It is an odd feeling to find out that although the land we are walking on seems pretty solid, the interior of the island is actually riddled with a large network of lava tunnels, so much of it hollow!
These amazing features are formed by flowing lava. The eruptions in oceanic volcanoes, such as those in Hawaii or Galapagos, tend to be effusive rather than explosive, and are characterized by extensive flows (either Pahoehoe or aa) which often reach the coast. The upper layer of these flows is in contact with the air, and solidifies rapidly. As solid lava is such a good insulator, this solid exterior often surrounds a molten interior, which continues to flow until it empties out and leaves an empty tube or tunnel. In some cases, these tunnels are so wide that we can easily walk through them for long stretches at a time. It is an eerie feeling to be walking inside the island, following the path of these great rivers of molten rock.
