Long Valley Caldera

Long Valley Caldera is a large volcanic caldera in eastern California, United States. It formed about 760,000 years ago in a major explosive eruption that produced the Bishop Tuff.

The caldera covers roughly 20 miles by 11 miles and contains a range of geothermal and volcanic features. On-site attractions and access points include hydrothermal areas, hiking and road-access viewpoints, and nearby mountain recreation including a ski area at Mammoth Lakes.

The caldera formed in the Pleistocene by the eruption that created the Bishop Tuff; subsequent volcanism produced features on and around the rim, including obsidian and rhyolitic eruptions. Episodes of ground uplift, seismic swarms and increased fumarolic activity have occurred in the late 20th and early 21st centuries and have been the focus of ongoing scientific study.

Geographically it lies on the eastern side of the Sierra Nevada in Mono County, immediately adjacent to the town of Mammoth Lakes and overlooking Owens Valley to the east. Major road access is via routes serving the Mammoth Lakes area.

• Size: About 20 miles long and 11 miles wide, making it one of the largest calderas in the United States.
• Age and origin: Formed in a major eruption roughly 760,000 years ago that produced the Bishop Tuff deposit.
• Monitoring: Actively monitored by the U.S. Geological Survey for seismicity, ground deformation and geothermal hazards.

What to See#

• Hot Creek Gorge: Hydrothermal canyon and spring system on the caldera floor known for hot springs, steaming pools and periodic access closures for public safety.
• Glass Mountain: A light-colored rhyolitic dome and obsidian ridge on the caldera’s eastern rim that is a prominent landscape feature visible from park roads.
• Mammoth Lakes: The town and year-round mountain resort on the western edge of the caldera that serves as the main visitor gateway and base for outdoor activities.
• Resurgent dome: The central uplift inside the caldera formed after the caldera-forming eruption; it is a focus of geothermal activity and scientific monitoring.

Long Valley Caldera is the broad volcanic depression centered on the Mammoth Lakes area in eastern California. By car you approach on US‑395 (the main north-south highway along the eastern Sierra) and turn onto CA‑203 to reach Mammoth Lakes; local paved roads such as Minaret Road, Convict Lake Road and Crowley Lake Drive give access to specific trailheads and viewpoints around the caldera. From the town of Mammoth Lakes the caldera’s western and central access points are within a few miles; from Bishop drive north on US‑395 (roughly a 45‑minute drive) and from Reno it is typically a 2.5-3.5 hour drive south on US‑395.

There is limited public transit directly into the remote parts of the caldera - regional buses and seasonal shuttles serve Mammoth Lakes, from where taxis, rental cars or local shuttles reach trailheads. Most popular viewpoints and trailheads require a short walk from parking areas (hundreds of meters to a couple of kilometres).

• Plan for early morning (shortly after sunrise) for the clearest, lowest-haze views of the caldera and surrounding Sierra peaks; approach from US‑395/Mammoth Lakes, which gives the best overlooks and visitor facilities.
• Check USGS Long Valley/Long Valley Observatory updates before you go - the area is actively monitored for seismic and geothermal activity and parts (e.g., Hot Creek) are periodically closed for safety.
• Don't skip the Glass Mountain/obsidian exposures along the caldera rim: pullouts along US‑395 give short viewpoints and interpretive signs that many drive‑by visitors miss, and parking is limited later in the day so arrive early.