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Long Valley Caldera Field Guide - Horseshoe Lake

Volcanic CO2 can kill trees and is a hazard to animal life.

Geologic Summary

Numerous small earthquakes occurred beneath Mammoth Mountain from May 1989 to January 1990. Data indicated that a small body of magma (molten rock) was rising (as a dike) beneath the mountain. During the next year, U.S. Forest Service rangers noticed areas of dead and dying trees on the mountain. After drought and insect infestations were eliminated as causes, scientists began looking for a geologic explanation. A soil-gas survey by USGS scientists in 1994 revealed that the roots of the trees were being killed by exceptionally high concentrations of carbon dioxide (CO2) gas in the soil. Today there are more than 100 acres of dead and dying trees around Mammoth Mountain. The town of Mammoth Lakes, just east of this volcano, has not been affected. Recent measurements indicate a gradual decline in CO2 flux over the past several years, although CO2 continues to pose a hazard in the high-flux areas such as Horseshoe Lake.

Horseshoe Lake parking lot showing dead trees near lake....
Horseshoe Lake parking lot showing dead trees near lake. (Public domain.)
Tree Kill Area near Horseshoe Lake at Mammoth Mountain where carbon...
Tree Kill Area near Horseshoe Lake at Mammoth Mountain where carbon dioxide in the soil is suffocates the trees. (Credit: Bergfeld, Deboray. Public domain.)

Analysis of the CO2 gas in the soil shows that the gas is of a magmatic origin. That does not mean that magma is currently rising to the surface. There are many examples around the world of magma stalling at shallow depths. When a magma moves from a depth towards the surface, the pressure on the magma changes. If the magma stalls at a shallow depth within the crust, it will start to cool. Both pressure and temperature changes can cause gas to exsolve (bubble out of the liquid) and start to rise, much like the CO2that forms bubbles in seltzer water when a bottle is opened. The CO2 rises until it hits an impermeable seal where it accumulates. Analyses show the CO2 that is killing the trees at Horseshoe Lake came from magmathat was cooling, but we do not know whether the CO2was from magma that rose during the 1989 - 1990 swarm or whether it had accumulated from a previous batch of cooling magma. In either case, earthquake activity is responsible for opening the pathway to the surface where the CO2 is escaping.


Although the leaves of plants produce oxygen from CO2 during photosynthesis, their roots need to absorb the oxygen directly. The high CO2 concentrations in some of the Mammoth Mountain soil are killing trees by denying their roots oxygen and interfering with nutrient uptake. In the areas where the trees are dying, CO2 makes up from 20 to 95% of the gas content of the soil; soil gas normally contains 1% or less CO2.

Danger sign indicating presence of CO2 gas at Horseshoe Lake, Mammoth Mountain. (Public domain.)
CO2 pathways above and below ground. (Public domain.)

When CO2 from the soil seeps from the ground, it normally mixes with the air and dissipates rapidly. However, because CO2 gas is heavier than air, it can collect in snowbanks, depressions, and poorly ventilated enclosures, such as cabins and tents, posing a potential danger to people. Breathing air with more than 7% CO2 can cause unconsciousness and death (CDC website). Therefore, poorly ventilated areas above and below ground can be dangerous in areas of CO2 seepage. Dangerous levels of CO2 have been measured in pits dug in the snowpack in tree-kill areas on Mammoth Mountain. In 2006, three members of the Mammoth Mountain ski patrol died on Mammoth Mountain when they fell into a snow pit while trying to reconfigure fencing around a known fumarole. It is important to be aware of CO2 hazards in the area.


2001, Invisible CO2 Gas Killing Trees in the Mammoth Mountain Area, U.S. Geological Survey Fact Sheet 172-96

Field Stop Location: Horseshoe Lake

Coordinates: 37°36.771' N 119° 01.279' W
Approximate elevation: 8955 feet.
Quadrangle: Old Mammoth, California 7.5 minute topographic quadrangle

Directions to Horseshoe Lake:

Directions from Mammoth Lakes exit U.S. 395 and CA-203 Go this distance
1. Start at Mammoth Lakes exit from U.S 395 and head west on CA-203. [If you are already in Mammoth Lakes, follow Main Street/Highway 203 up the hill towards Mammoth Mountain and intersection with Minaret Road.] Go 3.7 miles
2. Continue straight on Lake Mary Rd at the Minaret Road light (CA-203 turns right towards Mammoth Mountain). Take Lake Mary Road up into the Mammoth Lakes basin and follow all the way to the end at Horseshoe Lake. Go 4.9 miles