Discover the awesome Earth-shaping power of gigantic rivers of ice!
Glaciers are huge rivers or sheets of ice, which have sculpted mountain ranges and carved iconic peaks like the pyramid-shaped Matterhorn in the Swiss Alps. The secret of this awesome landscape-shaping power is erosion, the process of wearing away and transporting solid rock. Glacial erosion involves two main mechanisms: abrasion and plucking. As glaciers flow downhill, they use debris that’s frozen into the ice to sandpaper exposed rock, leaving grooves called striations. This is the process of abrasion. Plucking, however, is where glaciers freeze onto rock and tear away loose fragments as they pull away.
Today glaciers are confined to high altitudes and latitudes, where the climate is cold enough for ice to persist all year round. During the ice ages, however, glaciers advanced into valleys that are now free of ice. Britain, for example, was covered by ice as far south as the Bristol Channel.
You can spot landforms created by ancient ice. Cirques are armchair-shaped hollows on mountainsides, which often contain lakes called tarns. They’re also the birthplaces of ancient glaciers. During cold periods, ice accumulated in shady rock hollows, deepening them to form cirques. When two cirques formed back-to-back, they left a knife-edge ridge called an arete. Pyramidal peaks were created when three or more cirques formed. Eventually the cirque glacier spilled from the hollow and flowed downhill as a valley glacier. This glacier eroded the valley into a U-shape, with steep cliffs called truncated spurs. When the glacier melted, tributary valleys were left hanging high above the main valley floor.
Hard rock outcrops in the valley were smoothed into mounds orientated in the direction of ice movement. Rock drumlins are shaped like whalebacks, adopting a smooth, convex shape. Roche moutonnee have a smooth upstream side, and a jagged downstream side formed by plucking. Where valley rocks varied in strength, the ice cut hollows into the softer rock, which filled with glacial lakes known as paternoster lakes.
How does glacier move?
Glaciers can only move, erode and transport debris if they have a wet bottom. Polar glaciers are frozen to the bedrock all year round and typically move around 1.5 metres (5 feet) per year, as ice crystals slide under gravity. In temperate climes like the European Alps, however, glaciers can slide downhill at 10 -100 metres (30-330 feet) per year, due to the fact that meltwater forming under the glacier during mild summers acts as a lubricant.
If meltwater accumulates under a glacier, the ice can race forwards at up to 300 metres (990 feet) per day. During the fastest recorded surge, the Kutiah Glacier in Pakistan sped more than 12 kilometres (7.5 miles) in three months.
Spotter’s guide to lowland glaciers
When you stand at the bottom, or snout of a valley glacier, you can see landforms made of debris dumped by the ice. The debris was eroded further up the valley and transported downhill, as if on a conveyor belt. Meltwater rushing under the glacier sculpts the debris heaps.
The snout is the place in the valley where the glacier melts completely. This changes over time. If the glacier shrinks, it leaves a debris trail behind. Should it grow again, it collects and bulldozes this debris. To understand why the snout moves up and downhill, you need to see glaciers as systems controlled by temperature and snowfall. On cold mountain peaks, snow accumulates faster than the glacier melts. As ice flows into warmer lowlands, melting begins to exceed accumulation. The snout advances or retreats depending on whether inputs of snow exceed ice loss from the system by melting.
Most beautiful glacier – Landscape Arch, USA
This delicate natural arch -Earth’s third largest – is only 2m (6.5ft) thick at its narrowest, but spans a whopping 90m (295ft).
Most lively glacier – Transgondwanan Super-mountains, Gondwanaland
Nutrients eroded from a giant mountain range 600 million years ago may have helped Earth’s first complex life to develop.