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4.2.1.a Glacial Periods and Glaciers
Paleoclimatology traces the many periods of climate changes in Earth’s history when the temperature rose or fell and large areas were covered with ice sheets or large continental glaciers.
Long cool periods that lasted from tens to hundreds of millions of years are called ‘ice ages’ . During this time span, glaciers advanced and retreated. Glaciations are shorter cool periods lasting thousands of years. when glaciers cover the greatest area. Over 20 glacial advances and retreats have occurred during the last 2 million years.
The term ice age also refers to the last major glaciation that occurred in North America and Eurasia. We are still in that ice age today, the interglacial part. Our modern climate represents very short, warm periods between glacial advances.The graph below illustrates the last 600 million years.
Glaciers are influenced by two major processes – Plate Tectonics and the Milankovitch Cycles, as well as numerous minor processes. Across Earth history, ice sheets have periodically covered large continental regions. Evidence for such glaciations is largely tillites (glacial deposits) like the Oak Ridges Moraine and striations (glacial erosion) as on our Canadian Shield.
Plate Tectonics theory suggests that these glaciations relate to the gathering of continental land masses in the polar regions and to changes in the chemistry of the atmosphere. The atmospheric changes occur when a reduced greenhouse effect is caused by an increase in the use of carbon dioxide in the weathering of rock.
GLACIAL FEATURES AND LANDSCAPE PROCESSES
Continental glaciers are great ice sheets that move over large land masses under the influence of gravity. They form by the recrystallization of snow. Ice is a like a mineral and glacial ice is like a rock.
The conversion of snow to ice involves several steps:
- Accumulation of snow: snowfields grow in areas above the snow line where more snow accumulates in winter than melts in summer. Freshly fallen snow has about 80% empty space.
- Formation of ice granules: as snow accumulates and gets thicker, sublimation transforms the snow to water vapor. The pressure of its weight changes the vapor to ice granules called firn.
- Formation of glacial ice: more accumulation allows for compaction. The increased pressure of the ice granules melts and releases water that refreezes and cements ice granules together into glacial ice. Glacial ice is a mass of interlocking crystals with only about 10% empty space.
Glaciers are classified as:
- Temperate or wet-based: pressure-melting temperatures except near upper surfaces in winter; not frozen at base; water throughout its thickness and snow quickly changes to firn.
- Polar or cold-base : below pressure-melting temperatures; no meltwater; no calving, sublimation, or wind erosion.
- Subpolar : frozen at the base with the surface melting in summer.
Glaciers have different zones of activity:
- Accumulation: snow accumulates faster than it is removed
- Ablation: the loss of ice exceeds accumulation by frictional melting at sides and bottom, and warming during the summer
- Sublimation: ice converts directly to vapor and/or calving when the ends of glaciers break off into the ocean and become icebergs.
When ice reaches a thickness of about 40 meters, it begins to flow or advance through a combination of plastic flow and basal slip. The firn limit moves down the glacier and the ice mass increases.
- Plastic flow occurs when the glacier’s zone of flow, inside the glacier, under pressure begins to move and the zone of fracture on top of the glacier, doesn’t. This produces deep crevasses in the glacier.
- Basal slip results when friction melts the base of the glacier, lubricating it and allowing it to slip.
The average rate of movement of continental glaciers is a few centimeters to a few meters per day. Flow rates are fastest in the zone of accumulation and slowest below the firn line toward the margins. Thicker ice sheets have higher flow rates than thinner ones. These glaciers show little basal slip and may be frozen to the underlying surface.
When ablation is greater, the glacial front retreats. Eventually it melts or vaporizes and returns to the hydrological cycle.
EROSION
Glaciers erode by one or more methods as they move.
- Abrasion: a moraine in a glacier rubs and scratches like sandpaper to wear away valley sides and floor, producing very fine particles of pulverized rock or rock flour; long grooves and scratches cut into bedrock; or striations,and glacial polish, a very smooth surface produced by fine abrasion of bedrock by rock flour.
- Plucking: glacial ice meltwater refreezes onto and into solid rock; pries blocks of rock loose which become incorporated into the ice; and creates boulders or glacial erratics and moraines.
- Pressure Releasing: glacial ice melts during mini interglacials, easing its pressure. Layers of rock peel off.
- Bulldozing: water enters cracks during the day, and freezes into ice at night. It expands and jagged angular pieces of rock break off and fall into the ice as moraine.
These erosional processes produce many distinct landforms. Basic glacier terms include:
- glacial trough: formed when a glacier widens and deepens a valley from a V-shape into a U-shape. The upper slopes of glacial troughs tend to be very steep, while lower down there is a gentler descent. As the glacier gets smaller the trough will become shallower. Glacial erosion features associated with glacial troughs are truncated spurs which are a result of straightening the valley.
- truncated spurs: triangular cliffs that are formed by glacial erosion of ridges that once extended into the valley at stream meanders.
- hanging valleys: tributary glacier valleys, where the main glacier cuts its valley deeper than the tributary glaciers. After the ice melts, smaller valleys are left hanging above the main glacier valley. Streams in hanging valleys form waterfalls.
- ice-scoured plains: erosion strips soil and sediment away to expose bedrock.
- stream drainage patterns: braided drainage patterns with numerous lakes and swamps.
TRANSPORTATION AND DEPOSITION
Glaciers transported and deposited huge quantities of sediment and helped form the topography of the northern areas. All sediments of glacial erosion are known as drift and are identified as either ’till’ and ‘stratified’.
- Till is unsorted, unlayered material deposited directly by a glacier. During ablation, the ice can no longer transport its load because of it loss of energy. Ice, unlike rivers, does not sort its load into different sizes. The majority of moraine is deposited en masse as a mixture of angular unsorted debris of all sizes. These are termed glacial deposits. See the Oak Ridges Moraine north of Toronto, Interlobate Moraine around Bolton, Ontario.
- Stratified glacio-flvuial deposits are laid on top of a glacial outwash plain by networks of braided streams carrying the deposits to the edge of the glacier .
- Glacio-lacustrine drift is carried by the runoff from the end of a glacier and emptied into a lake. The heavier loads become deltas at the lake’s margin. The fine load is deposited across the lake bottom.
PRE-PLEISTOCENE GLACIATION
There have been were four major glaciation periods called Ice Ages. They were major causes of extinction of other organisms.
1st: late Proterozoic (800 and 600 million)2nd: Pennsylvanian/Permian (between about 350 and 250 million years ago)3rd: late Neocene to Quaternary (the last 4 million years).4th: parts of the Ordovician/Silurian (between about 460 and 430 million years ago) but somewhat less extensiveOver 20 glacial advances and retreats, minor glaciations, have occurred during the last 2 million years. The last glaciation of the North America had its maximum extent approximately 20,000 years ago but we are still in the ice age today – the interglacial part. Our modern climate represents a very short, warm period between glacial advances.
Activity
What does the y axis measure?
How many cool periods or ice ages are recorded here?
Locate the events on the graph and describe the climate characteristics.
What is the range between the average global temperature extremes of warm and cool ice ages?
Use the time scale to:label the various eras of the graph.
locate these glaciations on the time scale
identify other significant events (land, oceans, vegetation, animals) occuring at the same time.
Compare the length of warm spells to cool spells shown on this graph.
Research
What is our average global temperature today?
What is the mean of the average global temperatures shown here? why is it so warm?
ACTIVITY: RANDOM WALK GAME
The random walk game proposed by Leopold and Langbein gives an interesting insight into downward eroding stream drainage patterns.
Set Up:
- 4 players A, B, C, D each has a different color of marker.
- divide a piece of graph paper into 4 columns 5 spaces wide and draw a line across the top for the starting line.
- label columns A, B, C, D in their color.
Rules of the game:
In turn, throw one die and draw a line in the correct column on the graph paper to represent the toss. The lines form a path representing various rivers.
1 – advance one space straight ahead2 – advance one space forward left3 – advance one space forward right all other throws – no advance in any direction.If a player’s path intersects another’s, they must play as a team with one path from that point.
Examples of Pathways:
The forward movement represents gravity and the left or right, the random choice of a path a river can take through various kinds of rock and terrain. There are two possible outcomes to this game. Either the streams diverge indefinitely, or they converge or merge until a master stream predominates. The key word here is merge. The drainage patterns are for downward eroding streams: the paths diverge or merge, but they cannot cross.