Academic Reading — Test 152

Reading passage

Coober Pedy, a small town in the arid interior of South Australia, is widely regarded as the opal capital of the world. The settlement sits above a landscape that was, many millions of years ago, the floor of a vast inland sea. When this sea eventually retreated, it left behind thick layers of sediment that hardened into sandstone and a softer, clay-rich rock. The opals for which the region is famous formed within these layers, and understanding how they came to exist requires looking back across an immense span of geological time. Opal itself is not a true mineral in the strict sense, because it lacks the orderly crystalline structure that defines minerals such as quartz. Instead, it is composed of tiny spheres of silica packed together, with water trapped between them. A typical specimen contains between three and ten per cent water by weight. When the silica spheres are roughly uniform in size and arranged in a regular pattern, they bend and scatter light into the flashes of colour that make precious opal so prized. Where the spheres are irregular, the result is common opal, known to miners as potch, which displays no such play of colour and has little commercial value. The raw material for opal is silica, which was abundant in the sandstone country around Coober Pedy. Over long periods, rainwater seeping down through the surface dissolved silica from the weathered rock as it passed. This silica-rich solution travelled downwards until it reached cavities, cracks and the spaces left by decayed plant and animal remains. There, conditions were stable enough for the silica to be deposited slowly, layer upon layer, eventually filling the void. The process was unhurried; geologists estimate that opal may form at a rate of only a single centimetre over roughly five million years, which helps explain why large stones are so rare. A crucial factor in this story is the climate. The opal-bearing rocks of the region were exposed to repeated cycles of wetting and drying. During wetter intervals, water carried dissolved silica deeper into the ground; during drier intervals, the water table fell and the silica was left behind to settle and consolidate. This alternation appears to have been essential, for it concentrated the silica and allowed it to accumulate gradually rather than being washed away. The weathering of the sandstone also released the iron and other elements that occasionally tint opal with background hues of grey, brown or even black, the last of which commands the highest prices. Sometimes the silica solution entered the shells, bones or wood of buried organisms and replaced the original material entirely, producing what is called an opalised fossil. These specimens preserve the outward form of ancient creatures while consisting wholly of opal, and they are scientifically valuable as well as beautiful. The remains of marine reptiles and shellfish have been recovered in this opalised state, offering evidence of the sea that once covered the area. Such fossils are among the most remarkable products of the long chemical journey that began with rainwater and dissolved rock. Mining at Coober Pedy began in 1915, after a teenager named Willie Hutchison reportedly found opal while his party was searching for gold. The discovery drew prospectors to a place with a punishing climate, where summer temperatures regularly exceed forty degrees. To escape the heat, many residents built their homes underground, carving dwellings, shops and even places of worship into the hillsides. These subterranean houses, known locally as dugouts, maintain a comfortable temperature throughout the year without the need for air conditioning. Today the town remains a working opal field, though the conditions that originally created its treasures ceased to operate long before any human arrived to dig for them.