Variability of volcanic gas emissions during a crater lake heating cycle at Ruapehu Volcano, New Zealand

Abstract We present the first routine measurements of emissions (CO 2 , SO 2 , and H 2 S) from Ruapehu volcano, New Zealand during a crater lake heating cycle. Emissions were generally at a low level consistent with quiescent degassing of the volcano and the presence of a crater lake. Maximum concentrations (∼ 3–4 km downwind) during the highest measurement in May, 2004 were 14 ppm, 0.37 ppm, and 7 ppb for CO 2 , SO 2 and H 2 S, respectively. CO 2 emissions varied between not detectable to 900 t d − 1 over periods of months. SO 2 was not detectable until February, but gradually increased to 35 t d − 1 . Emissions of H 2 S were detected in April and May, although were − 1 . The CO 2 /SO 2 ratio in the plume was ∼ 37 by weight for each measurement during the peak of the cycle suggesting that significant scrubbing of SO 2 occurs through the crater lake, and that a common source exists for both gases. Magma volumes estimated from CO 2 emissions (0.001–0.004 km 3 ) are consistent with eruptive volumes given repose periods of 20–30 years. Delays between peaks in crater-lake heating and degassing suggests that volcanic emissions do not primarily reside in the shallow hydrothermal system directly beneath the crater. Further data is needed to adequately model the system, but first indications of the travel time associated with the degassing cycle suggests that the gas resides at minimally 300 m depth beneath the crater lake. Depths greater than 300 m are consistent with the top of the single-phase steam zone in heat pipe models (200–700 m depth) or perhaps even below the plastic–brittle transition zone above the cooling magmas (> 1 km deep).