These pillow basalts, seen on Dive 6 at Fina Nagu Caldera C, form when basaltic lava erupts underwater. Cold seawater chills the erupting lava, creating a rounded tube of basalt crust that looks like a pillow. As the newly erupting lava pushes through the chilled basalt crust, it can form scratches on the pillow surface, called striations. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download (mp4, 30.9 MB)
Dive 6 was the second of our three dives in the Fina Nagu Volcanic Chain. We expected that the age of this caldera would be between the ages of the older Fina Nagu D (visited on Dive 5), and younger Fina Nagu A (which will be visited on Dive 7). The remotely operated vehicle touched down at 2,575 meters, in a patch of lightly sedimented pillow basalts. The volcanic sample we collected in this area had well-defined ropy flow structure. We encountered several different types of lava morphologies as we moved upslope, including pillows, sheet flows, ropy flows, and columnar jointing related to a dike. We transited through an area covered in volcaniclastic sediment (produced by explosive eruption), and collected a second geological sample at an isolated pillow outcrop. While fauna was again scarce on this dive, we saw a rattail fish (Macrouridae, Kumba sp.), two halosaurs (Aldrovandia sp.), bamboo corals (Isididae), many transparent predatory sea squirts (ascidians), a benthic ctenophore living on a sponge (learn more about inter-species interactions), a few sea cucumbers (holothurians), sponges, and sea lilies (crinoids).