Excess degassing from volcanoes and its role on eruptive and intrusive activity

Abstract

[1] Volcanoes emit larger amounts of volcanic gas than can be dissolved in the volume of erupted magma during a variety of volcanic processes, including explosive and effusive eruption and noneruptive continuous degassing. Degassing of unerupted magma with a much larger volume than that of erupted magma caused such a large degassing; erupted magma represents only a small portion of the magma that drives volcanic activity. Evaluation of the magma-gas differentiation process causing the excess degassing is necessary to understand eruption processes, magma chamber evolution, and crustal growth by magma intrusion. Three mechanisms are proposed to explain various degassing modes, including eruption of bubbleaccumulated magma, degassing of a convecting magma column, and permeable gas transportation from a deep magma chamber. Examples of large degassing in excess of the erupted magma are common in subduction zone volcanism but are rare in rift-and hot spot -associated volcanism.Citation: Shinohara, H. (2008), Excess degassing from volcanoes and its role on eruptive and intrusive activity, Rev. Geophys., 46, RG4005,

A model of degassing for Stromboli volcano

“…This degassingdriven process (Shinohara, 2008) occurs in response to the sinking of the degassed (non-erupted) HP magma back into the conduit, and its replacement with ascending vesicular (and thus less-dense) magma blobs. The shallow convective overturning of the HP magma obviously gives rise to a second source of volatiles: degassing of dissolved volatiles in the ascending HP magma will produce gas bubbles which pressure-dependent compositional evolution is best described by curves 5 and 6 in Fig.…”

“…One of the most important though often overlooked aspects of basaltic volcanism is its exceptional gas productivity. The so-called "excess degassing" (Shinohara, 2008), the fact that basaltic volcanoes no doubt emit more gas than potentially contributed by erupted magma, implies an effective gas bubble-melt separation at some point during the ascent. However, while it is universally accepted that separate gas transfer exerts a key control on both quiescent (Burton et al, 2007a) and eruptive (Edmonds and Gerlach, 2007) degassing of basaltic volcanoes, the mechanisms (structural vs. fluid-dynamic control) and depths (shallow vs. deep) of such gas separation are still not entirely understood (Edmonds, 2008).…”

Evaluating the link between the sulfur-rich Laacher See volcanic eruption and the Younger Dryas climate anomaly

“…However, the cooling effects of a volcanic eruption are controlled by the amount of sulfur released and not necessarily the eruption size (Rampino and Self, 1982). In general, magnitude and erupted sulfur www.clim-past.net/14/969/2018/ (Shinohara, 2008). Red circles represent petrologic estimates of SO 2 release, and orange triangles represent estimated actual values, calculated from either satellite or ice core data (Shinohara, 2008).…”

“…Despite these unknowns, the amount of SO 2 released by the LSE was considerably higher than the petrologic estimates, and Schmincke et al (1999) speculated a maximum release of 300 Mt SO 2 , assuming the same relationship between petrologic and observed values as for the Pinatubo eruption. We utilise a similar but more comprehensive approach, taking the mean value of the relationship between petrologic and observed values of the 16 non-basaltic explosive eruptions catalogued by Shinohara (2008) plus estimated values for the large AD 1257 Samalas eruption (Vidal et al, 2016) to estimate that the LSE released ∼ 83 Mt SO 2 , although the range in possible values is substantial. A variety of estimated values clearly exist, but the eruption almost certainly released more SO 2 than the Pinatubo eruption (∼ 20 Mt) and possibly even more than the 1815 Tambora eruption (∼ 70 Mt) (Fig.…”

“…Red circles represent petrologic estimates of SO 2 release, and orange triangles represent estimated actual values, calculated from either satellite or ice core data (Shinohara, 2008). The grey bar represents the range of values for the Laacher See eruption as suggested by Textor et al (2003), Schenk et al (2018), and Bahr et al (2018), and the blue triangle in the Laacher See column represents the total estimated SO 2 emitted by the Laacher See eruption (83.6 Mt) assuming that the actual SO 2 emitted is 20.9 times the petrologic estimate, calculated here as the mean value of 17 non-basaltic explosive eruptions (the 16 listed in Shinohara (2008) plus the AD 1257 Samalas eruption; Vidal et al, 2016). amounts are well correlated (Oppenheimer, 2003;Carn et al, 2016), and therefore magnitude is often used as a surrogate for sulfur yield. However, variability of almost 3 orders of magnitude exists in the amount of sulfur released amongst equivalently sized explosive eruptions (Carn et al, 2016), and consequently eruption size is not the only predictor of total sulfur released.…”