Beyond the charts: geological highlights from NOAA’s 2013 hydrographic field season in Alaska

–By Christy Fandel, Coast Survey physical scientist

Have you ever wondered what lies beneath the charted soundings on a nautical chart? While surveying Alaskan waters during the 2013 hydrographic field season, collecting bathymetry to update NOAA’s nautical charts, hydrographers revealed many interesting geologic features on the seafloor.

NOAA focuses a significant portion of our ocean mapping effort along the Alaskan coast. The Alaskan coastline represents over 50% of the United States coastline and dated nautical charts are inadequate for the increasing vessel traffic in this region. NOAA surveys are essential for providing reliable charts to the area’s commercial shippers, passenger vessels, and fishing fleets.

This past season, NOAA-funded hydrographic surveys in Alaska revealed many interesting geological features on the seafloor. Three surveys, in particular, took place in southeastern Alaska in the Behm Canal, along the Aleutian Chain within the coastal waters surrounding Akutan Island, and around Chirikof Island.

These three areas were among the areas surveyed by the NOAA Ship Rainier and surveying contractor Fugro-Pelagos during the 2013 field season.

In May, hydrographic surveying conducted by NOAA Ship Rainier in the Behm Canal revealed two distinct geological features. In the northern region of the canal, scientists identified a long, meandering ancient river. This ancient submarine river is nearly 40 km in length with up to 50 m in relief. Further south, Rainier surveyed a large volcanic-like feature. The surveyed volcano appears to have a distinct caldera, or collapse-feature that most likely formed after the volcanic eruption.

Multibeam bathymetry of the northeastern portion of the Behm Canal shows a large, meandering submarine river. The cross-sectional inset highlights the relief of the channel, nearly 50 m, as shown by the red box.

Multibeam data acquired by NOAA Ship Rainier shows a large volcanic feature in the southern portion of the Behm Canal.

Directly following the Behm Canal survey, Rainier transited west to survey the coastal waters surrounding Chirikof Island. The acquired bathymetric data revealed a stark northeast-trending fault in the southeastern portion of the survey area. This surveyed fault is distinguished by a clear misalignment across the fracture.

The red box outlines the northeast-trending fault along the coast of Chirikof Island, shown with bathymetry acquired by the Rainier.

Concurrently, an Office of Coast Survey hydrographic surveying contractor – Fugro-Pelagos  – was surveying off the western coast of Akutan Island. Fugro’s hydrographers identified a large volcanic feature within the acquired bathymetric data. The surveyed volcanic feature is believed to be either a volcanic vent or cinder cone volcano. The multiple circular rings outlining this feature may represent the successive lava flows that formed the volcano.

Multibeam bathymetry acquired by Fugro, around Akutan Island, shows a large volcanic vent or cinder cone volcano, marked by multiple circular rings that represent the successive lava flows that formed the volcano.

With the upcoming 2014 hydrographic field season quickly approaching, the number of geologic discoveries will only increase. Extending all along the Aleutian Chain, from Kodiak Island to Bechevin Bay, the planned surveys for the 2014 field season will surely reveal many interesting and previously unknown geologic features.