When thinking of caves, it is common to think of voids under the earth's surface that formed as acidic water ate away at rocks like limestone or marble, and filled with stalactites and stalagmites. Hawaii has no layers of limestone to speak of, but it does have volcanoes. How can a volcano make a cave?
The more relevant question is: how can basalt lavas flow for miles from the erupting vent? Wouldn't the lava cool after just a few hundred yards and become solid? As it turns out, lava does cool quickly, but it also forms a crust, and volcanic rock is a great insulator. The lava can continue flowing beneath a solid crust for miles without any significant temperature loss. The molten rock travels through an extensive network of lava tubes to extend the length of the flow. An active lava tube system can be seen in the pictures below from Pu'u O'o on the Big Island of Hawaii.
In the picture above, the lava emerges at Pu'u O'o on the far right side of the picture. It immediately flows into the lava tube system, traveling for seven miles down the slope, to emerge on the coastline on the far left side of the picture. No active lava flows are visible in any part of the picture.
The picture below is from a point above the Pu'u O'o vent looking towards the ocean entry in the distance. The roof of the lava tube has collapsed in several places giving rise to the steam vents.
The caves form when the eruption ends and the lava tubes drain. The tubes are often big enough to walk (or crawl) through, and because they can split into multiple passageways, they can be a lot of fun to explore. Kazumura Cave starts near the summit of Kilauea at almost 4,000 feet, and has continuous passageways that reach the coastline near Hilo, 40 miles away. It is the longest known lava tube in the world.
Hawaii has a number of pretty incredible lava tubes, but isn't this post supposed to be about California? It turns out that the continental United States has numerous lava tubes, and California has some world class examples. The most extensive collection of lava tubes to be found is in Lava Beds National Monument, on the flank of Medicine Lake Highland. I've been exploring some of the surface features of the monument in recent posts, but the main attraction of the park is the system of lava tubes, some 700 of them totaling more than 30 miles of passageways. Many are open to exploration by park visitors.
Map from: Waters, Donnelly-Nolan, and Rogers, 1990, Selected Caves and Lava-Tube Systems in and near Lava Beds National Monument, California: U.S. Geological Survey Bulletin 1673
We will explore some of California's Volcano Underground in coming posts!