AVO increased the Volcanic Alert Level to Advisory and the Aviation Color Code to Yellow for Augustine on 22 September due to an increase in seismic activity below the summit over the previous week. During 22-25 September, the earthquakes were generally less than M 1 and were located at shallow depths beneath the summit.
AVO reduced the Concern Color Code at Augustine from Yellow to Green on 9 August. Seismic, satellite, and visual data indicated a decrease in activity to background levels. No changes were seen at the summit during the previous several months. AVO warned that the lava dome and surrounding area were still unstable despite the apparent cessation of lava-dome growth. Rockfalls and avalanches were still occurring, especially on the N flank, and may continue for several weeks or months.
Augustine is the most active volcano of the eastern Aleutian arc. It consists of several overlapping lava domes that frequently produce explosive eruptions and are notorious for collapsing, producing glowing avalanches (block and ash flows).
Augustine volcano had a devastating eruption 1883 when large parts of the summit dome complex collapsed. In the following century, several eruptions associated with dome growth have restored the volcano's height again. Historical eruptions after that date were typically explosive, producing pyroclastic pumice flows and block-and-ash flows, followed by regrowth of the lava domes.
The oldest exposed rocks on the flanks of Augustine are little more than 40,000 years old. At least 11 large debris avalanches have reached the sea during the past 1800-2000 years, and five major pumiceous tephras have been erupted during this interval.
Introduction to Plate Tectonics: The Earth's upper, rigid layer is broken into several plates which are in constant motion to one another. Most earthquakes and volcanoes occur as result of these plate boundaries.
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