St. George volcanic field description and information

From Wood and Kienle (1990) ^[1]: "St. George is the second largest of the Pribilof Islands, a group of late Tertiary and Quaternary volcanic piles on the western edge of the Bering Sea shelf. The volcanic rocks on St. George are Pliocene and early Pleistocene in age and are interbedded with marine sand and gravel, glacially derived sediments, frost breccia, and wind-blown sediments. Along the southeastern coast, the St. George volcanic field has an exposed basement of serpentinized peridotite intruded by quartz diorite. The lavas on St. George are older than those on nearby St. Paul, and the original volcanic topography has been subdued by weathering and later redefined by faulting, uplift, and glaciation. Vestiges of cinder cones, with filled craters and slopes reduced by frost and mass-wasting processes, occur as flat-topped or domical hillocks. Apparently alternating lava flows and pyroclastic eruptions in the eroded volcanic cones are actually evidence of much fountaining, which produced near-vent pumice interbedded with some flows in one eruptive event.

"Sea cliffs provide the best exposures of the products of repeated eruptions; for example, the cliff at Tolstoi Point on the eastern tip of St. George is made up of seven flows as much as 7.6 m thick, with associated pyroclastic layers. Similar to those on St. Paul Island, the St. George lavas consist of alkali olivine basalt and basanite, but the St. George samples and those from submarine dredge hauls near St. George have a broader range in SiO2 and include hypersthene-normative as well as nepheline-normative chemistries. Mafic inclusions are common, and some flows contain granitic inclusions. Many flows display a patchy zeolitization, which may have resulted from alteration during cooling shortly after emplacement. Several flows that erupted during high sea-level episodes display thick and extensive pillow breccias and palagonite tuff sequences."