Map of selected air routes above 18,000 ft.

Many air routes (gray lines) used by commerical aircraft pass over or near the Long Valley area (center of circle). The circle represents the tephra-fall hazard zone for a 1-cm thick layer of tephra, which spreads about 300 km from the Long Valley area. Mammoth Lakes is marked by "M.L."

Future explosive eruptions like those that occurred about 600 years ago from the Mono-Inyo Craters volcanic chain in the Long Valley area would generate eruption columns higher than 18,000 feet above sea level. The resulting eruption cloud would invade some of these air routes as far as several hundred kilometers downwind, depending on the size of the eruption and direction and speed of the prevailing wind. Airborne ash can diminish visibility, damage flight control systems, and cause jet engines to fail. Such an eruption would significantly affect air traffic over this part of the western United States.