Iceland’s Basalt Columns: Nature’s Artwork

May 11, 2020

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Photographer: Patti Weeks
Summary Author: Patti Weeks

Drive north from Iceland’s capital city of Reykjavík for about 75 mi (121 km) on Highway 54 to the eastern border of the Snæfellsnes Peninsula, then a short distance east, and you’ll encounter a geological marvel that’s stunning and looks almost man-made. This impressive natural phenomenon is referred to as the Gerðuberg basalt columns, a 1,640 ft (500 m) long cliff of hundreds of evenly-sized basalt columns, hugging each other closely. They range in height from 40–46 ft (12–14 m), and each pillar is about 5 ft (1.5 m) in diameter.

These cliffs were formed by flowing basaltic lava that originated within the Ljósufjöll volcanic system in the Snæfellsnes Volcanic Belt. Iceland’s 130 volcanoes have been erupting periodically over millions of years, but only 30-40 remain active, including Ljósufjöll. The Ljósufjöll system is classified as an alkalic rock series, which is found outside the active systems of the spreading central Mid-Atlantic Ridge. The Gerðuberg cliffs consist of a coarse-grained basalt called dolerite (also called diabase.)

Lava flows, such as the one that formed the Gerðuberg cliffs, are cooled rapidly by the sea, and as the lava solidifies, vertical cracks are formed by the stress of the rock’s cooling and contracting. The growth of the fractures, or joints, are perpendicular to the surface of the flow, and as they continue to grow, they form a closely-spaced regular array of columns or polygonal prisms. This process is called columnar jointing. The individual prisms can have 3 to 8 sides; the Gerðuberg columns have the more typical hexagonal shape. 333333333These relatively straight, vertical sets of columns are referred to as a colonnade. The horizontal surface bands, or chisel marks, develop due to the cooler exterior temperature of the column during its formation. Fallen blocks of stone, as seen in the photos near the base of the cliffs, have succumbed to erosion at weakened areas.

The bottom photo was taken on the steep, rugged southwestern coast of the Snæfellsnes Peninsula, about 60 mi (37 km) west of the Gerðuberg columns. This oceanside cliff, of the Snæfellsjökull volcanic system, reveals columnar jointing but with an irregular array of columns. Columns with smaller diameters are thought to have cooled faster than those with larger ones. Iceland, made of about 90 percent volcanic rock, may have the largest collection of basalt columns in the world. Photos taken September 4, 2018.

Photo Details: Camera: SONY DSC-HX400V; Exposure Time: 0.0025s (1/400); Aperture: ƒ/3.5; ISO equivalent: 80; Focal Length: 13.3mm. 2: Same except - Exposure Time: 0.0016s (1/640); Aperture: ƒ/4.0; Focal Length: 14.7mm. 3. Exposure Time: 0.0040s (1/250); Aperture: ƒ/5.0; ISO equivalent: 160; Focal Length: 39.6mm.

 

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