Electrical Aptitude Practice Test 3

Rivers of Ice: How Glaciers Move

¹A glacier begins as snow. In places where more snow falls in winter than melts in summer, the snow piles up year after year. The weight of each new layer presses on the layers beneath it. Snow that survives a full year becomes a dense, grainy material called firn, and as the pressure continues, the grains squeeze together until almost no air is left. Turning firn into solid glacier ice can take more than a century.

²Ice looks hard and brittle, yet a glacier flows. Once ice is thicker than about 30 meters, the weight of the ice above causes the deeper ice to deform slowly, much like very stiff putty. The crystals inside the ice shift and change shape, and the whole mass creeps downhill. Scientists call this process internal deformation.

³The ice near the surface behaves differently. The top layer, up to roughly 50 meters thick, is under too little pressure to flow. It stays rigid, and the ice below this brittle layer flows and carries it along. When a glacier passes over a bump in its bed or turns a bend, or when one part moves faster than another, this brittle surface cracks open, forming deep cracks called crevasses. Most crevasses are less than about 46 meters deep, because below that depth the ice bends instead of breaking.

⁴A glacier can also move by sliding. Water collects at the base of many glaciers, melted by pressure, by the heat of friction, or by heat from the earth. This water acts like a lubricant, letting the ice slide over the rock beneath it. Friction still slows the ice wherever it touches solid ground. The bottom of a glacier moves more slowly than its surface, and in a mountain valley the ice along the valley walls moves more slowly than the ice in the middle.

⁵Most glaciers move at an average of about one meter per day, too slowly to see. Some fast glaciers in Greenland travel 20 to 30 meters per day. A few glaciers also surge, speeding up for a short time to as much as about 90 meters per day.