There is a 0.5 mile loop trail at the Devastated Area at Lassen Volcanic National Park (CA) appropriately named the Devastated Area Trail. The trail is an easy hike with a number of information signs teaching about the rocks left behind from Lassen Peak’s 1915 eruption. One of the most interesting of these I found were the puzzle rocks which had the following description sign by them:
After the May 19 avalanche carried hot lava rocks, the surrounding air temperature quickly cooled them. As they cooled — from the outside in — some of the rocks fractured internally, breaking into pyramid-like shapes. Like a jigsaw puzzle, many of the pieces could easily be reconfigured — solving a hot puzzle from the past…
As the description indicates and the photos show, these rocks really do look like puzzles that could easily be shaped back together with a bit of effort. I always enjoy finding tidbits of information like this when I travel to the various National Parks.