Long Valley Caldera Field Guide
View from Lookout Mountain of Obsidian Dome and surrounding area.
The Long Valley Volcanic Center is an excellent place to learn about volcanoes. Many visitors hike, ski, mountain bike, and fish in the area without realizing they are inside, on, or next to a volcano. The following ten stops were chosen from published scientific field guides and from Long Valley Observatory field trip notes to represent the geology of the area. Each site highlights a specific geologic feature that is also accessible to the public and near major roads. We hope these sites provide a helpful introduction to the geology of the area.
Field Guide List with Stop Significance
| Site Name | Significance |
| Mono Lake | View the beautiful saline lake with tufa towers that formed under the water. |
| Panum Crater | Classic example of a crater, ejecta ring, and lava dome. Learn how volcanic eruptions produce these features. |
| Lookout Mountain | Stand on fragments of obsidian at the summit of a volcano that erupted shortly after the formation of the Long Valley Caldera. Excellent viewpoint of the Inyo chain. |
| Obsidian Dome | Explore the summit of a dome and learn how rock of the same composition can look different depending where it was during cooling. |
| Glass Creek Dome | Examine dome rocks with a marble-cake texture that was created by the mixing of two different magmas from separate sources. |
| Inyo Craters | View one or more of the three craters formed by steam-driven (phreatic) eruptions. Learn about failed magmatic eruptions. |
| Mammoth Mountain | Stand on a cluster of domes at 11,030 ft (3,362 m). Excellent viewpoint of Long Valley Caldera and field guide stops from far above. |
| Horseshoe Lake | Learn about how trees were killed by CO2 that has been rising to the surface and how the CO2 is related to earthquakes and cooling magma. |
| Devil's Postpile | Examine a classic example of columnar lavas and learn how basalts cool to form hexagonal patterns. |
| Hot Creek | View an active hydrothermal area where new thermal features are common. Learn about how magma at depth can heat water we see at the surface. |
Overview of Volcanic History
Simplified map of Long Valley Caldera showing
several field stops in blue.
About 760,000 years ago a catastrophic volcanic eruption blew out 600 cubic kilometers (150 cubic miles) of magma (molten rock) in eastern California over a period of six days (Hildreth and Wilson, 2007). As the magma was ejected, the roof of the magma storage area subsided by about 2 km forming a 17 by 32 km (9 by 18 mile), oval-shaped depression known as the Long Valley Caldera (dash and dot line on map). Over the next 100,000 years volcanism continued on a smaller scale inside the caldera. In the center of the caldera an area was built up by both volcanic flows and uplift caused by buoyant magma at depth. This area, called the Resurgent Dome (yellow on map), has also uplifted about 80 cm (about 2.5 feet) since 1980.
Volcanism continued outside the Resurgent Dome as well. Lookout Mountain, in the northwest moat (a depression around the ring fracture that formed the Long Valley Caldera) erupted 677,000-692,000 years ago. The Hot Creek (lava) Flow (brown on map), through which Hot Creek flows, erupted 288,000 years ago. The Hot Creek Flow is referred to on maps as Doe Ridge.
Mammoth Mountain (green on map) was erupted between 111,000 - 57,000 years ago as a pile of overlapping lava domes. Beginning about 40,000 years ago, many eruptions took place along the Mono-Inyo Crater Chain (orange on map), which extends north to Mono Lake. Several eruptions along this chain occurred around 1350 CE and the most recent eruption was just 300 years ago at Poaha Island in Mono Lake.
In addition to volcanic activity, small-to-moderate earthquakes frequently shake the area. In 1872, the great Owens Valley earthquake (magnitude 7.6) was felt throughout most of California and Nevada. The USGS Long Valley Observatory operates one of the most extensive volcano-monitoring networks in the U.S. and posts updates about activity, mainly earthquakes, in the area on the website activity page.
Below is a simplified timeline for use with the field guide.
Simplified Timeline of Long Valley Volcanic Field History
| Date (years before present) | Event | Significance |
| 3.5 - 2.5 M and 2.2 - 0.79 M | Eruptions prior to formation of Long Valley Caldera. | Basalt and dacite lavas erupt in the area between 3.5 and 2.5 million (M) years ago. Between 2.2 and 0.79 M years ago, Glass Mountain was formed as a series of domes and several ashfalls that deposited materials hundreds of miles downwind. All together some ~ 100 km3 magma erupted forming deposits over 1,000 m thick. |
| 760,000 | Long Valley Caldera formation | Long Valley Caldera was formed 760,000 years ago when 600 km3 of magma erupted in ~ 6 days. |
| 700,000 - 100,000 | Eruptions inside the Long Valley Caldera. | Over the 660,000 years following the creation of the Long Valley Caldera there were multiple subsequent eruptions including the formation of Lookout Mountain (677,000-692,000), the Hot Creek flow (288,000 years ago), Deer Mountain (115,000 years ago), and Mammoth Knolls (~100,000 years ago). |
| ~110,000 - 57,000 | Eruption of Mammoth Mountain and surrounding basaltic cones outside the caldera. | Approximately 25 to 30 overlapping domes erupted over a period of 50,000 years on the southwest rim of the caldera to form Mammoth Mountain. Numerous other eruptions formed smalled basaltic cones and flows including Devil's Postpile and Red Cones over a larger time period (~160,000 to 8,500 years ago). |
| ~50,000 - 300 | Mono-Inyo Chain and Mono Lake Vents | Eruption of ~ 30 domes and craters extending from Mono Lake south to just north of Mammoth Mountain. These eruptions include Panum Crater, Obsidian Dome, and Glass Creek Dome and the most recent eruption on Paoha Island (300 years ago). |
