Roger Weller, geology instructor
Columnar Jointing Around the World
What is columnar jointing? Larry Gedney, in his article in Alaska
Science Forum explains that as volcanic rocks cool after erupting onto the
earth’s surface, they shrink. This shrinkage produces stresses that strain the
rocks, causing them to crack. If the eruptive rock that cooled evenly is in
contact with a flat surface, the cracks will grow at right angles to the surface
and develop in three directions, each at 120 degrees to the other two. This
leads to hexagonal, six-sided columns. (1)
My interest in studying columnar jointing around the world can be
traced back to my home country of South Korea. Although born and raised in
South Korea, I never had the opportunity to visit Jeju Island, a small,
exclusive island 90 miles south of mainland Korea, until 2007. I was absolutely
fascinated by the dark rough hexagonal patterns of these rocks in the ocean. I
had no idea what columnar jointing was! They were just beautiful. This motivated
me to research different columnar jointing. Most are six-sided columns and a few
are three-sided and many are seven-sided formations. These are breathtaking and
it is hard to realize that these patterns are formed naturally. This is a
columnar jointing formation on Jeju Island, Korea. This columnar jointing is
approximately 30-40m height and is 1Km in width.
People are enjoying fishing while sitting on columnar jointing.
Another example is Fingals Cave found in Scotland. This formation shows larger
columns in height and form along the entrance to the cave. This cave is
associated with an Irish Folklore story. Finn Mac Cool and an Irish Giant went
to fight their Scottish enemy, Finn Gall. Mac Cool drove the columns in place,
one at a time, until the Causeway reached Scotland (Fingals Cave). Mac Cool was
so tired from the hard work that he returned home to rest. Finn Gall followed
the Causeway to Ireland. He found Mac Cool sleeping, and Mac Cool’s wife, a very
shrewd woman, told Finn Gall that this was her baby sleeping. Finn Gall thought
that this giant baby’s father must be of incredible size and that he would
surely lose any fight. Finn Gall fled back to Scotland. As he did, he destroyed
the Causeway; only the two ends of the Causeway survive to this day. (2)
This is a postcard featuring the columnar jointing at Fingal’s cave.
Another example of columnar jointing is Giant’s Causeway in Northern Ireland.
Giant’s Causeway is formed of lava flows, sills, dikes, ignimbrites, and shallow
intrusions of all compositions. Most columns are straight with parallel sides
and diameters from a few centimeters to 3 m. The Causeway consists of nearly
40,000 interlocking basalt columns, some as tall as 36 feet. The tops of the
columns from “Stepping Stones” lead from the foot of the cliffs before
disappearing under the sea. (3)
This postcard shows the columns at Grand Causeway.
Most columns tend to have 5 or 6 sides but some have as few as 3 and as many as 7.
Another example is Devils Post Pile in California. The high volcanic activity within Devils Post Pile is due to its geographic location. The monument is near the edge of the Long Valley Caldera, a 9- by 18-mile oval-shaped depression located 12 miles south of Mono Lake along the east side of the Sierra Nevadas in east-central California. This area of eastern California has produced numerous volcanic eruptions over the past three million years, including the massive eruption that formed the caldera 760,000 years ago. The formation of Devils Post Pile began when basalt lava erupted 100,000 years ago in the valley of the Middle Fork of the San Joaquin River. As lava flowed from the vent, it filled the valley near the Post Pile to a depth of 400 feet, possibly pooling behind a natural obstruction. (4)
This picture shows how the flaring joints pass smoothly into perfectly regular
Another example is Devil’s Tower in Wyoming. This Columnar Jointing is an
igneous intrusion in northwestern Wyoming that cooled and formed prominent
columnar jointing before being exposed by erosion. Devil’s Tower (Lakotaimato
Tipila, which means “Bear Tower”) is a monolithic igneous intrusion or volcanic
neck located in the Black Hills near Hulett and Sundance in Crook County,
northeastern Wyoming above the Belle Fourche River. It rises dramatically 1,267
ft (386 m) above the surrounding terrain and the summit is 5,112 ft (1,558 m)
above sea level. This Devil’s Tower has American Indian legends to tell. The
story is about six Sioux girls who were picking flowers when they were chased by
bears. Feeling sorry for them, the Great Spirit raised the ground beneath the
girls. The bears tried to climb the rock, but fell off, leaving their scratch
marks on the sides.(5)
From bottom to top can see flaring columns, extremely regular columns,
slightly undulose columns, and columns with horizontal jointing.
This is a map of how to get to Devil’s Tower
I found a columnar jointing on Mars by searching the web. According to an
Astrogeology team in Flagstaff, the unique fractures in lavas on ancient Mars
suggest water occasionally flooded portions of the Planet’s surface.
The fractures, known as "columnar joints," are the
first that have been observed on a planet other than Earth. "Columnar joints
form as cooling lava contracts," said Moses Milazzo, a geophysicist with the U.
S. Geological Survey Astrogeology team in Flagstaff. (6)
columnar jointing on Mars.
Another example is Hong Kong High Island and Sai Wan. The Geopark's key rocks - columnar jointed tuff - formed as a mighty volcano collapsed, in an area that includes the south of the present-day Sai Kung Peninsula in the north and the Ninepin Group of islands in the south. Incandescent volcanic ash formed thick
layers in the caldera, and as it cooled to become rocks, cracks formed, creating hexagonal columns. Occurring around 140 million years ago, this was perhaps the last major event of Hong Kong's four phases of volcanic activity during the Jurassic and Cretaceous periods. (7)
There are excellent exposures of the tuff, with the columns very
prominent, on the coast near the east dam of High Island Reservoir and around
nearby Po Pin Chau.
Around the world, naturally formed patterns of six-sided columnar jointing are
found to be amazing and beautiful. Columnar jointing across the globe, in high
mountains and in the sea, may be short, tall, or curved. All keep the same
pattern and basic characteristics of a hexagon, yet are unique in their
development and location. Our GLG 101 class afforded me the opportunity to study
these beautiful formations in detail. I have really learned a lot in this