Joints are fractures in rocks, and columnar joints are a specific type of joint pattern. Columnar joints contain aspects of both petrology and structural geology, but the discussion will address only the structural aspect. Columnar joints are defined as parallel, prismatic columns in basaltic flows and sometimes other rocks, and this specific pattern is a result of cooling (Bates and Jackson, 1984). The columns are normally found in shallow intrusive or extrusive igneous rock bodies, generally, basaltic, sills, dikes and lava flows.
The columnar fractures are a result of the cooling process. The basalt cools rapidly from the outside toward the center, causing shrinkage cracks to form, commonly, in a hexagonal pattern. There are examples of the more uncommon forms that display 3 to 12 sides. The shape of the columns is attributed to tensional stress. The columns vary from a few inches to several feet in diameter. The length of the column is ordinarily perpendicular to the contact. Hill (1972) explains that the sides of the columns commonly exhibit horizontal markings called chisel marks. Where a column may have broken, one side exhibits a concave surface and the other a convex surface - this is called cup and ball jointing.
There are several places in the United States where columnar jointing is exhibited, a few examples are, Devils Postpile in California, Devils Tower in Wyoming (pictured below), Sheepeaters Cliffs in Yellowstone National Park, and Palisades sill in New Jersey.
Julie Welch, 2/10/97Return to Structural Geology Notebook index page.>