YELLOWSTONE NATIONAL PARK — Following the hydrothermal explosion heard ’round the world at Biscuit Basin in 2024, the Yellowstone Volcano Observatory (YVO) is shedding some light on the way geologists research past explosions.
Analyzing the timeline of past hydrothermal explosions is notoriously difficult, according to University of Texas, Arlington, graduate student Karissa Cordero, who wrote the most recent installment of YVO’s Caldera Chronicles. Rocks and sediment disturbed by these explosions can continue to be moved around and affected by the elements in the thousands of years that elapse between incident and research. Cordero writes that luminescence dating methods can fill in the gaps in ways that other geochronologic techniques cannot.
“Luminescence dating methods determine the last time sediment grains were exposed to geothermal heat or sunlight,” the column reads.
Cordero writes that the luminescence signal can record the timing of the hydrothermal explosion due to the scattering and burial of debris.
“Before an explosion, these grains were constantly heated by the hydrothermal fluids flowing around them keeping the signal at zero,” Cordero writes. “But after an explosion, grains were ejected out onto ground surface, cooling them to air temperature marking the point at which the luminescence signal begins to build, like batteries beginning to charge. The luminescence signal that builds up after an explosion will not reset unless the grains are re-exposed to geothermal fluids or sunlight. This is typically the case for explosion deposits, where grains are ejected from the source area and then buried.”
Scientists collect sediment samples from explosion deposits and bring them to a laboratory with light-safe conditions. The dim amber lighting of these labs does not significantly affect the luminescence signal in the samples, per Cordero. Researchers will expose the sample to a controlled amount of light or heat to stimulate the emission of luminescence; the intensity of the light indicates the level of radiation absorbed since the explosion.
“This natural luminescence signal is compared to results from laboratory irradiation experiments, where the grains are exposed to known amounts of radiation,” Cordero writes. “This process helps determine the amount of radiation needed to reproduce the natural signal, known as the equivalent dose. The environmental radioactivity at the sample site, known as the environmental dose rate, is also measured to determine the rate at which the luminescence signal accumulates. The age of the explosion deposits is calculated by dividing the equivalent dose by the environmental dose rate. This age corresponds to the last time the grains were heated up — in the case of Yellowstone deposits, that’s the age of the hydrothermal explosion!”
Cordero points to Pocket Basin, one of Yellowstone National Park’s largest hydrothermal explosion craters, located in Lower Geyser Basin. The basin measures an estimated 1,200 by 2,600 feet (or 365 by 800 meters), and was believed to be the result of a glacial outburst flood from the Pinedale Glaciation ice age.
“Luminescence dating results help to refine this story,” Cordero writes. “Results from grains extracted from many samples taken around this crater rim suggest that the explosion took place 13,900 years ago (with an error of about 3,900 years), which is consistent with the time of deglaciation following the Pinedale period. This timing suggests that changing surface conditions associated with glacial retreat may have influenced the hydrothermal system, potentially triggering explosive activity.”
Fans of the park’s hydrothermal features won’t have to wait much longer to see them for themselves. Park entrances and roads are beginning to open to visitors for the season.
