Magma plumbing beneath Yellowstone.

Yellowstone is one of the most seismically active areas in the western United States. In addition to seismicity, Yellowstone experiences active ground deformation mainly concentrated at or near its caldera, a massive basin-shaped collapse structure formed shortly after the last explosive volcanic eruption 630,000 years ago. These characteristics, along with Yellowstone's hydrothermal features, are evidence of an active magma reservoir underlying the caldera. Previous studies indicated the accumulation of partially molten rocks (melt) beneath Earth's crust in Yellowstone (1–4). However, their origin and interaction have been elusive. On page 175 of this issue, Cao et al. (5) report that melt can form in Yellowstone by tectonic forces alone, without a mantle plume—narrow pillars of hot material that rises through the mantle to the base of the crust. The finding is crucial for evaluating hazards at the Yellowstone volcanic system and other similar volcanic systems around the world. [ABSTRACT FROM AUTHOR]