Glaciers Chapter 15.1

Glaciers Glaciers A huge mass of ice slowly flowing over a land mass, formed from compacted snow in an area where snow accumulation exceeds melting and sublimation. Sublimation is the process in which a solid changes directly into a gas, or a gas changes directly into a solid.

Sublimation

Formation Formation of Glaciers Generally, rain water quickly runs of and flows to rivers and oceans or seeps into the earth as ground water. However, at higher elevations, and in polar regions, water reaches the earth as snow and will remain as snow all year long.

Snow Line These elevations are above the “snow line”. The snow line is defined as the elevation above which ice and snow remain throughout the year. The snow line varies around the earth. At the equator the snow line is about 5,500ft above sea level and at the poles the snow line is at sea level.

Snow Line

Snowfield A snowfield is an almost a motionless mass of permanent snow and ice. Snowfields are formed by an accumulation of ice and snow above the snowline. They cover most of the land near the poles and the tops of some mountains at lower latitudes.

Snowfield

Firn Due to slight fluctuations in temperature, ice partially melts and re-freezes which changes the snow crystals into small grains of ice. This grainy ice is called snow firn.

Firn

Glacial Formation Due to layers of accumulated snow, the pressure on the lower layers of the firn is great enough to flatten and squeeze out the air between the molecules. This forms glacial ice, which is a bright blue color that is a common characteristic of glaciers. When the snow and ice accumulate to a great enough thickness, that the ice starts to move downslope due to gravity, a glacier is formed.

Glacier

Types of Glaciers Types of Glaciers There are two main types of glaciers: Valley Glaciers and Continental Ice Sheets.

Valley Glaciers A valley glacier is formed in mountainous areas, but is named after the ice has moved down into the valley. Valley glaciers are defined as long, narrow, wedge-shaped masses of ice, and are common in high mountain regions such as Alaska, the Himalayas, the Andes, the Alps, and New Zealand.

Continental Ice Sheets Continental ice sheets cover millions of square kilometers and are found only in Greenland and Antarctica. The Antarctica continental ice sheet is one and a half times as large as the mainland US, and is as thick as 4,000m.

Antarctica

Continental Ice Sheets The Greenland continental ice sheet covers 90% of Greenland and is over 3,000 m at its maximum thickness. If these two ice sheets were to melt, the water would raise worldwide sea levels by more than 60 meters.

Greenland Ice Sheet

Glacial Movement Movement of Glaciers On average, a glacier moves about 100m per year. However, some only travel a few centimeter and some a kilometer or more. Glacier movement is categorized into two different processes: Basal Slip and Internal Plastic Flow.

Basal Slip Basal Slip occurs when the pressure of the ice is great enough to melt the ice at the bottom of the glacier that is in contact with the land. This, now water, acts as a lubricant and allows the glacier to “slip” forward. Basal slipping also allows water to melt, move around a barrier, and refreeze to move the glacier forward.

Internal Plastic Flow Internal plastic flow is the process in which solid ice crystals slip over each other, causing a slow forward motion. The speed of internal plastic flow varies throughout the layers of the glacier. Areas of less friction, such as top and center, flow quicker than areas that are in contact with the earth.

Glacial Features Glacial Features Crevasses are large cracks formed in the brittle surface of the glacier by buckling of the ice from tension and compression.

Crevasses

Ice Shelves Ice shelves are areas of continental ice sheets that have moved out into the ocean. The largest ice shelve is the Ross Ice Shelf which covers an area the size of Texas.

Ice Shelf

Icebergs Icebergs are large chunks of ice that break off at the end of an ice shelf or glacier and are carried out to sea. Approximately 90% of the iceberg is below water.

Iceberg