Our knowledge on how and when these three calderas formed is a result of many years of extensive field work and geologic mapping led by U.S. Geological Survey (USGS) scientist Bob Christiansen. A caldera is large basin-shaped volcanic depression commonly formed when magma is withdrawn or erupted from a shallow underground reservoir. The removal of large volumes of magma results in loss of structural support for the overlying rock, thereby leading to collapse of the ground.

In the 1950s Joe Boyd, a doctoral student at Harvard University, carried out research on the rhyolite rocks in Yellowstone. Boyd recognized that some of the rhyolites were not lava flows, but rather tuffs (solidified deposits of ash that formed from explosive eruptions). He also identified a depression in the Yellowstone Plateau and suggested that it was related to the explosive eruptions. Boyd published his novel finding in a journal paper entitled "Welded tuffs and flows in the Rhyolite Plateau of Yellowstone Park, Wyoming". The causal link between the tuffs and the depression, however, remained unexplained.

Bob Christiansen was part of a USGS group that was tasked in 1965 with creating a comprehensive geologic map of the Yellowstone Plateau. Christiansen and his colleagues were asked to map the volcanic rocks that erupted during the Quaternary Period of Yellowstone, now recognized as the last 2.6 million years. The study was funded by several federal agencies including NASA, the National Park Service and the USGS. At the time, NASA was developing remote sensing instruments for satellites and needed test areas with good geologic maps to compare with the images captured from space.

Christiansen and his colleagues tried to understand when and how the volcano erupted, how much magma was vented, and where it went. Every year between 1966 and 1971 they spent three summer months in Yellowstone carrying out field work, often in very remote areas. In other times of the year, they analyzed aerial photographs, used microscopes to scrutinize slices of sampled rocks thinned to less than the width of a human hair (called "thin sections"), and measured the chemical composition of the rocks and their relative ages.

Sources/Usage: Public Domain. View Media Details

By 1972 the bulk of the geologic mapping was complete, and Christiansen and his colleague Dick Blank published the first report describing the volcanic history of the Yellowstone Plateau. Their findings revealed many of the details of Yellowstone's explosive volcanic past. They found that three large explosive eruptions were associated with formation of calderas. Based on potassium-argon dating by their USGS colleague John Obradovich, it was determined that the three large explosive eruptions occurred at 2.0, 1.2 and 0.6 million years ago. More recent results using more accurate dating methods revealed that these caldera forming eruptions took place 2.08 1.30, 0.63 million years ago.

Christiansen continued his work in Yellowstone for the next 25 years, mainly focusing his research on specific questions related to the volcanic history. His goal was to improve our understanding of the possible conditions that led to large eruptions. The comprehensive description of Yellowstone's volcanic history and the geologic map were published in 2001.

During that same year, Bob Christiansen became the founding Scientist-in-Charge of the Yellowstone Volcano Observatory. After 40 distinguished years with the USGS, he retired in 2004. Bob has described his time at Yellowstone in this interview. Today, Bob still continues his research as a scientist emeritus with the USGS in Menlo Park, California.