Lecture 14 Glaciers

Goals

• Determine how glaciers are formed

• Characterize the different types of glaciers

• Quantify how glaciers move

• Identify the effects of glaciers on land

During the last several lectures we have investigated the flow of fresh waters in surface streams and underwater aquifers. The other large reservoir of fresh water are glaciers the topic of today's lecture. We will primarily investigate the effects of these frozen reservoirs of fresh water as they occur over land or as ice sheets over the oceans. As the recent popular movie, Titanic, illustrates when these ice sheets break up icebergs are formed that float into the oceans. There are four primary motivations for studying glaciers besides their widespread occurrence over the Arctic Ocean and Antarctica. We will discover that they are important to (1) landscape evolution; (2) surface water budget; (3) feedback to earth climate; and (4) as a record of past climatic changes. A new term that represents that portion of the earth's surface that is perennially frozen is the cryosphere.

  1. How to Make a Glacier
    2.

    In order to get a glacier going today, we need a continental surface at polar latitudes, cold temperatures year round and a ready supply of moisture in the form of snow fall. The snow accumulates over time with little or no melting or sublimation under the correct circumstances. The build-up of snow results in compaction and ultimately recrystallization into ice. We define the snowline as the elevation in the mountains above which one finds year round snow. It is above this line that the process of glacier formation occurs.

     

    If you take a cube of ice out of the refrigerator, it seems hard and in fact brittle for one can break the ice with a blow from a hammer. When a glacier forms it has these characteristics over short times but can exhibit quit a different characteristic to stresses (forces) applied over time periods of hours to months. Over these longer time periods we find that ice deforms plastically or flows and stretches when a force is applied. It is this property that will lead to the movement of glaciers un der the force of gravity and lead to erosion of the earth's surface.

  2. Types of Glaciers

    3. Valley or alpine glaciers

    Continental ice sheets

    Piedmont glaciers

    Ice sheets

     

  3. Movement of Glaciers
    4.

Ice is plastic and therefore it can flow. We have already introduced the concept of viscosity when we were talking about magmas. In that case we defined viscosity as the resistance to flow in the material. One can think of ice as a very viscous fluid and so over very large time scales it can flow slowly. The driving force for the ice is gravity and so the glacier will always flow downhill.

This flow of ice is analogous to stream flow we have recently studied during our field trip. As a result of our flow we will find that there are variable speeds of the ice flow just as in the river. Additionally the ice will erode the landscape as it flows over the land.

There are two primary ways in which glaciers move:

The velocity profile of the ice changes with position in the glacier reaching its largest values at the center of the ice and its smallest velocities along its edges as it encounters frictional resistance with the surrounding rocks and soils. Velocities vary from a few centimeters to a few meters per day.

We will find it important to classify all glaciers in terms of whether they are warm or cold as follows:

Warm - meltwater and ice, enhanced basal sliding, temperate glaciers

Cold - no melting, frozen to bed, reduced motion, polar glaciers

D. Components of a Glacier

    The mass budget of a glacier can be quantified by dividing the glacier into two regions known as the accumulation area and the ablation area. In the accumulation area mass is gained by the glacier due to the introduction of snow. Mass is lost by the glacier due to melting in the ablation region. The equilibrium line separates the two zones. The difference between mass added in the accumulation region and mass lost in the ablation area is equal to the total mass gained or lost by the glacier in a year.

E. Glacier Features

Valley glaciers

Erosional Landscape

Depositional Effects

Moraines