Academic Reading — Test 61

Reading passage

Glacial calving is the process by which blocks of ice break away from the edge of a glacier and fall into the sea or a lake. In the steep-sided fjords of Norway and the southern reaches of Patagonia, this process is among the most dramatic spectacles in the natural world. A calving event may release only a thin sliver of ice, or it may dislodge a mass larger than a building, sending a wave surging across the water. Although the sight is sudden, the conditions that make it possible accumulate slowly, over weeks and sometimes years, as a glacier creeps downhill towards the point where it can no longer support its own weight. The fundamental cause of calving lies in the balance of forces at the glacier's leading edge, known as the terminus. As ice flows forward, it eventually reaches water deep enough to lift the floating front, or thin enough that the unsupported overhang cracks under gravity. Two opposing influences govern the outcome. The first is the forward motion of the glacier, which continually pushes fresh ice towards the terminus. The second is the rate at which that ice is lost through melting and breaking. When the rate of loss exceeds the rate of supply, the terminus retreats inland; when supply dominates, it advances. Calving is therefore not merely an accident but a regular mechanism by which a glacier sheds the ice it can no longer carry. Water plays a decisive role in determining how and when calving occurs, and here the fjords of Norway and Patagonia offer instructive contrasts. A fjord is a long, narrow inlet carved by a glacier during a previous ice age and later flooded by the sea. Because fjords are typically very deep, the ice that reaches their heads often meets water that buoys up the glacier's snout, weakening its grip on the bed beneath. Warmer water is particularly effective at undermining the ice, because it melts the submerged face from below and carves out a notch. Once this notch grows large enough, the ice above it loses support and topples forward. In Patagonia, where some glaciers terminate in freshwater lakes rather than the sea, the same principle applies, although the lower density of fresh water alters the precise balance of buoyancy. Temperature is not the only factor; the internal structure of the ice matters greatly. Glacial ice is riddled with fractures called crevasses, which form as the ice bends and stretches over uneven ground. These weaknesses act as natural lines along which the ice will eventually split. Meltwater that trickles into a crevasse can deepen it dramatically, since water is denser than ice and exerts pressure that prises the crack further open. On a warm afternoon, the surface of a glacier may be laced with streams that vanish into such openings, quietly preparing the ground for the next collapse. For this reason, calving tends to intensify during the warmer months, when meltwater is abundant and the submerged face is bathed in comparatively mild water. The consequences of calving extend well beyond the immediate drama. Each detached block becomes an iceberg that drifts away and slowly melts, returning fresh water to the ocean and, over long periods, contributing to the rise of global sea levels. Scientists monitor calving glaciers closely because a sudden acceleration in the process can signal that a glacier has become unstable. When the terminus retreats into deeper water, the buoyant force on the ice increases, which can trigger still more calving in a self-reinforcing cycle that is difficult to halt. Some Patagonian glaciers that were stable for centuries have, in recent decades, begun to thin and retreat at a quickening pace, a pattern that researchers attribute partly to warming air and water. Studying these processes is far from straightforward. The terminus of a calving glacier is one of the most hazardous places a researcher can approach, since a collapse may occur without warning and the resulting wave can capsize a small boat. Modern investigators therefore rely heavily on remote methods, including time-lapse cameras positioned on the surrounding cliffs, satellite images that track the position of the terminus from week to week, and instruments that listen for the distinctive sounds that ice makes as it fractures. By combining these observations, scientists hope to predict when a major calving event is likely and to understand how the glaciers of Norway and Patagonia will respond to a changing climate. The answers carry weight far beyond the fjords themselves, for the ice stored in glaciers worldwide represents a vast reservoir whose fate will help shape the coastlines of the future.