|Start:||December 1955 ||Observed|
|Stop:||December 1956 ||Observed|
|Fumarolic or hydrothermal activity: ||
|Eruption Type:||Not an eruption.|
Nichols and Yehle (1961) provide a description of Shrub mud volcano as they saw it during the summers of 1955 and 1956. "A vegetation-bare basin or crater about 120 feet in diameter and floored by clayey silt is present on the southwest side of the cone about thirty-five feet below the crest. In 1955 this basin had seven pools, two to eighteen inches in diameter, from which gas and silt-laden water were discharged. Only a trickle of water from the pools drained across the basin and down the steep-sided cone. The flow of water must have been considerably greater in the past as indicated by the presence of two barren drainageways on the south and west sides of the cone where non-salt-tolerant plants have been killed.
"In 1956 only four pools, one to two inches in diameter, remained and both gas and water emanations were sharply reduced in volume. There was no surface drainage away from the pools. The surface of this and other cones has been much trampled by moose and, possibly, caribou which graze in the area and drink the salty water. These animals, by stepping in or near the vents may cause them to be temporarily, or perhaps even permanently clogged. This may result either in a shift in gas outlets or, in the case of weak activity, complete cessation of discharge."