Ash/Tephra Fall

A good vantage point for viewing the region.

A tephra ring and dome, plus pumice and banded obsidian.

Volcanic unrest through the 1980's to 1990's in the southern part of the Long Valley caldera reminds us that the volcanic system is young. Volcanic activity and related hazards are likely in the future. USGS scientists closely monitor the area and research past activity to better understand what might happen in the future.

Possible eruption sequence for the Long Valley-Mono Lake area.

During a typical explosive eruption of a Mono-Inyo vent, tephra (volcanic ash and larger rock fragments) may accumulate near the vent to a thickness of tens of meters (yards).

The Bishop Tuff refers to the deposits of ash and pumice ejected during the enormous eruption that created Long Valley Caldera.

Downwind deposits of ash produced by an explosive eruption could reach thicknesses of at least 20 cm at a distance of 35 km (8 in. at 22 mi), 5 cm at a distance of 85 km (2 in. at 53 mi), and about 1 cm at a distance of 300 km (0.5 in. at 185 mi).

The broad depression of Long Valley Caldera that we see today is much shallower and a little larger in diameter than it was immediately after its formation about 760,000 years ago.

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.

The resurgent dome is a broad area of the central caldera floor that was pushed upward within 100,000 years or less of the caldera-forming eruption 760,000 years ago.

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.

Since the enormous explosive eruption formed the caldera about 760,000 years ago, hundreds of smaller eruptions have partially filled the initially deep depression.