Lava tubes are a type of lava cave formed when an active low-viscosity lava flow develops a continuous and hard crust, which thickens and forms a roof above the still-flowing lava stream. Tubes form in one of two ways: by the crusting over of lava channels, and from pahoehoe flows where the lava is moving under the surface. Lava usually leaves at the point of eruption through channels. The channels stay very hot although their surroundings are cool. When lava begins to flow away from its eruption point the surrounding lava of the pahoehoe flow cools down, resulting in the underground channel known as the lava tube.
- Surtshellir – For a long time, this was the longest known lava tube in the world.
- Víðgelmir – Another long lava tube of Iceland.
- Gruta das Torres – Lava tube system of the Azores.
- Cueva del Viento – 17 km lava tube. Tenerife, Canary Islands
- Lava River Cave – Located near Flagstaff, Arizona inside Coconino National Forest
- Lava Beds National Monument – The site of the largest concentration of lava tubes, located in Tulelake, California
- Kazumura Cave – Not only the world’s most extensive lava tube, but has the greatest linear extent of any cave known and is located in Hawaii County, Hawaii
- Lava River Cave – A lava tube in Oregon’s Newberry National Volcanic Monument, located in Deschutes County, near Bend, Oregon
- Ape Cave – Lava tube located in Gifford Pinchot National Forest, just south of Mount St. Helens in Washington state