What evidence from pillow basalts shows that oceans existed at least 3.5 billion years ago?

The presence of pillow structures in 3.5-billion-year-old basalts serves as critical geological evidence that oceans existed during that time. These distinctive structures form when basaltic lava is extruded underwater, causing the exterior of the flow to cool rapidly and form rounded, pillow-like shapes.12

The identification of these relict pillow structures within amphibolites—which share a trace element chemistry similar to modern seafloor basalts—demonstrates that the lavas were erupted in an aquatic environment, likely around volcanic islands in the early ocean. This evidence, combined with the existence of 3.5-billion-year-old stromatolites, confirms that Earth's surface had cooled sufficiently to support liquid water by this period in the Archean Eon.12