From Molten Rock to Landmark: Nine Volcanic Formations

Devils Tower is an igneous intrusion in the Black Hills of Wyoming that cooled underground about 40 million years ago and was exposed when the softer surrounding rock eroded. It fractured into mostly hexagonal columns as it contracted, and geologists still debate whether it is a volcano neck or a laccolith.

The Giant's Causeway in County Antrim shows the same columnar jointing on a far larger scale: about 40,000 interlocking basalt columns from a fissure eruption 50 to 60 million years ago, some up to 12 meters tall, listed by UNESCO in 1986.

Fingal's Cave on Staffa is cut into basalt columns of the same Paleogene lava flow as the Giant's Causeway. Wave action carved a chamber roughly 20 meters high and 75 meters deep, and the pairing suggests the two sites were once one lava field before the Atlantic widened.

The Organ Pipes in Victoria, Australia, are a much younger case, about one million years old. Lava pooled in the valley of Jacksons Creek, cooled evenly from the surface and cracked into hexagonal columns up to 20 meters tall, which the creek later exposed in a cliff face.

Shiprock in New Mexico is the opposite case to a lava flow: it is the eroded throat of a volcano active about 30 million years ago. Its volcanic breccia and radiating minette dikes survive because the softer surrounding rock wore away, leaving a 483 meter monadnock.

The fairy chimneys of Cappadocia are built from volcanic ash, or tuff, deposited roughly 9 to 3 million years ago. A harder basalt cap shields the softer column beneath it from erosion, and the tuff is soft enough that people have carved homes, churches and underground cities into it.

Manjanggul on Jeju Island is volcanic rock seen from the inside. Its outer lava crust hardened while molten lava kept flowing beneath, and when the eruption ended the lava drained away, leaving a tunnel about 7.4 kilometers long in a UNESCO World Heritage Site.

The Old Man of Storr on Skye shows what happens to volcanic rock after it is in place. A basalt plateau slipped and rotated downhill over weaker Jurassic sedimentary rock in a rotational landslip, leaving isolated blocks, of which the Old Man is the tallest.

The Quiraing completes the path on the same Trotternish escarpment as the Old Man of Storr. Its basalt cap is still slumping, producing the Table, the Needle and the Prison, and the road below needs regular repair because the ground is still moving.