Amazing Rocks!

On a recent hike in the Shenandoah National Park with my friend Kathy, we saw some rocks that were truly amazing!  They were unlike any I’d ever seen.

A sign along the main trail directed us to a side trail that would lead us to an unusual rock formation called columnar jointing.  We took this side trail down a steep hill, past a massive rock.  It was only when we turned to look back up at that rock that we suddenly understood why these rocks were not to be missed!

The downhill side of the giant rock face looked like a bundle of hexagonal columns, all sliced crosswise to similar but not identical lengths.  The hexagonal shape of each column was so distinct that these rocks looked like crystal formations.  Here is a photo I took of the rocks:

columnar jointing

Like many hikers before us, no doubt, Kathy and I were intrigued by these rocks and wondered how they had been formed.  So a few days later, I did a little research.

Apparently, these rocks are well-preserved cooling columns from major lava flows that occurred some 570 million years ago.  At that time, two tectonic plates began to spread apart along a system of rifts thousands of miles long.  Molten basalt from deep inside the earth rose through these rifts, spilling out onto the earth’s surface in vast quantities that eventually covered more than 4,000 square miles.

As the liquid basalt cooled, it solidified, forming very angular, polygonal cracks similar to those found in drying mud.  Under the right conditions, these cracks can extend many tens of feet and produce a structure that looks like long, polygonal columns of rock, which geologists call columnar jointing.–April Moore


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