Glaciers and Climate Change

Lecture notes Glaciers & Climate Change Glaciers are also part of the hydrologic cycle and the rock cycle as glaciers act to erode and transport sediments. We don’t usually see too many glaciers in the United States today, outside of the high mountainous areas, although continental glacial ice sheets used to cover a large portion of the northern part of the United States. The last glacial ice age ended approx. 10,000 years ago – not that long ago. We are currently in an interglacial period, and are more focused on global warming, both natural and man‐made. In this section we will cover the basics of continental and alpine glaciations and climate change. The global ice caps we see today are actually the extent of the present day continental ice sheets. We know that ice forms in our freezer at home, but how does it form into glaciers. After snow falls and is incorporated into a glacier it recrystallizes to dense grains called “firn”. When ice thickness is over 50 meters – interlocking crystals form glacial ice. The only way ice gets out of our freezer is by removing the cubes to cool our drinks. How does glacial ice move? Movement of Glacial Ice • • • Rates of glacial movement Rates of up to several meters per day Some glaciers exhibit extremely rapid movements called surges. Budget of a glacier ‐ Balance, or lack of balance, between accumulation and loss is referred to as the glacial budget. • • • • The zone of accumulation is the area where a glacier forms. Elevation of the snowline varies greatly. The zone of wastage is the area where there is a net loss of ice due to melting and calving —the breaking off of large pieces of ice (icebergs where the glacier has reached the sea) If accumulation exceeds loss (called ablation), the glacial front advances. If ablation increases and/or accumulation decreases, the ice front will retreat. Important note: A retreating glacier continues to move downhill Movement within a glacier is referred to as flow. There are two basic types Plastic flow ‐ This occurs within the ice, under pressure, as ice behaves as a plastic material. Basal slip ‐ The entire ice mass slipping along the ground. Most glaciers are thought to move this way by this process. Zone of fracture ‐ This zone occurs in the uppermost 50 meters. Tension causes crevasses to form in brittle ice. Landforms Created by Glacial Erosion – Alpine Glaciers. These are some of the main Alpine features. Learn these features Aretes (long sharp ridges between valleys) Horns (pointed peaks where arêtes meet), Cirques (bowl shaped depressions near at the high point of the u‐shaped trough) Tarns (lakes in cirques), Hanging Valleys (at bottom end of glacial valley where it meets another main glacial trough) U‐Shaped Troughs (U shaped instead of V shaped as in river systems), etc. – Glaciers are capable of great erosion and sediment transport. Glaciers erode the land primarily in two ways: 1. Plucking — lifting of rocks (e.g. glacial erratics) 2. Abrasion • • Rocks within the ice acting like sandpaper to smooth and polish the surface below Glacial abrasion produces: Rock flours (pulverized rock) Glacial striations (grooves in the bedrock) Glacial Deposits. Glacial drift refers to all sediments of glacial origin. Types of glacial drift • • Till —material that is deposited directly by the ice Stratified drift — sediments laid down by glacial meltwater Glacial Till Is Typically Unstratified and Unsorted Landforms made of till Moraines ‐ Layers or ridges of till Moraines produced by alpine glaciers • • • Lateral moraine Medial moraine Other types of moraines End moraine—terminal or recessional Continental Glaciers ‐ Motion of Ice in Continental Glacier ‐ from higher to lower thickness. Continental Glacial Deposition Features – Learn these features Landforms made of stratified drift Ice‐contact deposits ‐ Deposited by meltwater flowing over, within, and at the base of motionless ice Features include • • • Kames Kame terraces Eskers Outwash plains (with ice sheets) and valley trains (when in a valley) • • • Broad ramp‐like surface composed of stratified drift deposited by meltwater leaving a glacier Located adjacent to the downstream edge of most end moraines Often pockmarked with depressions called kettles Landforms made of till Drumlins • • • Smooth, elongated, parallel hills The steep sides face the direction from which the ice advanced. Occur in clusters called drumlin fields. • Formation not fully understood. Glaciers of the Past Indirect effects of Ice Age glaciers • • • • • • Forces migration of animals and plants Changes in stream courses Rebounding upward of the crust in former centers of ice accumulation Worldwide change in sea level Climatic changes Pluvial lakes –Glaciers trigger more precipitationt/wetter climates in dry areas away from glaciers. Example: Pluvial Lakes of the Western U.S. Climate Change Why study global climate change? We hear about it on the news, but what is it all about, and why should we care? These are some of the past and possible future effects of glaciations and global warming. • • • Global Sea Level Drops‐ during ice ages, sea level drops allowing advance of glaciers from Greenland across shelf; Bering land bridge. Global Sea Level Rises‐ between ice ages, sea level rises Flooding of coastal cities and agriculture belts. The amount of land that would be covered by a rise in sea level depends on the steepness of the shoreline. Broad low lying coastal areas would experience the most flooding. Global Weather Patterns‐ affects crops, water supplies, etc. Predicted Sea Level Change for the next century high vs. low prediction (top) & contribution vs. source (bottom) Some possible causes of glaciation • • • Plate tectonics ‐ Continents were arranged differently in the past. Changes in oceanic circulation Variations in Earth’s orbit The Milankovitch hypothesis • • • • Shape (eccentricity) of Earth’s orbit varies. Angle of Earth’s axis (obliquity) changes. Earth’s axis wobbles (precession). Changes in climate over the past several hundred thousand years are closely associated with variations in the geometry of Earth’s orbit. • Other factors are probably also involved. More Causes of Climate Change • Global Warming: CO2/O2 ratio and the “greenhouse effect” ‐‐ Increase of CO2 traps solar radiation. Possible Causes ‐ Examples • • • Rain forest changes CO2 ratio. Air pollution increases CO2 concentration. Global Cooling: Nuclear Winter/ Volcanically induced “mini‐ ice ages” ‐‐ Atmospheric dust blocks sunlight Possible Causes ‐ Examples • • Mt. Tambora eruption. Demise of the dinosaurs at K/T boundary