A mount of pillow lava. These pillow basalts 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.

A mound of pillow lava. These pillow basalts 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. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download larger version (jpg, 1.0 MB).

Dive 9: Young Lava Flows
April 30, 2016
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Glassy Lava

On Dive 9, we dove on a new pillow lava flow. Comparison of bathymetry collected in 2013 and 2015 indicated an eruption over 100 meters thick. We visited three pillow mounds that were composed almost entirely of glassy pillow lavas. The glass forms when molten lava erupts on the seafloor and comes in contact with very cold seawater, immediately quenching the lava, forming long tubular or round shapes, and creating glass. The glass is important for geochemists because it allows them to determine the composition of the erupting lava. Video courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download (mp4, 57.9 MB)

For Dive 9, we explored part of the Mariana back arc spreading center where there was a recent volcanic eruption. Comparison of seafloor bathymetry collected in 2013 and 2015 showed over 100 meters of new lava flows. We were interested in documenting the young lavas and exploring for diffuse hydrothermal flow. When we first reached the bottom at 4,064 meters, we encountered fresh, glassy pillow basalts, confirming that these were formed recently. Some had a yellow iron precipitate that was likely formed by microbes in the recent past. We made our first rock collection in this area, and then began our transit south to observe a series of pillow mounds. There was an incredible diversity of lava morphologies throughout this dive, which may have been reflective of different temperatures and/or gas contents of the original magmas. We came across a pile of what we first thought were exploded pillow lavas covered in white mineral deposits, and collected one of these fragments. As we continued on, we found a 60-meter-high sheer cliff of pillow basalts, and realized that the fragments below had actually broken off of this mound. We then jumped to the southernmost of the three pillow mounds, where we saw pillows with very glassy textures and some strange morphology (sticking out at 90 degrees into the air). We collected a particularly glassy and curly pillow extrusion. Because the seafloor is so new in this area, it has not yet been colonized by many animals. We saw only a few species: a Synallactidae holothurian (sea cucumber), a Munidopsis squat lobster, and many swimming polychaete worms. One surprise of the dive was finding an area of diffuse hydrothermal flow, where several vent-endemic Chorocaris shrimp were living.