Jennifer R. Reynolds

Near‐ridge seamount chains in the northeastern Pacific Ocean

High-resolution bathymetry and side-scan data of the Vance, President Jackson, and Taney near-ridge seamount chains in the northeast Pacific were collected with a hull-mounted 30-kHz sonar. The central volcanoes in each chain consist of truncated cone-shaped volcanoes with steep sides and nearly flat tops. Several areas are characterized by frequent small eruptions that result in disorganized volcanic regions with numerous small cones and volcanic ridges but no organized truncated conical structure. Several volcanoes are crosscut by ridge-parallel faults, showing that they formed within 30–40 km of the ridge axis where ridge-parallel faulting is still active. Magmas that built the volcanoes were probably transported through the crust along active ridge-parallel faults. The volcanoes range in volume from 11 to 187 km3, and most have one or more multiple craters and calderas that modify their summits and flanks. The craters ( 1 km diameter) range from small pit craters to calderas as large as 6.5×8.5 km, although most are 2–4 km across. Crosscutting relationships commonly show a sequence of calderas stepping toward the ridge axis. The calderas overlie crustal magma chambers at least as large as those that underlie Kilauea and Mauna Loa Volcanoes in Hawaii, perhaps 4–5 km in diameter and ∼1–3 km below the surface. The nearly flat tops of many of the volcanoes have remnants of centrally located summit shields, suggesting that their flat tops did not form from eruptions along circumferential ring faults but instead form by filling and overflowing of earlier large calderas. The lavas retain their primitive character by residing in such chambers for only short time periods prior to eruption. Stored magmas are withdrawn, probably as dikes intruded into the adjacent ocean crust along active ridge-parallel faults, triggering caldera collapse, or solidified before the next batch of magma is intruded into the volcano, probably 1000–10,000 years later. The chains are oriented parallel to subaxial asthenospheric flow rather than absolute or relative plate motion vectors. The subaxial asthenospheric flow model yields rates of volcanic migration of 3.4, 3.3 and 5.9 cm yr−1 for the Vance, President Jackson, and Taney Seamounts, respectively. The modeled lifespans of the individual volcanoes in the three chains vary from 75 to 95 kyr. These lifespans, coupled with the geologic observations based on the bathymetry, allow us to construct models of magma supply through time for the volcanoes in the three chains.

Habitats and Demersal Fish Communities in the Vicinity of Albatross Bank, Gulf of Alaska

Publisher Summary Albatross Bank is located in the Gulf of Alaskas outer continental shelf near Kodiak Island. Geologically, it sits in a broad forearc above the Alaska subduction zone, 50–100 km north of the trench and 250 km south of the Alaska arc volcanoes. Earthquakes and vertical tectonic processes from active plate collision form the general morphology of the region. Bedrock at the outer shelf is composed of Miocene and younger sedimentary strata, mostly flat lying, and hosts seeps that vent methane-bearing fluids. Albatross Bank is influenced by strong ocean currents. The Alaskan Stream flows along the shelf break and continental slope in a generally southwest direction. Assessment of biological communities is based on analysis of submersible observations of fishes and macroinvertebrates. The focus was on the abundance and community composition patterns of fishes relative to substrate types and water depth. Rockfishes were the most abundant group, accounting for 69% of the fishes observed. This study employed statistical methods to study associations between fish communities and benthic habitat, and hence identified habitat types that may be useful as surrogates in a search for potential habitat of these fish communities.