March 26, 1999

By Jed Schneider and Kathryn Clapp

Return to Table of Contents

Glacial Systems

Mass
Mass is the accumulated crystalline water that makes up a glacier. Common sources of mass are: Direct snowfall Avalanches Wind blown snow Over time glacial ice is formed through melt-freeze and compaction. This process can take anywhere from one year to hundreds of years depending the climate of the area. Steps to form Glacial ice: Snow recrystallized to granules = Firn Névé = 50% the density of liquid water Glacial Ice = 90% the density of liquid water Example: At Bridger Bowl ski resort in Montana, it takes one year to form firn while in Antarctica it ten years or more - more than a century to produce ice! Properties of Glacial Ice: Glacial Ice can deform like silly putty or cold taffy. When about 10 meters of glacial ice has accumulated, the ice begins to flow down hill in alpine regions or out to the sides.	The formation of glacial ice from from new snow.

*Mass Budget*
Mass Budget = Accumulation - Ablation The mass balance can be measured over 1 year or over long spans of time. One year's worth of data for the accumulation and ablation on a glacier is called a "budget year". The term "mass balance" is used for the total accumulation and ablation recorded over a period of years - the longer the better! If ablation exceeds accumulation for a number of years the glacier is said to have a negative mass balance, and it will thin and/or retreat. Most glaciers will have a high-elevation region where the mass balance is positive and a low-elevation region where it is negative. Accumulation Zone - area where snow can accumulate and is not melted in the summer Firn Line - seasonal boundary between old ice (below) and recrystallized new snow (above) Ablation Zone - area where snow is melted during the summer. The long-term average position of the highest (late summer) firn line is termed the Equilibrium Line Altitude (ELA): The long term balance point along a glacier. The ELA is where the amount of accumulated snow and ablated water are equal. The ELA is climate and location dependent. The ELA can be a significant indicator of glacier health. Warming will raise the elevation of the ELA. Because there is a greater area of the glacier below the ELA, a mountain glacier will retreat uphill. This is why many mountain valleys are not presently occupied by glaciers, but have been in past ice ages. - "Highest firn line" describes the zero balance point in one year's worth of accumulation and ablation. It is usually marked by a distinct separation of old ice (dark) and recrystallized snow (see right). It is different from the ELA because it only describes the ablation and accumulation for one year. The ELA describes the balance point of the glacier over several years.	Accumulation and ablation zones on the Vatnajokull Ice Cap, Iceland. *Source*

A model depicting the location of the annual firn line with regards to the zones of accumulation and ablation. The dots indicate the direction of ice flow due to the displacement of mass over time(after Sharp, 1960).

**Lab Exercise**
Click for a GLACIAL BALANCE EXERCISE (Locke,1999)

*Energy*:
The basic types of Energy in Glacial systems: Short Wave radiation from the Sun Long Wave radiation from clouds and valley walls Heat conducted from the atmosphere Geothermal heat, conducted from the Earth's interior Latent heat from the transition from vapor to water, water ice, and the reverse The Sun (through short wave, reradiated long wave, and conducted ("sensible") heat is the main source of energy in glacial systems. It is the primary energy source for summer ablation. How much radiant energy is absorbed by a glacier surface depends on its reflectivity, or albedo. Light surfaces have very high albedo. Think about why we wear white of light colored clothing in the summer instead of dark colored clothing. Snow's high albedo helps keep temperatures low by reflecting solar radiation away. Fresh snow in winter, higher albedo --> less melt-off. Lower albedo in summer --> more ablation because of rock debris, dust, and ice at the surface.
(after Paterson, 1994)

**Longwave and Shortwave Radiation**
The two types of radiation, short and long wave radiation, have a large influence on glaciers. Short-wave Radiation- Direct sunlight Long-wave Radiation- Absorbed and reradiated energy Dark rocks and debris can absorb more thermal energy (Think about how hot a dark car gets on a sunny day) then re-emit that energy as long wave radiation. This causes more melting even after sunset. Clouds and pollution help the keep reflected energy from escaping the atmosphere and causes regional heating of areas.

Flow:
Glaciers flow as a result of the gravity deforming the ice and pulling it downhill. The deformation makes glacial flow resemble rivers of Ice. Glaciers have laminar flow properties. Glaciers do not have turbulent flow within them like a stream. (See Glacial Flow Animations) Ice can "bend" around corners and obstacles but it is still brittle which causes crevasses to form as it moves. The brittle nature of the ice also causes calving at floating termini and ice falls at steep drops on land. Flow rate is intimately related to the mass balance of a glacier. High amounts of accumulated mass increases flow rates. Likewise, low mass accumulations slow the rate of flow. Rates of flow can vary from 10-200 m/yr. Flow makes the glacial mass budget possible by transferring mass from the accumulation area to the ablation area If glaciers did not flow , then global water would accumulate in the polar and alpine regions of the world. All the liquid water on Earth would freeze and life (as we know it) could not survive. For example, consider Mars, where sub freezing temperatures have caused sublimation of water and locked the remaining available water in the North polar ice cap. (MARS PATHFINDER 1998 )	Internal flow of a glacier showing differential flow rates as a function of stress above the bed (after Sharp, 1960).
A specific type of flow: Regelation
The specific mechanics of flow will be covered in depth in the later section on flow. Here it is important to understand that the glacier can flow not only by deformation if ice, but also by sliding at the bed surface. One process that can facilitate such a process over an irregular bed surface is regelation. Regelation is the refreezing of melted water at the base of the glacier (Drewry, 1986). As a glacier flows over an irregular surface, pressure builds on the up-glacier side of the obstacle. IF the glacier base is at the pressure melting point, melting, thus free water, will will occur. The presence of water helps the glacier to slide on the bed. On the down-glacier side of the obstacle, there is an area of relatively low pressure where the water can refreeze. The refrozen lice is known as a regelation layer. Refreezing in bedrock fractures helps to mechanically weather the down-ice area of bedrock and also freezes loose debris into the glacier; creating material for glacial erosion and depositional processes (Drewry, 1986).	A depiction of regelation at the bed surface of a glacier. High pressure on the up-glacier side of an obstacle exceeds the pressure melting point. The liquid water migrates around or through the obstacle, then refreezes in the low pressure area on the down-glacier side of the obstacle.

back to Introduction

Up to Systems2

Down to Table of Contents

On to Morphology