The island is made up mainly of past eruption deposits. Scientists have been able to discern that past dome collapse has resulted in large avalanches.
The nearly circular uninhabited island formed by Augustine Volcano is 12 km (7.5 mi) wide east-west, 10 km (6 mi) north-south; a nearly symmetrical central summit peaks at altitude 1,260 meters (4,134 ft).
Augustine's summit consists of several overlapping lava dome complexes placed during many historic and prehistoric eruptions. Most of the fragmental debris exposed along its slopes comprises angular blocks of dome-rock andesite, typically of cobble to boulder size but carrying clasts as large as 4 to 8 meters (10 to 25 feet), rarely as large as 30 meters (100 ft). The surface of such deposits is skeet, a field of steep conical mounds and intervening depressions with many meters of local relief. En route to Katmai in 1913, Robert F. Griggs had briefly inferred landslide (debris avalanche) as the origin of Augustine's hummocky coastal topography about Burr Point, by geomorphic analogy with the hummocky and blocky deposit of a 1912 landslide near Katmai.
The hummocky deposits on Augustine's lower flanks resemble both topographically and lithologically those of the great landslide or debris avalanche that initiated the spectacular May 18, 1980 eruption of Mount St. Helens. The deposit of that landslide revealed the origin of coarse diamicts with hummocky topography at other strato volcanic cones. Since 1980 many hummocky coarsely fragmental deposits on Augustine's lower flanks have come to be interpreted as deposits of numerous great landslides and debris avalanches.
