GeoLog

Polygons

Imaggeo on Mondays: Giants Causeway

Imaggeo on Mondays: Giants Causeway

Since its discovery back in the late 1600s the origin of the spectacular polygonal columns of the Giants Causeway, located on a headland along the northern coast of Ireland, has been heavily debated. Early theories for its origin ranged from being sculpted by men with picks and chisels, to the action of giants, through to the force of nature. It wasn’t until 1771 that Demarest, a Frenchman, suggested that the origin of the world-famous headland was indeed volcanic.

“The myth goes that the Irish giant Finn MacCool once constructed a land bridge from Northern Ireland to Scotland in order to meet a rival giant,“ explains Bernhard Aichner, author of this week’s featured imaggeo image. “It is said that he used basaltic rocks from the surrounding cliffs to construct the bridge. Finn´s rival later destroyed most of the causeway, but the remnants still can be seen today as basalt columns descending into the sea.“

We now know that the hexagonal columns along the rugged Irish coastline formed some sixty million years ago, during a time when the ancestors to modern plant species started to emerge and the Earth was going through a period of warming. At the time, Antrim (a modern-day county of Ireland), was subjected to an intense period of volcanic activity as a result of the opening of the Atlantic Ocean.

The Giants Causeway is only a small part of a vast network of lava flows which extended over the Antrim landscape; much of which, through the passage of time, has been eroded away. There were three distinct periods of volcanic activity which resulted in a thick succession of lava flows which has been subdivided into the Lower, Middle and Upper Basalts.

The lavas of the Giants Causeway belong to the Middle Basalts and comprise over 40,000 vertical columns. Recently, a new model was proposed for the formation of the striking polygonal pattern formed by the columns. They formed within a lava flow, which contracted and fractured while cooling very slowly. If the loss of heat is steady, then the pattern formed is uniform, but if areas cool faster than others, the fractures develop unevenly, meaning the columns form in a variety of sizes and shapes: expect anything from pentagons to heptagons if you get a chance to visit the Causeway!

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: Polygon ponds at sunset.

Thinking of the Arctic conjures up images of vast expanses of white icy landscapes punctuated by towering icebergs and a few dark rocky masses; certainly not a green landscape with a series of water pools amongst rolling hills. The image below is perhaps more reminiscent of the temperate Scottish or Welsh countryside; but don’t be fooled, out Imaggeo on Monday’s image was captured by Reinhard Pienitz  (Laval University, Canada) in western Bylot Island, part of the Canadian Arctic Archipelago.

Polygon ponds at sunset. (Credit: Reinhard Pienitz, via imaggeo.egu.eu)

Polygon ponds at sunset. (Credit: Reinhard Pienitz, via imaggeo.egu.eu)

The uniqueness of Bylot Island is due to the convergence of a number of ecosystems. It lies to the north of Baffin Island and is dominated by high mountain peaks and glaciers. The southern plain of the island is at relatively low-elevations and covered by tundra vegetation. Wetlands are common in the low lying terrain where grasses, brown moss and sedges carpet the landscape. Think of them as ‘polar oasis’ which support hundreds of plant species and tens of animal and bird species. In contrast, the slopes of the hills are much drier and support shrubs, grasses and forbs.

The ground in the low-lying areas is perpetually frozen which means the drainage of water from the melted snow is hampered; it is the presence of this permafrost which allows the formation of the widespread wetlands. Year on year, organic matter accumulates, rising upwards, as the permanently wet and cold conditions mean it is very difficult for organic material to break down. As time passes, cyclic layers of peat and permafrost build up. The importance of the wetland ecosystem in the Arctic cannot be underestimated. The peat-rich soils constitute a net sink for carbon during the Holocene and are thought to store 97% of the tundra carbon reserve (Ellis et al., 2008).

Freezing over the winter months and thawing over the (slightly) warmer spring months drives the formation of the polygonal pattern seen across the wetlands of Bylot Island (and many other Arctic regions). As the ground freezes it contracts resulting in the formation of vertical cracks which penetrate the layers of peat and permafrost. The moist soils and meltwaters mean that there is plenty of water available to infiltrate the cracks, especially during spring. As the cold winter months approach the water freezes and the cracks expand, forming what are known as ice wedges, commonly connected at the surface, which give rise to the well-ordered polygonal pattern.

 

References

Ellis, C.J., et al. (2008), Paleoecological Evidence for Transitions between Contrasting Landforms in a Polygon-Patterned High Arctic Wetland, Arctic. Antarctic, und Alpine Research., 40, 4,624-637

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.