Devils Postpile National Monument
The Devils Postpile in California may be one of the younger rock formations on this list, as recent studies show it was formed less than 100,000 years ago when a lava flow cooled and cracked into multi-sided columns. This particular type of lava — basaltic lava — is rich in iron and magnesium and is normally much hotter than other types of lava, according to the National Parks Service. Because of this, basaltic lava flows more quickly, and that's exactly what it did tens of thousands of years ago as it flooded Reds Meadow Valley until it was blocked by a natural dam, creating a lava lake about 400 feet deep.
The lava cooled at different rates, with shallow parts of the lake hardening first. As it cooled, the solid lava contracted away from the liquid lava, causing cracks or joints. These joints formed columns that now stand about 60 feet high but used to be much taller. Today, this national monument stands as a stunning example of a basalt column landscape.