Academic Reading — Test 8

Reading passage

Occupying much of the southern interior of Western Australia, the Yilgarn Craton is one of the largest and oldest stable blocks of continental crust on Earth. A craton is a region of the planet's outer shell that has resisted deformation for an immense span of time, having long since cooled and stiffened. The rocks of the Yilgarn formed mostly between about 3.0 and 2.6 billion years ago, during the Archaean eon, and they have remained comparatively undisturbed ever since. Because such ancient terrains are rare and have escaped the mountain-building and recycling that reshaped younger parts of the crust, the Yilgarn offers geologists an unusually clear window onto the early evolution of the continents. The craton is not a single uniform mass. It is conventionally divided into a number of distinct terranes, each with its own assemblage of rocks and its own geological history, which were welded together along major fault zones as the early crust assembled. Two broad rock types dominate. The first consists of vast expanses of granite and gneiss, pale crystalline rocks that make up the bulk of the craton's volume. The second comprises narrow, elongated belts of greenstone, a name applied to sequences of volcanic and sedimentary rocks that have been altered by heat and pressure, giving many of them a characteristic greenish tint. These greenstone belts, though far smaller in area than the surrounding granite, are of disproportionate economic importance, for it is within them that the great majority of the region's gold is found. The formation of the gold deposits is closely tied to the structural history of the greenstone belts. During the later stages of the Archaean, the rocks of the Yilgarn were subjected to intense compression, which fractured them and created networks of faults and shear zones. Hot fluids, rich in dissolved silica and metals, migrated upward through these fractures from deep in the crust. As the fluids cooled and chemically reacted with the surrounding rock, they deposited quartz together with small quantities of gold, filling the cracks to form what miners call reefs or lodes. A reef is essentially a sheet or vein of mineral-bearing quartz; where several reefs occur close together, they may constitute a substantial ore body. The gold itself is often invisible to the unaided eye, being finely distributed through the quartz or locked within sulphide minerals such as pyrite. The exploitation of these reefs transformed the colony of Western Australia in the final decade of the nineteenth century. Although small finds had been reported earlier, it was the discoveries at Coolgardie in 1892 and, more spectacularly, at Kalgoorlie in 1893 that triggered a rush of prospectors from across the world. The ground around Kalgoorlie proved extraordinarily rich, and one concentration of reefs there became known as the Golden Mile, a name that reflected the density of gold rather than any precise measurement of distance. Towns sprang up almost overnight in a harsh, arid landscape, and the chronic shortage of water was eventually relieved by an ambitious pipeline that carried fresh water hundreds of kilometres inland from the coast. Mining at Kalgoorlie has continued, with interruptions, ever since, and today a single vast open pit, worked in place of the original underground shafts, dominates the town. Not all of the Yilgarn's gold remained locked in solid rock. Over the long ages since the reefs first formed, the land surface above them was gradually worn down by weathering and erosion. Gold liberated from decomposing reefs was sometimes carried short distances by water and concentrated in gravels, forming what are termed alluvial or placer deposits. In other places the metal accumulated within the deeply weathered, rust-coloured soils that mantle much of the craton. Because gold is dense and chemically unreactive, it tends to survive these processes when lighter and more soluble materials are washed or dissolved away, so that surface deposits can be surprisingly concentrated. It was loose gold of this kind, easily gathered with simple tools, that the earliest prospectors sought before the deeper reef mining began. Interest in the Yilgarn has by no means faded. Modern exploration relies on sophisticated geophysical surveys, geochemical sampling and computer modelling to locate ore bodies that lie hidden beneath layers of barren cover and give no sign at the surface. The craton continues to yield not only gold but also nickel, iron ore and other valuable commodities, and it remains a principal focus of mineral research in Australia. For scientists, however, its value extends beyond economics, since the ancient rocks preserve a record of conditions on the early Earth that can be read almost nowhere else in such detail.