UNDERSTANDING THE YELLOWSTONE CALDERA AND ITS VOLCANIC SYSTEM

The earliest scientific studies of Yellowstone National Park and vicinity clearly recognized the vast scale of its volcanic activity. Those early studies, however, including work as recent as the 1930s and 1940s, did not explicitly recognize the considerable gap in time and contrasting origin between what are now recognized as the early Tertiary, predominantly andesitic Absaroka volcanism, and the Quaternary bimodal, predominantly rhyolitic volcanism of the Yellowstone Plateau. The nature of the Yellowstone Plateau volcanic field only became clear with the first comprehensive geologic mapping of the region since the nineteenth century, undertaken by geologists of the U.S. Geological Survey in the 1960s and 1970s.

Important advances in understanding Yellowstone Plateau volcanism shortly preceding that geologic mapping were made by F. R. Boyd, published in 1961, and Warren Hamilton, published in 1960, both of whom recognized the extremely voluminous pyroclastic rhyolitic volcanism and suggested the possibility of major subsidence in its source area. It was not, however, until our detailed geologic mapping that the specific existence, character, and size of the Yellowstone caldera was recognized and documented. Furthermore, these studies showed that the Yellowstone caldera is only one of three such great rhyolitic caldera systems in the Quaternary volcanic field. A popular misconception, repeated many times in the popular literature, is that the Yellowstone caldera was initially recognized in imagery from space obtained by NASA satellites. That misunderstanding perhaps originated because the USGS studies of the 1960s were funded in significant part by NASA; that funding, however, was premised on demonstrating “ground truth” for satellite imagery that was yet to be obtained as part of a learning process for planned satellite imagery of the Earth and moon.

Understanding of the Yellowstone Plateau volcanic system has been greatly advanced by the geophysical studies of Bob Smith and his students and colleagues at the University of Utah. Much of the interpretation of both the geologic and geophysical work has benefitted from close collaboration between the detailed and sophisticated geophysical work of Smith and his colleagues with USGS and other geologists.