Columnar joints

The basalt here is broken up into consistent linear features that at first may appear to be depositional layers because they are currently horizontal. However, these are not bedding layers from successive lava flows or from river or lake deposition. Instead, these are parallel joints that were originally vertical in orientation! The fracture planes were produced through process known as columnar jointing, and it is a relatively common occurrence as lava hardens and contracts as it cools. A dramatic example of this can be seen at Devil’s Postpile near Mammoth in the eastern Sierra. Other famous instances of columnar jointing include Devil’s Tower in Wyoming and Giant’s Causeway off the Irish coast. In these above listed cases, columnar jointing has formed striking, hexagonal pillars, but the joints here are not as neatly geometric. For more on the formation and structure of columnar joints, follow this link.