Helium Discharge at Mammoth Mountain Fumarole (MMF)

Mammoth Mountain fumarole in summer, Long Valley caldera, California

MMF in summer

Mammoth Mountain fumarole in winter, Long Valley caldera, California.

MMF in winter

Mammoth Mountain fumarole (MMF), located on the north side of the mountain, is a vent from which gas escapes into the atmosphere. The chemical and isotopic compositions of gas discharging at MMF are similar to those in the soil gas at the Mammoth Mountain treekill areas in terms of carbon dioxide and helium. However, the treekill soil gas contains more air (oxygen and nitrogen), as the soil is saturated with air, then "diluted" with gas that is nearly pure carbon dioxide. Additionally, the treekill soil gas contains no methane, hydrogen, or hydrogen sulfide because these components react with soil minerals or water, or are consumed by microbes. Similarly, the treekill gas contains more helium and nitrogen and less carbon dioxide than MMF gas because carbon dioxide is most soluble of the three components, such that when the gas encounters ground water, it is taken preferentially to helium and nitrogen.

The similarities of He and CO2, and the persistent high rate of CO2 emissions from these areas, strongly indicates that they are derived from the same long-lived reservoir of magmatic and crustal gas trapped at depth beneath the mountain. The data in the follow table was derived from samples collected in August 1997, and are representative of many other similar pairs of samples.

Gas component

MMF fumarole gas

(volume percent)

Treekill area soil gas

(volume percent)

Carbon dioxide (CO2)
Helium (He)
Oxygen (O2)
Nitrogen (N2)
Methane (CH4)
Hydrogen (H2)
Hydrogen Sulfide (H2S)

The amount of helium present is quite small, but is much higher than atmospheric concentrations. Changes in the composition of this gas give us information about magmatic conditions deep below the surface. Helium has two isotopes: helium-3 (3He) and helium-4 (4He). 3He is referred to as primordial because it comes from deep in the Earth's crust and mantle, trapped there during the Earth's formation. 4He is mostly derived from radioactive decay of uranium and thorium minerals. The ratio of the two isotopes can then be used to infer the origin as either mostly from radioactive decay or from degassing of the Earth's interior—helium gas with a relatively high isotopic ratio of 3He/4He is derived from magma. A large increase in the 3He/4He ratio in MMF was first detected in 1989, following a 6-month period of seismic swarms beneath Mammoth Mountain. The rise in 3He/4He from 3.5 RA (where RA represents the ratio in air) in July 1989 to values near 7 RA in July 1990 is consistent with the arrival of gas released from an intrusion of new magma beneath the mountain.

Graph showing helium-3 to helium-4 ratios during 1982-2003 at Mammoth Mountain fumarole, Long Valley caldera, California.

After July 1990, the 3He/4He ratio at MMF declined to values near 5 RA, with the exception of a short-lived spike (6.4 RA) following the magnitude 7.3 Landers earthquake in southern California in June 1992. The most likely explanation for this correlation is that seismic energy derived from this far-reaching earthquake caused a release of magmatic gas from a gas reservoir beneath Mammoth Mountain.

The magmatic gas component of MMF increased again in the fall of 1993 following shallow earthquake swarms beneath Mammoth Mountain, and in the fall of 1997 soon after the onset of long-period earthquakes at greater depth beneath the mountain. These long-period earthquakes are thought to reflect inputs of basaltic magma from mantle sources.

Selected sources of additional information: