Another well-known rift zone.
And, some pertinent data:
"A preliminary estimate of the current rate of CO2 gas emission at Mammoth Mountain is 1,300 tons per day. Similar rates of CO2 emission have been measured from the craters of Mt. St. Helens (Washington) and Kilauea (Hawaii) volcanoes during periods of low-level eruptive activity. Past eruptions at Mammoth Mountain, such as the phreatic (steam-blast) eruptions that occurred about 600 years ago on the volcanoÕs north flank, may have been accompanied by CO2 emissions. Scientists think that the current episode of high CO2 emission is the first large-scale release of the gas on the mountain for at least 250 years, because the oldest trees in the active tree-kill areas are about that age."
And, please note: they refer to this as similar to other volcanoes during "periods of low-level eruptive activity".
The levels aren't always low.
Some bright lad or lass at the USGS has probably added all the worldwide volcanic emissions up. Hopefully, we'll find her/his work. Otherwise, we'll keep digging.

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