Aniakchak National Monument and Preserve - Geology

Aniakchak"s origins are uncertain but the caldera was probably formed by a nuee ardente explosion similar to the Katmai eruption of 1912. The circumference of the base of the volcano is approximately 100 miles. The Aniakchak River passes through the east wall at a point known as “The Gates,” where erosive action has helped form a narrow canyon between 2,000-foot cliffs. The Gates were described as: towering walls streaked with pink, black, orange, and green and eroded into turrets, buttresses, and soaring columns that looked like totem poles. On the western rim is a less spectacular gap consisting of a pass at an altitude of about 2,000 feet between rim elevations over 3,000 feet. The eastern and southern rim, including The Gates, as mostly feldspathic sandstone of the Upper Jurassic Staniukovich formation. The remainder of the caldera walls are flows and breccias of Pleistocene and Recent age. Sedimentary rocks exposed along the rim of the caldera, are Upper Jurassic, predominately marine clastic rocks. The surface of the area is covered by volcanic materials probably distributed during a prehistoric explosion which hurled 15.4 cubic miles of debris out of the volcanic vent and scattered it over a 184-square-mile area.

The 1931 volcanic eruption, which probably took place in the southwestern section of the caldera near Half Cone, added to the ash blanket in the vicinity of the volcano. Since 1931, the volcano has not been known to be active, though a U.S. Geological Survey researcher found areas of high-ground temperatures in the western portion of the caldera. This, plus the warm springs that are feeding Surprise Lake, indicate potential for future volcanic activity.

The principal structural feature of the area is a series of complex anticlines and synclines aligned subparallel to the trend of the Alaska Peninsula. The Wide Bay-Bear Creek anticline plunges southwestward from the vicinity of Wide Bay to Aniakchak Crater and involves rocks of Permian to Oligocene ages. The Main Creek anticlinal complex is characteristic of the tight accessory folds associated with the major arches of the Alaska Peninsula southwest of Wide Bay. The eastern margin of this folded zone is marked by a reverse fault dipping steeply to the northwest, separating this area from the Meshik River-Amber Bay syncline on the south. The structure of this anticlinal complex is exposed in high mountains. Minor complex folds are present also east of the head of Amber Bay. These minor structures expose rocks as old as the Upper Jurassic-Lower Cretaceous Staniukovich and Upper Cretaceous Chignik formations. The Wide Bay anticline is separated from the eastern plunge of the Elephant Mountain-Aniakchak anticline by a gentle saddle which is interrupted by abundant volcanic necks and sills. The Tolstoi formation of Eocene age rests directly upon Jurassic rocks on the south flank of Aniakchak volcano. The north flank of this anticline is covered by recent volcanic rocks from Aniakchak crater. Tertiary intrusive igneous rocks are exposed at 21 localities within the proposed monument. These intrusive centers penetrate the thick section of folded sediments. The western lower slopes of the volcano are composed of volcanic flows and breccias of Pleistocene and Recent ages. The western coastal plain and the Meshik and Cinder River valleys are composed of Quaternary dune sand, beach deposits, and Pleistocene glacial debris, also of Pleistocene and Recent times.