Final answer:
Pillow lava structures indicate that the lava was extruded under water.
Step-by-step explanation:
Pillow lava structures indicate that the lava was extruded under water. When lava is extruded under water, it cools rapidly and solidifies quickly, forming characteristic pillow-like shapes. This occurs because water rapidly cools the lava, causing it to contract and form a solid outer crust while the interior continues to flow.