Academic Reading — Test 13

Reading passage

In the arid interior of South Australia lies Coober Pedy, a small mining town that produces a substantial share of the world's precious opal. The gemstone for which the region is celebrated did not arrive from elsewhere; it formed slowly within the local rock over an immense span of time. Understanding how opal came to occupy the weathered sandstone beneath the town requires looking back many millions of years, to a period when the landscape was profoundly different from the dusty plains seen today. Roughly 120 million years ago, much of central Australia was covered by a shallow inland sea, sometimes called the Eromanga Sea. Layers of sand, mud and the skeletons of marine creatures settled on its floor and were gradually compacted into sedimentary rock. When the sea eventually retreated, these marine sediments were left exposed at or near the surface. Over the following ages, the climate shifted repeatedly, and long phases of intense weathering began to alter the upper layers of rock. It is within this weathered zone, rather than in fresh unaltered stone, that opal is almost always discovered. The chemistry of opal itself helps to explain why the gemstone is bound so tightly to these conditions. Opal is a form of hydrated silica, meaning that it consists of microscopic spheres of silicon dioxide together with a proportion of water trapped within its structure. Unlike quartz, opal is not a true crystal; its silica spheres are arranged in a more disordered manner. In precious opal, however, these spheres are remarkably uniform in size and are stacked in an orderly, repeating pattern. When light passes through this regular lattice, it is split into the spectrum, producing the shifting flashes of colour known as the play of colour. Common opal, which lacks this internal regularity, appears milky or dull and carries little value. The favoured explanation for how the silica gathered and set involves groundwater moving through the weathered rock. Rainwater, though scarce in the region, occasionally percolated downwards and dissolved silica from the surrounding sandstone and clay. As this silica-rich solution travelled through the rock, it eventually reached cavities, cracks and the spaces left by decayed fossils. There, under stable conditions and over very long periods, the dissolved silica was deposited and slowly hardened into solid opal. Because the process depended on existing voids, opal is frequently found filling fractures or replacing the shells and bones of ancient animals, occasionally yielding spectacular opalised fossils. A second, more recent hypothesis links the formation of opal to the gradual acidification of the landscape. According to this view, the breakdown of pyrite and other minerals in the drying sediments released acidic fluids. When these acidic waters were later neutralised by alkaline rock, the resulting chemical change is thought to have encouraged silica to settle out of solution and harden into opal. Whichever mechanism is correct, both rely on the same essential ingredients: a source of dissolved silica, suitable spaces in which it could accumulate, and a stable environment lasting long enough for the deposit to set. Mining opal at Coober Pedy remains a notoriously uncertain pursuit. The opal-bearing layer is generally shallow, lying within roughly thirty metres of the surface, which makes small-scale digging feasible. Yet the precious material occurs in unpredictable pockets, and a miner may shift a great deal of earth without finding anything of worth. The fierce summer heat has driven much of the human activity underground; many residents live in excavated dwellings called dugouts, which maintain a comfortable temperature throughout the year without artificial cooling. This blend of geology, climate and human ingenuity has given Coober Pedy a character found in few other places, and it continues to draw both miners and curious visitors to its harsh but rewarding terrain.