A mound of pillow lava.
Pillow Lava
Image courtesy of NOAA Ocean Exploration, 2016 Deepwater Exploration of the Marianas. Download larger version (jpg, 1.1 MB).

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.

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