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Okmok reported activity

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EVENT SPECIFIC INFORMATION

Start:February 13, 1997 Observed
Stop:May 23, 1997 Observed

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Eruption Type:Explosive
Eruption Product: basalt BibCard
MaxVEI: 3 BibCard
ColHeight: 9000 m BibCard
Duration: Ash clouds and lava flow for 6 weeks, all activity ceased in 3.5 months BibCard
ChemYes
ModalYes
Othermafic

Description: From Grey (2003): "The latest eruption at Okmok began on or shortly before February 11, 1997 and originated at Cone A. An excellent synopsis of the eruption is found in U.S. Geological Survey Open-File Report 99-448 (McGimsey and Wallace, 1999) and is paraphrased here. Possible precursory activity included a steam plume observed by a passing pilot on November 11, 1996. The next report was of a dark plume, witnessed by a pilot and passengers en route from Atka to Dutch Harbor on February 11, 1997. Because these observations were not immediately reported, and because AVO did not yet have seismic instruments deployed on Umnak Island, AVO first became aware of unrest at Okmok with the detection of a thermal anomaly in Advanced Very High Resolution Radiometer (AVHRR) satellite imagery on February 13, 1997. The beginning of the eruption was confirmed with a phone call from ranchers at Ft. Glenn, 15 km E of Cone A, who reported a dark plume rising to 3000 m ASL and drifting to the SW.

"Inclement weather prevented further observation until February 18, when pilots reported an ash plume to 4600 m, and ranchers at Ft. Glenn reported a red glow reflected on the underside of clouds over the caldera. This was the first verification of lava at the surface. The thermal anomaly detected in AVHRR imagery continued to grow to 22 pixels by February 22. At this time, A Ft. Glenn rancher who had climbed to the caldera rim reported a lava flow extending 0.8 to 1.5 km northeast from Cone A. On February 28, a National Oceanic and Atmospheric Administration (NOAA) research crew passing by in a helicopter spent about twenty minutes flying inside the caldera, capturing six minutes of video footage and several still photographs of strombolian fountaining activity at Cone A. The photos document that the first (NE) lava flow lobe had reached its full length of 5.5 km from the base of Cone A by February 28 [ see figure 4.9 in original text], and the second (N) lobe had not yet begun to emanate from the cone.

"The satellite thermal imagery suggest that the second (N) lobe began to form sometime between March 1-4 (Moxey and others, 2001). Over the next three weeks, several pilot reports (PIREPs) were issued and satellite imagery continued to show high thermal activity and occasional ash plumes (AVO, unpublished data). On March 11, a PIREP reporting ash to 9000 m prompted the Federal Aviation Administration (FAA) to issue a NOTAM (Notice to Airmen; contents unknown). The largest thermal anomaly in AVHRR was on March 12 with 19 saturated pixels (Patrick, 2002). This probably coincides with the maximum areal extent of the lava flow, while low-level effusion, perhaps reactivated on the first (NE) lobe, likely continued until sometime between March 26-31 (Moxey et al., 2001; Patrick, 2002; Patrick et al., 2003). The presence of multiple flow units within the distal portion of the first (NE) lobe, as seen on Ikonos imagery and in the field suggest multiple pulses of effusion on this part of the flow. Three units were initially recognized, but field investigations suggest that the third (top) unit is perhaps an apparent additional unit, the result of lava flowing over a drop in the underlying 1958 flow. Intermittent ash bursts and low plumes continued for the next several months (McGimsey and Wallace, 1999).

"When the 1997 eruption of Okmok was over, the resulting lava flow covered a total area of 8.8 square km or about 10% of the caldera floor, based on the lava flow map produced in this study. Previous estimates range from 7.5 square km (Moxey et al., 2002) to 9.8 square km (Lu et al., 2003). With thickness varying from 5-45 m (Lu et al., 2002 and Lu et al., 2003), the calculated bulk volume is 1.5 x 10 ^8 cubic m (Lu et al., 2003)."

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