperature at which they crystallize from molten to solid rock. So the chemical composition of crystals in the ash indicates the temperature of the magma when it erupted and started to cool.

Not only have caldera eruptions become less frequent over time, the erupting magma has also become cooler, decreasing from more than 1830 degrees Fahrenheit about 16 million years ago to as little as 1470 degrees Fahrenheit in the past 7.5 million years.

Yellowstone hotspot caldera eruptions are believed to stem from molten basalt rising from depth, and then melting and mixing with the overlying granitic crust, which in turn erupts. Nash believes the decrease in magma temperatures over time means that less high-temperature basalt is incorporated in the melting process.

The three eruptions at Yellowstone during the past 2 million years were, respectively, 2,500, 280, and 1,000 times larger than the 1980 Mount St. Helens eruption. Some researchers speculate the earlier caldera eruptions were "a bunch of little guys," but Nash says some of their ash deposits were just as thick and widespread, "so we conclude they were large too."

Nash says there are a few possible explanations for why caldera eruptions have become less frequent. First, the hotspot's heat source within the Earth could be cooling down. Second, as North America drifts over the hotspot, the crustal rock above the hotspot may be thicker, cooler and harder to melt, although Nash says there is no persuasive evidence of that.

A third possibility, which Nash believes is most likely, links the hotspot's behavior to two kinds of motion of the overlying rock: the southwest movement of the North American plate of Earth's crust, and the east-west stretching apart of the crust in the western United States during the past 17 million years.

Hotspot volcanism occurs when molten basalt rises from Earth's mantle and melts granite in the crust, feeding caldera eruptions. If the crust remained still instead of moving, caldera eruptions would continue until the basalt ran out of fresh material to melt. On the other hand, if North America moved too quickly over the hotspot, then the hotspot wouldn't have time to melt the overlying crust and there would be no eruptions. Between these two extremes is an optimum plate speed that allows the maximum amount of crustal melt to be produced, resulting in more frequent and larger eruptions.

North America drifts southwest at a constant rate of almost 14 miles per million years. But the east-west stretching of Earth's crust in the West has slowed in the past 8.5 million years. The net effect, says Nash, is that crustal rock is now moving over the hotspot more slowly than it did prior to 8 million years ago, so crustal rock melts less efficiently and big eruptions are becoming less frequent.

"There is no reason to expect any sudden change" in the current rate, which has produced three caldera eruptions in the past 2 million years, she says. "I anticipate there will be future large-scale eruptions at Yellowstone, but not in my lifetime or not in the foreseeable future. That doesn't mean there couldn't be smaller eruptions as there have been during the last 600,000 years - lava flows, small eruptions, and steam events."