|Start:||July 31, 1998 ||Observed|
|Stop:||December 1999 ||Observed|
|Fumarolic or hydrothermal activity: ||
|Eruption Type:||Not an eruption.|
Staff from the Alaska Volcano Observatory visited Shrub on July 31, 1998, and reported "The level of activity remains high. Total amount of mud production at the volcano is probably about the same as in August, 1997. However, production at the main vent area [see fig. 4 in original text] is considerably less than the rate observed in 1997. A very noticeable change is the nature of the eruptive activity. The violent discharge of mud and gas that was typical of the 1997 activity was not observed during this visit. Rather all presently active vents were quietly discharging mud and gas from bubbling mud pools or ponds. Temperatures as high as 49.9 degrees C - more than 2 degrees C higher than those of June and August 1997 - were measured (McGimsey and others, 2003)."
From McGimsey and others (2004): The eruption of Shrub mud volcano continued throughout 1999. "Shrub mud volcano was visited on June 23 and 24, 1999 as part of an informal joint National Park Service (NPS) and U.S. Geological Survey project to monitor the activity that began in the spring of 1997. All of the known vent areas were visited and remapped. Temperature of the mud was measured at numerous vents, [see fig. 5 in original text], CO2 discharge was determined for several vents, diffuse CO2 flux in the soil was measured in a grid section, and several new vents were discovered and documented (Sorey and others, 2000). As in 1998, all of the active vents were quietly discharging mud and gas from bubbling mud pools or ponds. Temperatures as high as 54°C (5-7°C higher than those recorded in June 1998) were measured (Sorey and others, 2000). Surprisingly, several new vents were discovered that were discharging mildly turbid water at ambient temperatures (9.5-14°C). The new vents were located only a few meters away from those discharging highly turbid, warm water (48°C). All vents, regardless of mud content and temperature, were discharging gas comprised 98% CO2. An estimated 10 metric tonnes of CO2 per day is discharging from spring vents on Shrub, and along with measured concentrations of bicarbonate, a total CO2 upflow from depth of 20-40 metric tonnes per day was estimated by Sorey and others (2000). Measurements of diffuse degassing rates from soil in an area of recent vegetation kill indicated high CO2 concentrations in the root zone (26% CO2 at 10 cm depth). Several small, dead animals near the vents and newly killed vegetation were indicative of the continuing CO2 hazard at Shrub.
"More than 500,000 m3 of mud have been erupted since activity began in the spring of 1997 (Richter and others, 1998, OF 98-128). New aerial photography flown on August 14, 1998 was used to more precisely map the extent of mud deposits and main vent locations [see fig. 5 in original text; after Richter and others, 1998; addendum to OF 98-128). The extent of mud deposits in 1999 was largely the same as in 1998."
Activity similar to that observed in 1999 continues at Shrub to the present (2015).