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Columnar Jointing on Turtleback Mountain

The formation of regular shapes by natural processes makes some of the world’s most intriguing wonders.  The presence of long, mostly six-sided columns (called columnar joints) in cooled lavas is one of those wonders.  No one who has seen the awesome grooved structure in Devil’s Tower in Wyoming or the Devil’s Postpile in California, can forget this feature.  What is it about a cooling mass of magma that could cause such symmetrical columnar joints?

A similar phenomenon can be seen when a mud puddle dries.  As it loses water, the mud cracks in symmetric shapes, forming mud polygons.  This occurs because the loss of water causes the mud to shrink.  That contraction is relieved by breaking as the dry mud becomes brittle.  When things contract like this, they naturally seek the most stable position.  For mud, that is a system of natural polygons, separated by vertical cracks.

Basalt and other lavas behave similarly.  As lava cools, it doesn’t dry out like mud, but it does shrink.  As it becomes cold and brittle, the lava contracts and relieves the stress by cracking.  The cracking produces a polygonal pattern that extends through the lava flow.  As weathering cuts into lava, the rock breaks along the joints, exposing this geometric regularity.  Although many of the polygons are six-sided, four, five, seven, or eight- sided columns are also relatively common.  The degree and perfection to which this phenomenon is developed depends on the thickness and composition of the lava and how fast it cools.

The long sides of the column form parallel to the direction of heat loss as the lava cools.  In lava flows, the joints generally run vertically through the flow.  This is because the lava was losing its heat upward to the air and downward to the ground.  Sometimes columnar joints are found in veins or dikes of magma cutting the rock.  These joints may lie horizontally in the dike, like stacked firewood.  This is because the magma in the dike was losing its heat to the cold rocks to its sides.  At Devil’s Tower, the joints through much of its length run vertically, indicating that heat loss was to the surface.  Toward the base of the Tower, however, the joints curve horizontally.  Here heat loss was easiest to the sides, probably because the tower base was far enough below the ground surface that most of the heat loss was to the sides.

Devil's Tower (Wyoming)
Devil's Postpile (California)

Anywhere that lava has been active is a potential place to find columnar joints.  The Devil’s Postpile in California, Fingal’s Cave along the sea coast of Scotland, and the Giant’s Causeway along the coast of Northern Ireland are world famous for this feature.  Yellowstone Park and the Columbia River plateau in Washington State are other good places to look.  In the Northeast and Midwest, good columnar jointing can be seen in the lava flows in the Palisades along the Hudson River, on Isle Royale in Lake Superior, and on the Keweenaw Peninsula of Michigan.

The appearance of these features often has them related, in name and legend at least, with giants or the devil.  Yet there is nothing infernal about them.  They are just good examples of the workings of the laws of physics and geology.

– Dr. William S. Cordua, University of Wisconsin–River Falls

Turtleback Mtn. (Ossipee Mtns.)

The columnar jointing seen on the summit of Turtleback Mountain is not as perfect an example of columnar jointing as Devil’s Tower or the Devil’s Postpile – the conditions during the cooling of the lava that caused the Turtleback Mountain rock formation were not ideal, as described above, so the columns have irregular numbers of sides and the polygons formed are not perfect polygons (meaning equal on all sides), causing the surface we see here to be much rougher and more irregular than the example of the Devil’s Postpile shown above, where conditions had been much closer to ideal and the tops of the columns are often regular enough to look like tiles laid on a floor.  The Devil’s Postpile surface was planed flat by a glacier – so was the Turtleback Mountain formation, but not quite as completely.