Gas sampling at Mount Shasta builds a long-term record of volcanic emissions

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One of the three main ways that California Volcano Observatory scientists monitor the state’s threatening volcanoes is by sampling the gases that are released from magma deep below (in addition to monitoring seismicity and deformation).

A scientist stands on a bare, rocky slope of a mountain, punching a steel tube into the ground with a glass sample bottle on top

USGS scientist Sara Peek collects fumarole gas at Sulphur Springs near the summit of Mount Shasta as a part of CalVO’s long-term monitoring at very high threat volcanoes.

(Credit: Lauren Harrison, USGS. Public domain.)

The proportions, types, and amounts of gases give insights into the depth, physical, and chemical state of the magma. Mount Shasta, the largest volcano in the Cascades volcanic arc, had its last eruption around 3,000 years ago, but it is still considered a high threat volcano because its gas emissions tell us that there’s hot magma in its ‘plumbing’. In addition to its 590,000-year history of volcanic hazards such as lava flows, debris flows, pyroclastic flows, and lahars, this means that Shasta is still an active and dangerous volcano.

Last week, two USGS scientists hiked to the summit of Mt. Shasta to collect gas samples from Sulphur Springs as a part of the long-term monitoring of volcanic gas composition. Volcanic gas samples were first collected at the summit of Mt. Shasta in the late 1970s, and regular monitoring has been conducted since 2012. Samples are collected from Sulphur Springs near the summit because this fumarole has one of the most direct connections to the hydrothermal system, and therefore the underlying magmatic system, at Mount Shasta. The volcanic gas collected at the summit consists of a mixture of water (H2O), carbon dioxide (CO2), hydrogen sulfide (H2S), and helium (He), in addition to other minor components.  The helium isotopic composition of the fumarole gas is a particularly important sign when new magma moves into the deep volcanic edifice, since the light isotope of helium (3He) is only stored deep in the Earth. Although the gas activity at Mount Shasta is considered to be at ‘background levels,’ USGS scientists need to know the gas ‘signature’ when the volcano is not erupting so we can spot when activity increases. This sampling campaign helps build the foundation for identifying future volcanic unrest!

By Sara Peek and Lauren Harrison