Description: From McGimsey and others (2007): "After several years of quiescence following an explosive eruption in 2001, AVO remote sensors observed a 3-pixel thermal anomaly at the summit of Cleveland on March 13, 2005 (see fig. 38 in original text). On April 27, 2005, the FAA alerted AVO of a pilot report of eruptive activity - "ash cloud * * * 15,000 to 18,000 ft high" - in the vicinity of Cleveland (based on coordinates from the pilots). Satellite images showed no evidence of activity. AVO seismologists checked seismic data from the nearest stations (Nikolski, located 75 km [45 mi] east, and at Okmok Volcano, 150 km [93 mi} east of Cleveland), and found nothing unusual. CWSU issued a one-time Urgent Pilot Report, and AAWU issued a one-time SIGMET. Although time-series thermal data did not record any evidence of activity, short-lived minor explosive activity would not be considered unusual for Cleveland and could go undetected if it occurred during periods between acquisitions of satellite images or if concealed within the frequent cloud cover.
"Following the detection of a 1-pixel thermal anomaly at the summit on June 28, evaluation of before and after satellite images suggested the presence of a lahar deposit on the northeast flank, inferring that minor activity persisted at Cleveland. Then, on July 5, the entire upper flanks of the volcano were observed dusted with ash in a satellite image (see fig. 39 in original text). AVO rasied the Level of Concern Color Code from Unassigned (UA) to Yellow in an Information Release on July 7, 2005 (see table 6 in original text). The presence of ash, minor blocky avalanche-like deposits, and thermal anomalies was consistent with low-level Strombolian eruptive activity (D. Schneider, AVO logs).
"Thereafter, although a thermal anomaly was observed on August 11, the activity appeared to wane. AVO reduced the Color Code from Yellow back to UA on August 27. But the volcano remained restless, and a summit thermal anomaly again was observed on August 31. By mid-September, AVO was ready to test a new automated system that detects thermal anomalies and raises an alert. On September 21, this new system successfully detected a thermal anomaly at the summit of Cleveland. For the next few weeks, the volcano remained quiet. Then, on the morning of October 7, AVO detected in satellite images a small drifting ash cloud located about 150 km (90 mi) east-southeast of Dutch Harbor. On the basis of regional seismic data at Nikolski (75 km [45 mi] east of the volcano), and backtracking the ash cloud, AVO concluded that a small eruption had occurred at Cleveland at approximately 01:45 ADT (0945 UTC). AVO and the NWS worked together to determine that the ash cloud was at an altitude of no more than 15,000 ft (4,600 m). No ash fell in Nikolski. AVO immediately raised the Color Code from UA to Orange and NWS issued a SIGMET indicating that the ash cloud was moving east. The next day, October 8, ther was no sign of ash emission or a summit thermal anomaly, and on October 10 the Color Code was downgraded from Orange to Yellow. The last thermal anomaly was seen on November 6, and steam plumes were occasionally visible in satellite data for the next several weeks. Because there was no evidence of ash emissions on November 25, AVO reduced the Color Code for Cleveland from Yellow to UA. As fate would have it, a few days later, evidence for minor eruptive activity was observed; however, the activity did not continue and the volcano remained quiet for the rest of the year. AVO issued five special Information Releases about Cleveland activity between July 7 and November 25, 2005."
A chronology of this event is available at: http://www.avo.alaska.edu/archives/Cleveland2005.php
From the Smithsonian Institution (2005): "Mount Cleveland produced significant ash plumes during March 2001 (BGVN 26:04). Volcanic unrest continued through 4 May 2001, and signals consistent with volcanic seismicity were detected by an Alaska Volcano Observatory (AVO) seismic network 230 km E. By the end of May, neither eruptive activity nor thermal anomalies were observed. Until July 2005, no alert level was assigned, and AVO monitoring produced no reports on Cleveland.
"Cleveland lacks a real-time seismic network. Accordingly, even during times of perceived quiet there is an absence of definitive information that activity level is at background. AVO's policy for volcanoes without seismic networks is to not get assigned a color code of Green.
"Satellite imagery of Cleveland taken during 24 June to 1 July 2005 showed increased heat flow from the volcano and a possible debris flow. AVO stated that although observations were inhibited by cloudy weather, they indicated the possibility of increased volcanic activity. AVO did not assign a Concern Color Code to Cleveland due to the lack of seismic monitoring and limited satellite observations.
"Satellite images during 1-8 July showed increased heat flow, thin ash deposits, and possible debris flows extending ~ 1 km down the flanks from the summit crater. AVO assigned a Concern Color Code of Yellow on 7 July. On 18 July satellite imagery showed steam emanating from Cleveland's summit and evidence of minor ash emissions. Meteorological clouds obscured Cleveland during the third week of July. During 22-29 July satellite images showed minor steaming from the summit, possible fresh localized ash deposits, and a weak thermal anomaly.
"On 4 August satellite images showed a thermal anomaly. On 27 August AVO reduced the Concern Color Code at Cleveland from Yellow to "Not Assigned" because there had been no evidence of activity since a thermal feature was observed on satellite imagery from 11 August. A thermal feature was detected on several satellite images obtained on 31 August, and one on 19 September, but there was no evidence of eruptive activity.
"On 7 October, AVO raised the Concern Color Code to Orange after detecting a small drifting volcanic ash cloud. The cloud was seen in satellite data at a spot ~ 150 km ESE of Dutch Harbor at 1700 UTC. Based on data from a regional seismometer at Nikolski, AVO concluded that the ash came from a small Cleveland eruption at approximately 0145. AVO, in consultation with the National Weather Service, estimated the top of the ash cloud to be no more than 4,600 m altitude. The ash cloud dissipated and was not detected via satellite after 1800 UTC. Three days passed during which there were no new observations of eruptive activity at Cleveland from satellite data, pilots, or ground-based observers. Accordingly, on 10 October the Concern Color Code was reduced to Yellow."