The eruption of Lassen Peak in northern California was a landmark geological event that began on May 22, 1915. This volcano, located within Lassen Volcanic National Park, experienced an explosive outburst that sent ash clouds high into the atmosphere, visible from considerable distances. Prior to this eruption, Lassen Peak had been dormant for thousands of years, with no recorded activity since the last eruptive phase, which concluded around 1850. The powerful eruption produced a significant amount of volcanic ash, which blanketed the surrounding landscape and created a dramatic new topography. This eruption was characterized by explosive activity, pyroclastic flows, and sulfur dioxide emissions, challenging the understanding of volcanic behavior at the time.

Following the initial eruption, lava flows and smaller eruptions occurred over the next several years, with significant activity continuing until 1921. Scientists and volcanologists flocked to the area to observe and assess the changes occurring within the volcanic system. Lassen Peak's eruptions attracted considerable public attention, and the nearby region saw an increase in tourism as people were eager to witness the volcanic activity firsthand. This period of eruptions not only advanced the scientific community's understanding of volcanology, but it also highlighted the need for monitoring volcanic activity, a practice that became standard after this and similar events.

The eruption was part of a series of volcanic events across the western United States, offering insights into the geology of the region. The ongoing observations made during the eruption years enabled prominent volcanologists, including Joseph Pardee, to study ash deposits, gas emissions, and eruption patterns. The legacy of Lassen Peak's activity remains pertinent, as it serves as a case study for volcanoes in the continental U.S. and their potential impacts on both local ecosystems and human activities there.