Lava Along the Limberlost

Ranger programs can dive deeper into the rich geology of the Limberlost

What have we stumbled upon? The old ruins of a former building? An artfully crafted stone bench? The petrified honeycomb of a ginormous beehive?

This is called Columnar Jointing, but to understand this fascinating stone, we must go back to the beginning. The story of Shenandoah would not exist without the geology underfoot. More than one billion years of formation, erosion, and uplift have sculpted the mountain range into how we see and experience today.

Most of the stone that visitors hike along is greenstone, a metamorphic rock, meaning that it started as one type and slowly changed under the influence of pressure, heat, and time into another. Greenstone actually shows evidence of two different types of rock - sedimentary rock from the former sea that left behind sand and shells, and igneous (volcanic) rock left behind by active lava flows and underground magma. These volcanic roots are seen in several areas throughout Shenandoah, but Limberlost holds one of the best and easiest to access examples!

How would you describe this rock formation to somebody who has never seen it? Does it resemble anything you've seen before? Run your hand along its corners, cracks, and flat edges--what do you notice?

The movement of lava has been a very active ingredient in the recipe of Shenandoah and the Blue Ridge.The most common volcanic rock that existed before the mountain metamorphosed, was basalt...but what exactly is basalt, and why is it so iconic? Lava that forms basalt is very liquid (low viscosity) in nature, which means that it can both move and cool very quickly. Because of this rapid cooling, cracks form.

 

Have you ever noticed when an ice cube is dropped in a glass of water, it crackles and cracks? This is because of the instant change in temperature!
 

Those cracks continue to grow and spread throughout the rest of the lava chamber, eventually becoming hexagonal column-like features. Over time, the softer stone and loose soil around the now hardened basalt erodes away which leaves behind the beautiful formations we see today.
 

How much deeper into the mountain might the roots of this columnar basalt stretch? What might Park visitors see here in another fifty, hundred, or thousand years as the ground erodes?