Lava caves only form when fluid basalt flows out of a volcano forming a river of molten rock. The sides of the flow slowly crust over forming a roof over the flow. When the eruption stops the lava pours out the end of a lava tube forming a cave.
Entrance to a cave on Mount Newberry
in Oregon, Myrna Martin
Where caves form
Lava tubes form on the flanks of shield volcanoes. Pahoehoe lava flowing from a volcano vent on the summit of a shield volcano will form channels as the outside edges of the lava flow cool. The lava channels are like rivers with riverbanks. The outside edges of the flow crust over forming a roof over the channel as the eruption continues. When the eruption stops the lava tube drains out at the flow front leaving behind a cave
If the roof of a lava tube is weak, parts of it may fall into the lava flow and be carried away forming a skylight. Scientists often take samples of the lava during an eruption by dipping containers into the flowing lava through a skylight.
Formation of skylights
After the eruption ceases the roof will continue to cool and contract. If the roof over the molten rock is thin it sometimes creates a new skylight. Skylights are often used as entrances to underground caves by people who explore caves.
Big Island of Hawaii lava caves
There are many places in the world where you can find these caves. The most important place to study them is the Hawaiian Islands because of the frequent eruptions of Kilauea and Mauna Loa. There are miles and miles of lava tubes that have formed caves on the Big Island of Hawaii.
Thurston Cave, Hawaii
Thurston Cave on the summit of Kilauea is the most famous caves. Several companies now offer tours of lava tubes on the Hawaiian Islands.
Ape Cave, Washington
Ape cave on the flanks of Mount Saint Helens in Washington is over two miles long. It is the longest cave in continental United States and is open to visitors year around.
Ignimbrite Learn more about ignimbrite that forms during large volcanic eruptions that produce pyroclastic flows.
Coal Formation Find out how leaves falling into a swamp can turn into coal.
Pahoehoe-Lava Find out how pahoehoe lava forms in the Hawaiian Islands.
Pyroclastic Rocks Find out how pyroclastic rocks form during volcanic eruptions.
Aa Lava Learn more about aa lava flows that occur on the Hawaiian Islands and the lava balls that often form on their surface.
Lava Caves Find out how pahoehoe lava flows form these caves.
Rock Cycle Learn more about igneous, sedimentary and metamorphic rocks and how they form.
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