Californians live on the edge . . . of a tectonic plate, that is. In this geologically tenuous location, where a tsunami, earthquake, or volcanic eruption is just another hazard, the rocks and landforms are dynamic too. From erupting geysers and boiling mud pots to collapsing sea arches and crawling landslides, California is a land in motion. In fact, rocks on the west side of the San Andreas Fault have moved northward nearly 200 miles in the last 20 million years.

With lively prose and beautiful photographs, California Rocks! explores sixty-five geologic sites at parks and other publicly...