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Lava
Lava tunnel on Isabela Island. As lava flows, it can form lava tunnels up to several kilometers long. The surface of the lava cools rapidly and forms a crust that protects the lava that continues to flow beneath. If the lava under the hardened crust eventually drains out, it can leave a hollow tunnel behind.
vesicles
Elongate vesicles in basalt from the Bonito Lava Flow, west of Sunset Crater, AZ. Gas bubbles trapped in the lava originated these cavities, with the stretching related to a preferential direction of flow. This example of volcanic activity is about 1,000 years old, and is part of a larger Volcanic Field (San Francisco Volcanic Zone, AZ, USA).