Wednesday, 17 July 2013

Tip of the Inselberg


It appears as a striking anomaly in a landscape of low dunes and flat horizons. The dome of Uluru looks unreal as it seems to be sitting as though it had been dropped from somewhere else onto the vast and flat desert of Australia’s Red Centre. Uluru is, however, not alone. A fraternal twin lies some 40 kilometres away – Kata Tjuta – quite different in appearance but born of the same mother, the Peterman Ranges.

As the geologists' story goes, the Peterman Ranges were much higher 550 million years ago, and erosion of these grand mountains produced two great alluvial fans, one of them consisting largely of river-rounded stones and the other of mostly sand. Over the next 50 million years, these fans built up layer upon layer, becoming kilometres thick. Then the whole area became covered by a sea and new layers of sediments fell on top of the fans. The weight of the new seabed turned the fans into rock – conglomerate for the rocky fan and sandstone for the other. Then some 400 million years ago, the Australian continent underwent considerable tectonic movement. The sea drained away and the layers of sediments were folded and tilted. The sandstone fan was tilted so that one end bent up at nearly 90 degrees. From 300 million years ago and on, the covering of sand and soil has slowly been blown away, exposing the harder rock of the ancient alluvial fans to the air. But what you see “sitting” on the landscape is just the tip of the iceberg, so to speak, or more accurately, just the tip of the inselberg.

Inselbergs are, according to the Encyclopaedia Britannica, an “isolated hill that stands above well-developed plains and appears not unlike an island rising from the sea.” Indeed, the name comes from German for “island” and “mountain” and was coined by early German explorers of southern Africa who were impressed by such landforms they encountered there. Inselbergs are often granite and other rocks of igneous origin, or other rocks that are more resistant and whose base lies well below the surface. While the surrounding soils and sands are gradually removed by the powers of erosion, the harder rock remains, slowly “rising” above the plains. As in the case of Uluru and its sibling Kata Tjuta, most of the petrified sediments still lie below the ground, possibly extending for several kilometres. Only a small portion of the total mass is exposed, making both landforms the lithological equivalent to an iceberg.


Incidentally, the sediments are not red in their unexposed state but rather light grey as the main components are feldspar and quartz, a type of sedimentary rock known as arkose. The exposed rocks turn reddish as their iron components oxidize with the air. This brings to mind the scene in the movie “The Hitchhiker’s Guide to the Galaxy” when for but a couple of seconds we see workers busy painting the grey rocks of Uluru red in preparation for the completion of the new Earth. Someone’s shot at a gag was not so far off from reality.

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