Young pillow basalts on Vailulu'u, with an age of perhaps five year or less.

Young pillow basalts on Vailulu'u, with an age of perhaps five year or less. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download larger version (jpg, 1.3 MB).

While we did not see many fish during the dive, we did catch a glimpse of a thresher shark as it circled near Seirios, above ROV Deep Discoverer.

While we did not see many fish during the dive, we did catch a glimpse of a thresher shark as it circled near Seirios, above ROV Deep Discoverer. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download larger version (jpg, 305 KB).

Dive 09: Vailulu'u Seamount
February 24, 2017
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Dive 9: Ctenophore

This beautiful ctenophore was seen in the water column while exploring Vailulu'u seamount. Rows of cilia moving in waves refract light to create rainbow-like patterns as the animal moves through the water. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download (mp4, 24.8 MB)

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Vailulu'u Volcano

On February 24, 2017, the ninth dive of the expedition, we explored Vailulu'u seamount, an active volcano lying in the eastern region of the Samoan hotspot.

Prior to our dive, the last visual survey of the volcano had taken place in 2005, but the seamount has been mapped since. Comparison of multibeam data collected in 2012 with new bathymetric data collected during this expedition show that the volcanic cone in the crater, called Nafanua, has grown substantially since the seamount was last mapped in 2012, having formed two distinct volcanic features.

The dive represented an exciting opportunity to observe a very dynamic environment both in terms of geology and biology. Much can be learned in terms of succession and how species colonize the seafloor over time. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2017 American Samoa. Download (mp4, 123.1 MB)

During Dive 09 of the expedition, we explored Vailulu'u seamount, an active volcano lying in the eastern region of the Samoan hotspot. Comparison of multibeam data collected in 2012 with new bathymetric data collected during this expedition show that the volcanic cone in the crater, called Nafanua, has grown substantially since the seamount was last mapped in 2012, having formed two distinct volcanic features. The new mapping data also showed plumes (likely composed of carbon dioxide gas) rising from the location of previously identified hydrothermal vents.

Today's dive started at a depth of ~900 meters, within the crater of Vailulu'u and approximately 50 meters east of the hydrothermal activity. However, upon reaching the seafloor near the hydrothermal vent field, the water was so turbid that visibility was severely limited and we had to move to a different location with better visibility. This extreme turbidity is a change from the last visual remotely operated vehicle (ROV) observation of the seamount, which occurred in 2005.

In exploring the youngest volcanic feature on Nafauna, we consistently observed spectacular pillow basalts as the ROV moved along the bottom, some (likely older) pillows partially covered with sediment. However, all lavas on Nafanua are less than 18 years of age, as Nafanua did not exist in the crater when mapped in 1999. Carnivorous sponges, anemones, ophiuroid brittle stars, and stalked hydroids were seen on some of the pillows. Upon reaching the second new volcanic feature on Nafauna, we saw a significant change in the morphology of the flows. Instead of the classical pillow basalt structures observed earlier, the flows were chaotic "broken" structures; it is not clear what is responsible for this change in lava morphology. On the summit of Nafanua, where flows are known to be older, rock surfaces had an orange-brown color, suggesting significant hydrothermal alteration. Fresher (grey) surfaces were juxtaposed with the orange-brown surfaces, suggesting a recent disturbance, possibly a landslide. Here, we observed several Anthomatus corals, larger carnivorous sponges and anemones, and hydroids. The large degree of sediment cover, in addition to the greater degree of biological diversity and the larger size of the fauna, is consistent with a lack of recent growth on the summit of Nafanua.