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If you’ve ever visited the Big Island of Hawaii, you’ll get a chance to see one of nature’s most amazing formations: a lava tube. Lava tubes are natural tunnels where lava flows underneath the ground, sometimes for many kilometers. After the eruption is over, you can be left with a long empty tunnel that seems almost man made.
A lava tube happens when low viscosity lava forms a continuous hard crust that gets thicker and thicker, while lava is flowing inside it. Eventually the lava forms a thick hard crust above, but low viscosity lava continues to flow inside. In fact, the thick sides act like insulation to keep the inner lava hot and molten. When the eruption finally ends, the lava flows out of the tube, emptying it out.
Lava tubes can be many meters wide, and typically run several meters below the surface. One tube on Mauna Loa starts at the eruption point and then flows about 50 km to the ocean. Inside the tube there can be various formations, like lava stalactites known as lavacicles (named after icicles). You can also get pillars that stretch from the top to the bottom of the lava tube.
Some of the most well known lava tubes are Thurston Lava Tube in Hawaii Volcanoes National Park, and Lava Beds National Monument in Northern California.