Geo-Joint: The Devil’s in the Details of Columnar Basalt

Not too long ago, the Geo-Joint looked into the history and nature of flood basalts. These massive flows of volcanic material over long periods of time had some drastic planetary-scale effects on atmospheric chemistry and the evolution of life itself. So these were some scary, big-deal events. But aside from nearly wiping out life on Earth, basaltic lava flows do have some charming qualities. Under the right circumstances, as a mass of basaltic lava cools, it may do a very curious thing. Over the course of perhaps 100 years in the case of big basaltic bodies, the surface-to-depth cooling will cause cracking, or jointing, usually in a hexagonal pattern, a geometry that pops up in many natural processes. In this case, the pattern emerges because the hexagonal shape is the most efficient for releasing the heat energy coming off evenly spaced cells of cooling contraction within the lava mass. If nodes of cooling are more randomly distributed, due to uneven heating or variable density in the melt, the cracks, or joints, in the lava may form 5- or 7-sided polygons. As the cooling proceeds deeper into the rock, the fracture pattern persists. This cracking may penetrate for many tens of feet into the rock body, and when fully hardened and eventually weathered out for us to see, what results is an astonishing display of what is called columnar basalt. These tall, multi-sided columns stand looking like a man-made wall of broadly faceted stone.

Devils Tower is the most prominent feature for miles around its Wyoming home.

Big bad bears couldn’t climb it, but people find a way—though Native American groups are sometimes disapproving of the activity.

Bear-claw scratches or a cooling fracture pattern? Pretty amazing either way.

Places where this process has occurred have become well known landmarks, and in this country two of them have been honored as national monuments: Devils Tower in Wyoming (America’s first national monument), and Devils Postpile in California’s Sierra Nevada. Native peoples have lived alongside these puzzling structures for centuries and have ascribed them mythic tales to account for their existence. Several tribes in the region of Devils Tower each have their own narrative. The details vary, but they mostly involve an enormous bear, or bears, in pursuit of people who take refuge on a rock that grows skyward by the aid of helpful spirits. The bears rake the sides of the huge rock with their claws in a futile attempt to reach their prey, and the giant linear grooves now seen on the sides of the rock column are those scars. Despite the variations in the native stories, all the tribes consider Devils Tower to be a place of great spiritual importance, and still gather there for ceremonies. The diabolical name the landmark now goes by came from white settlers and may have been the result of a misunderstanding or translation error.

Somewhat less thrilling are the how-it-got-there explanations provided by geologists. Devils Tower, unlike the masses of basalt found in surface flows, is basalt that pushed up into overlying sedimentary rock which has since eroded away. Several theories have been posited as to the particular form of that intrusion, ranging from a simple stock of basalt not greatly different in shape from today’s tower, to the laccolith model which would have the magma body as more of a blob which later weathered to a thinner spire. Some think it may be the remnant of a once active volcano, an artifact called a volcanic neck or plug. Scientists wonder, though, why there is no other evidence of volcanism in the area, such as ash, lava flows, or cinders. Another scenario, called a maar-diatreme volcano, suggests a rising molten basalt body could have encountered groundwater, resulting in a massive steam explosion and a crater on the surface, which then filled with lava. The cooling, jointing, and later erosion then led to the Devils Tower we now see. Unfortunately, the erosion involved in all these scenarios has wiped away the clues that might have allowed for a definitive conclusion. By whatever mechanism of formation, Devils Tower is the biggest example of columnar jointing in the world.

Devils Postpile, California’s smaller but fascinating example of columnar basalt. Broken pieces at its foot gave some to call it the Devils Woodpile.

Glaciers sheared off and polished the upper surface of the basalt body.

The origin of Devils Postpile seems to be in less dispute. Somewhere between 80,000 to 100,000 years ago, a great expulsion of basaltic lava came forth out of the seismically and volcanically active Eastern Sierra region, forming a sizeable lake, possibly dammed by old glacially deposited ridges. The rocky lake had a depth of 400 feet in places, a hot bath that slowly cooled, allowing for the formation of the spectacular basalt columns. Once solidified, the mass of stone endured a variety of erosive forces beyond rain and wind, including the destructive power of ice expansion in the joints, earthquakes knocking the columns down, and rivers eating into the base. Most effective, however, were the glaciers that periodically chewed away at the lava body, basically providing a view that would otherwise still be locked inside the lava deposit. As a fingerprint of their handiwork, the glaciers ground away at the upper reaches of the structure and left scour marks and a shiny polished surface on some of the column tops.

The homes of Finn Mac Cool and Finn Gall.

The Giant’s Causeway, on the coast of Northern Ireland. Fans of Led Zeppelin and the Game of Thrones may recognize Finn Mac Cool’s home turf.

Fingal’s Cave, on Scotland’s Staffa Island, is at the other end of the mythic causeway, and features the same hexagonal motif.

Examples of columnar jointing in basalt can be found in many places worldwide, never failing to impress those fortunate enough to encounter them. Two iconic occurrences of columnar basalt can be found at the Giant’s Causeway in Northern Ireland, a broad display of over 40,000 seaside columns, and Fingal’s Cave on Scotland’s Staffa Island. Like Native Americans, early denizens of Ireland found the magical structures required an origin story. The legend is told that two giants, Finn Mac Cool in Ireland, and Finn Gall of Scotland, had a rivalry. Finn Mac Cool drove the huge, basaltic, hexagonal columns into the seafloor, creating a causeway to the Scottish island, where he hoped to fight Finn Gall. But the work so tired him that after he reached the far shore he went back home to rest. Meanwhile, Finn Gall discovered the causeway and followed its path to the sleeping Mac Cool. Mac Cool’s wife was quick-thinking enough to tell Finn Gall that the slumbering giant was her baby, and that of course made Gall think Mac Cool himself must be enormous. He beat it back home, destroying the causeway behind him as he went. And thus, the columnar basalt at the Giant’s Causeway on Northern Ireland’s coast, and Fingal’s Cave on Staffa, are all that remain. The inventiveness of storytellers is only rivaled by the ingenious deductions of those who have applied science to the mystery. Whichever explanation you find more intriguing, columnar basalt is one of nature’s more delightful and scenic stone creations.

Stunning displays of columnar basalt, like this one in Iceland, occur worldwide.

They’ve even been spotted on Mars.


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PHOTO CREDITS:

caption: Devils Tower is the most prominent feature for miles around its Wyoming home.
source: Wikimedia Commons: Arunth 1992 (CC by SA 4.0 International)

caption: Bear-claw scratches or a cooling fracture pattern? Pretty amazing either way.
source: Wikimedia Commons: Colin.faulkingham (Public domain)

caption: Big bad bears couldn’t climb it, but people find a way—though Native American groups are sometimes disapproving of the activity.
source: National Park Service: Unknown (Public domain)

caption: Devils Postpile, California’s smaller but fascinating example of columnar basalt. Broken pieces at its foot gave some to call it the Devils Woodpile.
source: Flickr: Curtis Barnard Jr (CC by SA 2.0 Generic)

caption: Glaciers sheared off and polished the upper surface of the basalt body.
source: Wikimedia Commons: EKH photography (CC by SA 4.0 International)

caption: The Giant’s Causeway, on the coast of Northern Ireland. Fans of Led Zeppelin and the Game of Thrones may recognize Finn Mac Cool’s home turf.source: Pixabay: hhach (Pixabay license)

caption:  Fingal’s Cave, on Scotland’s Staffa Island, is at the other end of the mythic causeway, and features the same hexagonal motif.
source: Wikimedia Commons: Luk—commonswiki (CC by SA 4.0 International)

caption: Stunning displays of columnar basalt, like this one in Iceland, occur worldwide.
source: Pixabay: Barthwo (Pixabay license)

caption: They’ve even been spotted on Mars.
source: Wikimedia Commons: NASA Earth Observatory/Mars Reconnaissance Orbiter (Public domain)

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