Final answer:
Lava vesicles are formed when gas becomes trapped and cannot escape while the lava solidifies into igneous rock, resulting in the creation of these unique structures.
Step-by-step explanation:
Lava vesicles develop when gas is unable to escape as the lava solidifies. This occurs because the lava flows from a volcano and hardens into igneous rock. During the cooling process, gases that were dissolved in the molten rock become trapped as the lava solidifies, forming vesicles.
For example, in the basaltic lava flows observed on Earth and similar basaltic structures spotted on the Moon, where the maria represent ancient hardened lava flows, these gas bubbles can be preserved as vesicles. When observed in a cross-section as in the case of the basaltic lava flow from Kilauea Volcano in Hawaii, these structures are evident.