Waimea Canyon

Waimea Canyon: layers of solidified lava on the island of Kauai, HI

Waimea Canyon on the island of Kauai in Hawai'i is impressive. Beyond the surreal beauty of standing on the edge of a 3000 ft (900 m) cliff, there are a some fascinating details in the rock.

First off, these aren't layers of sedimentary rock, but igneous basalt! They formed by the solidification of lava flow upon lava flow upon lava flow over several million years! That's pretty darn unusual - every other layered canyon I know of cuts through layers of sediment.

Secondly, the layers are remarkably horizontal and undisturbed. What's so special about that? Well when lava flows down the side of a volcano and solidifies, the solidified rock layer dips with the slope of the volcano (you can't see it in the photo, but the west side of the canyon actually has layers that slope gently).  Horizontal layering of lava only happens when lava is fills in a hole; the lava flows down, settles and solidifies. So what happened here? It turns out that about 4 million years ago, the volcano collapsed, leaving a huge depression and then over the next several million years lava flowed in and filled the depression.