Pillars at the bottom of the world [Australia]

When it comes to pockets of land, there is far and few in-between in the remote south. Before you hit Antarctica the closest you’ll get is the southern tip of South America, the last gasp of south island New Zealand, and finally, the Australian island state of Tasmania.

Having recently temporarily relocated to this island for university, I’ve noticed how its southern isolation results in bizarre and spectacular weather patterns. The locals say you get four seasons in one day, with chilly Antarctic winds howling from the south and warm high pressure systems creeping from the north. Because the island is so far south, making an effort to get out to the south-easternmost tip of the island, as I did on a short trek to Cape Raoul, is worthwhile alone (even if just for southerly bragging rights). Once there, however, a spectacular geological wonder awaits, and is a reward in and of itself! Giant sea columns riddle the shoreline, and although these columns may not be rare to Tasmanians, they are a wonder to be seen by any foreigner and worth a little explanation on how/why they formed and their geological ties to Tasmania.

Nature’s Hexagons

The coastline of southern Tasman Peninsula is composed by giant dolerite sea columns that protrude up to 300 metres from the sea. The distinct elongated shape are the property of the dolerite; a mafic (i.e. oceanic) rock that intruded beneath the surface of the earth as sill (parallel to the bed) or as dykes (across the bedding planes).  

The hexagonal, prismatic column shape is termed columnar jointing. Dolerite forms this way because the magma cooled from the outside toward the centre surrounded by consistent temperatures (more or less…). These joints radiate outward from the centre of each column, which themselves form perpendicular to the flow base of the source magma (or lava if the rock was extruded at the surface, such as basalt).

A column can be broken into three sections; a lower and upper colonnade that from via conductive cooling, and a middle entablature that formed via rapid convective cooling (Spry, 1962; McPhie et al., 1994). This characteristic shape and orientation of columnar jointing provides a useful indication of where any feeder pipe for the dolerite would have been located, which in the southern Tasman Peninsula appears to be due north of Cape Raoul (Leaman, 1999).

While dolerite sea columns do exist elsewhere in the world, Tasmania has the largest number of exposed columns. These intrusions were likely formed at depth (~ 2 km deep) from a massive volcanic event in the Jurassic (~185 million years ago) that covered up to a third of Tasmania, intruding into mainly Permian and Triassic sedimentary rocks. Since then, the Tasman Sea and Southern Ocean have formed, and in the last 10 million years (Tertiary) rocks were exhumed by uplift and erosions, brought to the surface to experience breaking and cracking due to the elastic stress relief (Leaman, 1999). Consistent abrasion from the sea has undercut and shaped these spectacular cliffs to what we see today. The cliffs would have been even more spectacular as well, if the Navy didn’t at one point use the Cape Raoul columns as target practice!

Final thoughts

To me, what’s most striking about these bastions of southern Tasman Peninsula is how they protrude from the sea in almost perfect geometric shapes. Usually rocks and sea landscapes have been rounded and eroded to beautiful but odd and mismatched shapes. Even with millions of years of harsh conditions the columnar joints of the Tasmanian dolerite remain prismatic hexagonal pillars. Maybe this is a bit of my love for math coming through, but something about symmetry within nature always impresses me. These dolerite sea columns provide evidence of a major geological event in Tasmania, and are a must-see feature of the Southern Tasmanian coast.

– Stephanie

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