What do pillow structure indicate about the environment of valcanism?

Pillow structures, or pillow lavas, indicate that volcanic activity occurred in a subaqueous environment, typically underwater. These formations arise when lava erupts and cools rapidly upon contact with water, creating rounded, pillow-like shapes. The presence of pillow lavas often suggests that the volcanic activity took place at mid-ocean ridges or in volcanic island arcs, highlighting the role of water in shaping volcanic features. Their occurrence can also imply that the region has a history of tectonic activity associated with oceanic crust formation.