When you visit Hawai’i Volcanoes National Park you’re inundated with hazard warnings. There are warnings about hazardous fumes, there are warnings about steam vents, earth cracks, and cliffs, and there are warnings about invisible cows. Oh wait, that’s Mauna Kea.

There are also warning signs about lava delta collapses, where huge chunks of land can just break off and fall into the ocean. To most visitors, this is a rather abstract warning, as the land looks incredibly stable. It’s rock, after all!

The main problem is that the lava bench isn’t built on a stable foundation. It’s typically formed covering jumbles of sand and smaller rocks formed by the lava entering the ocean. This unstable foundation eventually gives way under the enormous weight of the lava covering it, and the whole thing drops into the ocean.

Not surprisingly, the USGS has an excellent description of how lava deltas form and collapse. Pictures and descriptions still don’t give any sense of magnitude. Videos are much better.

Luckily the USGS caught a lava delta collapse on November 28, 2005. A 34-acre delta at the East Lae`apuki ocean entry collapsed in five hours, taking an additional ten acres of land with it.

Even more amazingly, the entire bench reformed in only four days.