Amlia Island, located in the central part of the Aleutian Island chain, exhibits excellent shoreline exposures of Paleogene sedimentary and volcanic rocks, which record the earliest observable history of the volcanic arc. Volcaniclastic sedimentary rocks deposited by debris flow, turbidite, and grain-flow processes interfinger with volcanic flows and breccia. Pillowed flows as well as massive columnar flows ranging in composition from basalt to rhyolite record inter-fingering episodes of submarine and subaerial deposition.
The occurrence of Reticulofenestra reticulata and Dictyococcites scrippsae in laminated siltstone and sandstone beds indicates that the rocks on Amlia are late middle Eocene to early Oligocene in age. Whole-rock radiometric ages of 32 and 39 m.y. from igneous rocks that interfinger with the sedimentary rocks corroborate the fossil ages and are correlative in age with the Hidden Bay pluton on Adak Island.
Some of the rocks on Amlia Island are only slightly altered and contain authigenic zeolite phases; other rocks are highly altered and contain prehnite, pumpellyite, and laumontite. Areal distribution of highly altered rocks may reflect proximity to shallow-seated plutons, such as those that crop out on nearby Atka Island, which probably underlie much of Amlia.
The island is structurally simple, with dips rarely exceeding 10° or 15°. High-angle faults with small displacement are common, however, and may reflect emplacement of shallow-seated intrusive rocks. Wave-cut terraces around much of the island appear to be in equilibrium with present sea level and suggest recent tectonic stability.