The Earth Story

Tessellated Pavement in Tasmania

Eaglehawk Neck, located in Southeastern Tasmania, is a narrow isthmus connecting the Tasman Peninsula to mainland Tasmania. The area boasts many natural wonders to observe, from the southern lights to rare geologic formations. The most famous of these geologic formations is the Tessellated Pavement, an area of flat rock that gives off the impression it was molded by human hands. Yet, nature’s ability to create should never be underestimated and at Eaglehawk Neck nature proves it is capable of producing landscapes eerily similar to those made by humans.

Tessellated pavement gets its name from its resemblance to Roman mosaic floors, which are also called tessellated pavement. At Eaglehawk Neck two types of formations can be observed: pan and loaf formations. Pan formations form when saltwater erodes the central surface of a stone block creating shallow pools. This usually happens at a safe distance from the shore, allowing saltwater to crystallize in the concave depressions of the stone. Loaf formations are typically located closer to the shore and are inundated with water for longer periods of time. Water, carrying abrasive sand, is funneled through the joints eroding them faster than the rest of the pavement. Protruding structures resembling loaves are eventually created. The image below is a pan formation.

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Photo courtesy of JJ Harrison[_

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Tessellated pavement at Eaglehawk Neck

The tessellated pavement at Eaglehawk Neck, a narrow isthmus that connects the Tasman mainland to the Tasman Peninsula appears to be man-made. In this case, looks can be deceiving. In the first place tessellated pavements are merely inter-tidal rocky platforms. The geological circumstances of the coastal landform at Eaglehawk neck on the other hand have created quite a rare site.

Between 160 million and 60 million years ago this siltstone platform fractured into rectangular blocks under stress of the earth’s crust. Eventually seawater covered the plateau and in the process the pavement eroded much faster than the rims of the pavement which resulted in a polygonal appearance. This phenomenon is also known as a pan formation where erosion/seawater wears away the center portion of the stone into a pool. The pan area dries out at low tide and this allows salt crystals to form. In turn salt crystals speed up the erosion process of the pan leaving only the edges visible to the eye. The opposite is a loaf formation in which the cracks between the siltstone erode faster leaving a structure that has the appearance of a loaf of bread.

Although the tessellated pavement at Eaglehawk Neck displays similarities with the Giant’s Causeway in appearance, the latter was formed in a completely different geological process namely extrusion of basalt.

As a man-made structure, tessellated pavement is known a specific mosaic floor covering particularly popular throughout the Roman Empire for paving baths, courtyards and the interior of houses. Here, individual mosaic tiles were called tessera.

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Image: Copyright JJ Harrison. The tessellated pavement at Eaglehawk Neck at sunrise.

References:

http://www.parks.tas.gov.au/file.aspx?id=7038

http://www.atlasobscura.com/places/eaglehawk-neck-tessellated-pavement

Archaeology or geology?

Hoary with age, these patterns look like fields separated by dry stone walls submitting to a rising ocean. They are however an entirely geological phenomenon called tessellated pavement, caused by the interaction between erosion and the joints that form naturally in rocks as they are slowly uncovered and the pressure from the disappeared overlying rocks releases. The lower rocks then expand and split, sometimes in amazing geometrical forms like these orthogonal joint patterns.

Such pavements, named after the Latin for tiles, only form when some kinds of flat lying layered sedimentary rocks are exposed to marine erosional forces. This example is from the Australian island of Tasmania, at a site called Eaglehawk Neck that links the Tasman peninsula to the rest of the isle. The erosion is differential, the salty sandy seawater erodes the less resistant central areas between the joints, resulting in these field like hollows separated by the more resistant cracks in the rock. It might seem counterintuitive that the cracked areas survive better, but the areas near the cracks were reinforced by precipitated minerals from hydrothermal fluids that circulated through the system when it was still buried underground.

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Image credit: J.J. Harrison, Wikimedia Commons