Once lava erupts from a vent, its potential pathway is predictable because lava moves down hill according to the local topography. These short flows often build mound-shaped features called lava domes. The sides of a growing lava dome are steep and unstable, and sometimes collapse to form pyroclastic flows that can move outward at least 5 km from the dome. Indonesia Explosive eruptions inject large amounts of volcanic ash and corrosive gases high into the atmosphere, especially into cruising altitudes used by commercial jet airplanes. Volcanic eruption clouds may drift for hundreds to thousands of kilometers from an erupting volcano, and can contaminate heavily used aeronautical routes over wide areas.
Volcanic ash can damage aircraft flying surfaces and electronics, and cause engine failure.
Eruption clouds also may cause flights to be diverted, delayed, or canceled. Since winds above the Long Valley area blow toward an easterly direction more than 80 percent of the time, air routes east of the volcano are likely to be contaminated with volcanic ash during a future eruption. For example, air routes within a few hundred miles east of Long Valley that accommodate jets either approaching and departing San Francisco or flying in a north-south direction toward or away from Las Vegas, Nevada.