Assignments

1. Understanding Earth: Chapter 21 - Glaciers: The Work of Ice
2. Earth Revealed: movies. (http://www.learner.org/resources/series78.html)  Requires Windows media Player.  Sign in and view film 23.

Terms: glacier, continental ice sheet, ice shelf, alpine glacier, zone of accumulation, zone of ablation, sublimation, calving, abrasion, plucking, cirque, U-shaped valley, esker, striation, groove, roche mountonnee, drumlin, glaciofluvial, glaciolacustrine

Introduction

Living in New England one becomes accustom to a glaciated terrane. The outcrops are fresh with no covering of weathered rock, called saprolite. The soil is typically stony except in valleys where it may be sandy or clayey. The stones in the soil are commonly varied in composition and unlike the underlying bedrock. Throughout a drainage basin streams seem to disappear swamp and bogs and later reappear down valley. All of these subtle characteristics are the product of glaciation. Formation and flow of glaciersWhen the climate is such that more snow and ice accumulate than can be melted then glaciers is formed. Glaciers can range in size from huge continental ice sheets to small alpine cirques. One the accumulated ice reaches a thickness of 40 meters or greater the ice will begin to flow by slow plastic deformation, driven by gravity acting the glacier's mass. 

How Glaciers Move

Mechanisms:

1. Brittle Deformation (surface)
2. Plastic Deformation (internal deformation) rate is determined by ice thickness, temperature and slope
3. Basal Slip (warm-based glaciers)

What causes glaciers to advance and retreat? - Glacial Budget

1. Accumulation > Ablation : Glaciers advance
2. Accumulation = Ablation :  Equilibrium Conditions - glacier neither advances or retreats
3. Accumulation < Ablation : Glaciers retreat

Types of Glaciers

Continental glaciers

• ice sheets and ice caps
• Continental glaciers flow radially from the source area. The flow is governed by the slope of the ice surface which is largely independent of topography.
• Causes of continental glaciation

Alpine glaciers

• cirque and valley glaciers

Continental glaciers overwhelm the topography and tend to subdue the landscape through regional erosion. On the otherhand the localized erosion of alpine glaciers result in over steepening of the landscape and an increase in local relief.

Processes of erosion

Glaciers erode by mechanically plucking and abrading their bed.

• Abrasion: The grinding down of the underlying rock is accomplished by the rock fragments embedded in the sole of the glacier.
• Plucking: The removal of block of loosened material that becomes frozen to the bed.

Features of glacial erosion

• Continental: Through-going valley
• Alpine: cirque, U-shaped valley, arete, horn, etc.
• Small scale features common to both environments: striations, grooves, roche mountonees, etc.

Processes of deposition and their deposits

• Release of debris directly from the ice. (glacial till). Drumlins are eliptical streamlined hills molded beneath the glacier.
• Deposition of debris from glacial meltwater
  • glaciofluvial sand and gravel deposits such as outwash and eskers
  • Glaciolacustrine and glaciomarine deposits are laid down where meltwater enters a lake or marine environment

Internet Sites to Explore

• National Snow and Ice Data Center / Glaciers in the News
• United States Geological Survey / Glaciers in the News