Here's why:

* Shape: The distinctive pillow-like shape is formed when molten lava erupts underwater. The rapid cooling of the lava in the cold water creates a hardened outer layer while the interior continues to flow. This creates the characteristic rounded, bulbous shapes.

* Glassy margins: The outer surface of pillow basalts often exhibits a glassy texture, a result of the rapid cooling process.

* Fractures: Pillow basalts often have fractures that form as the lava flows and cools, creating a distinctive pattern on the surface.

* Absence of weathering: Pillow basalts typically lack the weathering features that are common on terrestrial lava flows.

In contrast, lava flows that erupt on land have different features, such as blocky surfaces, ropey flows (pahoehoe), and extensive weathering.

Therefore, the presence of pillow basalts is a strong indicator that the volcanic activity occurred underwater, potentially along mid-ocean ridges, oceanic hotspots, or submarine volcanic chains.