" Visible degassing vs. invisible degassing" dur ing magmatic evolution.
Kenji Notsu
Graduate School of Science, University of Tokyo,Hongo, Bunkyo-ku, Tokyo 113-0033, Japan(e-mail: notsu@eqchem.s.u-tokyo.ac.jp)

Geochemical studies on volcanic gas during the last 15 years have shown that large
amounts of CO2 are released not only from their active craters but also from their flanks asdiffuse soil emanations. Now it becomes possible to discuss general features of the diffuse
degassing through soils compared with plume degassing from the craters. Although thediffuse degassing is observed to be highly variable in space and time, the high diffuse CO2
efflux is typically observed on volcanoes when active plume degassing is low, corresponding
to a low-activity stage, as exemplified by the Hakkoda volcano (Hernandez et al, 2003) andMammoth Mountain (Gerlach et al., 2001). On the other hand, it is not unusual to observe avery week efflux on volcanoes exhibiting intense plume activity from a central crater, as
shown in White Island, New Zealand, (Wardell et al., 2001) or Nisyros volcano, Greece(Brombach et al., 2001). The Popocatepetl volcano in Mexico provides an extreme exampleof negligible diffuse CO2 emissions of magmatic origin throughout the extensive volcanicedifice (Varley and Armienta, 2001) despite huge emissions of CO2 from the summit crater.
Considering these observations on the two types of volcanic gas release, a five-stage
evolutionary model for the release of volcanic gas can be proposed, as shown in Figure 1.
Stage I: When magma pools at a significant depth (e.g., >10 km), neither plume degassing nor
diffuse degassing of volcanic gas is typically observed.
Stage II: As magma rises into the subsurface, diffuse degassing through the permeable ground
begins, and the efflux at the surface increases as the magma migrates upward. Such increases
in efflux have been observed at Mt. Usu, Japan, prior to the 2000 eruption (Hernandez et al.,2001).
Stage III: When magma reaches the surface, volcanic eruptions occur with associated lava
extrusion or violent explosions accompanied by an instantaneous release of a large amount of
volcanic gas and ash as a plume. This causes a sudden drop in gas pressure within thevolcanic body and results in a rapid decrease in diffuse degassing throughout the volcanicedifice. This was also observed just after the 2000 Usu eruption (Hernandez et al., 2001).
Stage IV: While new magma is being supplied to the surface, intense pluming continues anddiffuse degassing remains negligible. Examples of this stage have been observed at Mt.
Popocatepetl (Varley and Armienta, 2001) and at White Island (Wardell et al., 2001).
Stage V: As magma begins to drain away from the surface, the efflux of plume degassing
decreases. After degassing from the volcanic vent stops, if gases are still released from thedescending magma at greater depth, diffuse degassing increases considerably due to anincrease in the gas pressure of the volcanic body. Diffuse degassing from quiescent volcanoes
such as that at Mt. Hakkoda (Hernandez et al., 2003) may represent post-eruptive degassing.
This diffuse degassing will gradually recede as the volcanic system returns to Stage I.
[references] Brombach et al., GRL, 28, 69-72 (2001). Gerlach et al., CG, 177, 101-116 (2001).
Hernandez et al., Science, 292, 83-86 (2001). Hernandez et al., JGR, 108, 2210 (2003). Notsuet al., PAGEOPH, in press (2006). Varley and Armienta, CG, 177, 157-173 (2001). Wardell etal., CG, 177, 187-200 (2001)

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Plume degassing

Diffusedegassing～10～1Eruption～1～10

2～10year

Figure 1. Schematic illustration showing an evolutionary model of gas release fromvolcanoes. (Notsu et al., 2006).
Solid circle is magma rising and descending along the vent. Emission patterns of both diffuse
and plume degassing are roughly illustrated as two broken lines. Time scale of horizontal axis
is tentatively indicated in case that the recurrence time of eruptions is 102~103 years.