|Start:||June 12, 1997 ||Observed|
|Stop:||December 2, 1997 ± 6 Months||Observed|
|Fumarolic or hydrothermal activity: ||
|Eruption Type:||Not an eruption.|
From McGimsey and Wallace (1999): "The present  activity was first noticed by Alaska Department of Fish and Game personnel on June 12, 1997. AVO and National Park Service field crews visited the site on June 21, June 30, and on August 13, 1997, and found that most of the gas and mud emission was occurring from at leaset four areas on the summit and north, northwest, and southeast flanks [see fig. 7 in original text]. The activity ranged from quiet effusion of CO2 from collapse pits to vigorous gushing of mud and gas from fissures and mud pots -- some up to heights of 10 m (33 ft) above the vent -- which flowed in streams down the flanks forming mud fans along the base of the cone [see fig. 8 in original text]. Measured mud temperatures were between 43,8C (110.8F) and 46.4C (115.5F). Voluminous quantities of CO2 flowed as streams down the flanks in several locations, killing vegetation to heights of as much as 2 meters (6.6 ft) above the ground surface. Small mammals and birds perished around several of the CO2 vents, indicating a potential CO2 hazard.
"Following the August site visit and AVO hazard assessment, the National Park Service issued a formal public warning of potentially dangerous CO2 levels at the volcano. An aerial survey of the area on December 2, 1997, confirmed that mud and gas discharge continues."
From Richter and others (1998), concerning activity at Shrub during the summer of 1997: "The most active area was immediately below (southeast of) the cone's summit where a 65-m-long east-trending fissure was discharging copious amounts of mud and gas at 43 degrees C. Recently dead spruce trees on the slopes below this fissure suggest that area was also the site of summit activity in 1996. Mud and gas production was also vigorous a central vent, about 1 m in diameter, within a group of three vents, about 100 m west of the fissure vent. The other vents in this latter group were less active. Approximately a third of the way down from the summit on the north side of the cone, in dense alder-willow-birch growth, a 4- to 6-m-diameter pit, 1- to 2-m-deep, was filled with very active bubbling mud. Although the pit had not discharged mud, leaves of alder and birch downslope from the bit were browned to heights as much as 2 m above the ground surface suggesting that significant mounts of CO2-rich gas may have flowed out of the pit prior to our visit. We did observe a dense layer condensate (probably water mixed with CO2) up to about a meter above the roiling mud surface. Northwest of this gas/mud pit and lower on the con's flank a number of small vents were quietly discharging mud, with no apparent gas, through a thick surficial organic mat. Mud discharging from the largest of these vents had a temperature of 47 degrees C, the hottest recorded on the mud volcano."
They further report that on June 30 "the fissure was still active but activity was restricted to a deep (about 3-5 m) pit in the middle of the fissure; temperatures remained at 43 degrees C. The northernmost of the gushing vents was very active, noisily producing large bubbles of mud as much as 2 m in diameter that burst as high as 10 m above the vent. There was minor mud emission from the central vent and the southernmost vent was inactive. The mud/gas pit on the north flank was not visited, but we could see from the air that it was still active."
And on August 13 vigorous mud and gas emission continued. "The fissure vent was virtually inactive. In the deeper parts of the fissure some roily mud was present but none was reaching the surface. About 50 m south of the fissure a small pit (vent) was discharging a thin trickle of mud. At the gushing vents the former weakly active vent was now the major producer of mud and gas. The temperature was 46OC, ~3OC higher than measured on 21 June, and it was estimated that mud production was on the order of a cubic meter a minute. The formerly very active vent to the north was discharging some mud and a fourth - probably new - vent a few meters farther to the north was also active. The mud/gas pit lower on the north flank was also inactive, but about 10 m upslope from the pit was a new 3-m-wide collapse pit, at least 5 m deep, which appeared to have active mud at the bottom. At the mud vents lower down the flank of the cone it appeared that mud emission had decreased, but there were so many small vents, all hidden in the dense vegetation, that it was difficult to tell where and how much mud was being discharged. Between the 30 June and 13 August site visits there had been significant mud production from these lower vents as indicated by the covering of additional 1996 mud during this time frame. A temperature of 44OC was slightly lower than what was measured on 30 June.
"A low-level flight over the mud volcano on 2 December 1997 revealed that activity continued at about the same level as observed in August. At the original mud/gas pit on the north flank of the cone, which was quiet on August 13, a small stream of mud was observed issuing from the pit and running down the cone. At the gusher vents it appeared a depression as forming, maybe similar to the basinal feature observed by Nichols and Yehle (1961) in 1955-56."
Richter and others collected gas samples on June 30, 1997, and analyzed. Analyses show "the sample contains over 98% CO2 with minor to trace amounts of N2, O2, CH4, Ar, and He. H2, C2H6, H2S, and CO are below detection." Richter and others (1998) state that the "bulk composition of the 1997 Shrub gases [see table 1 in original text] is very similar to past analyses from Upper and Lower Klawasi mud volcanoes (e.g., up to 99% CO2; Reitsema, 1979; Motyka and others, 1989), suggesting the 1997 Shrub mud volcano gases come from the same origin. This is confirmed by the [little delta] [superscript 13] C in CO2 and [superscript 3]He/[superscript 4]He data [see table 1 in original text], which lie within the range of previous analyses of the Upper and Lower Klawasi gases."