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 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. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas.

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