G LACIERS M INI U NIT Types, Formation and Effects

INTRODUCTION: Glaciers are creeping ice masses. As they move, glaciers carve mountain valleys and cover continents, redistribute rock and sediment from high places to low ones and create landscapes that bulldozers can only dream about.

The key to building glaciers anywhere is to have more snowfall in winter than melts during summer. In mountainous areas, glaciers form in the higher, cooler elevations. In Greenland and Antarctica, the climate conspires to maintain cool average temperatures across the entire landmass, such that enormous ice sheets drape across the landscape.

Although it is difficult to imagine, just years ago, when mammoths (and humans) walked the earth, both northern Europe and northern North America were covered in ice as much as 2-3 km thick (just as Antarctica is covered today). North America’s Great Lakes were carved by glacial ice scouring into soft sedimentary rocks.

TYPES OF GLACIERS Glaciers come in many sizes and flavours. There are seven main types – the first five are confined glaciers – glaciers that are associated with mountains and restricted in their forms by the local topography. The last two (ice caps and ice sheets) are unconfined in form and typically larger, such that they cover much of an entire mountain range, or even a continent.

ALPINE GLACIERS: Glaciers that are associated with mountain ranges and that flow in confined areas, such as valleys, are called alpine glaciers, named for the Alps Mountains in Europe, where glaciers are quite common.

Alpine glaciers come in many styles; they form from an accumulating snowpack in mountain ranges and then flow downward through the mountain valleys. Cirque and valley glaciers are two common types of alpine glaciers.

CIRQUE GLACIERS: Small alpine glaciers that form near a mountaintop commonly carve out a bowl-shaped depression over long periods of time. These bowl-shaped erosional basins are called cirques, and a glacier that flows from one is called a cirque glacier.

Cirque glaciers are typically quite small (many are less than 1 km across). As they move down the mountain, however, they often join with other glaciers to form a more respectable valley glacier.

VALLEY GLACIERS: as it moved along. Commonly, rock debris piles up along the edge of the valley glacier, forming a lateral moraine Large alpine glaciers that flow down mountain valleys are called valley glaciers. Typically hundreds of meters thick, they carry large quantities of rock debris that have fallen on them or been plucked up by the ice

( moraine is a term describing an unsorted pile of rock fragments collected by a glacier). When two alpine glaciers flow into each other, a long, linear pile of debris forms where the two glaciers join. This medial moraine results when the two lateral moraines from the individual glaciers that merged are themselves joined together.

PIEDMONT GLACIERS: Where a large alpine glacier flows out of a valley and onto an unconfined plain at the foot of a mountain range, we term this ice mass a piedmont glacier. Piedmont glaciers are continuously fed by an alpine glacier, but they are no longer restricted on their flanks by the local topography. Thus, they can spread out radially from their valley source.

TIDEWATER GLACIERS: A tidewater glacier arises where an alpine glacier descends and terminates into an ocean body. Here it is common for icebergs to break off (calve) from the front of the glacier, leaving a steep cliff that may be hundreds of meters tall.

In Alaska, many tourist cruise ships venture into the waters near tidewater glaciers in hopes of watching an iceberg calve from the icy cliffs.

ICE CAP: An ice cap is a domed, unconfined glacial mass that is large enough to flow outward in all directions. In many cases, ice caps cover an entire mountain range or a small island. (When an ice cap exceeds 50, 000 sq km in area it is term an ice sheet). Numerous ice caps are found on islands in the arctic polar region.

ICE SHEET: An ice sheet is very large, unconfined ice mass that flows outward in all directions. Two large ice sheets today are the Greenland and Antarctic ice sheets. A large ice sheet covered about 50% of North America 18,000 years ago, extending from northern Canada to as far south as southern Ohio and Illinois. That ice mass has now completely melted away.