Glaciers - Amazing Ice Glaciers and Ice Ages Prepared by...

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Unformatted text preview: Amazing Ice: Glaciers and Ice Ages Prepared by: Ronald Parker, Senior Geologist Parker, Fronterra Geosciences Houston, Oklahoma City, Denver, Anchorage, Dallas, Midland, Aberdeen, Vienna, Buenos Aires, Neuquén Outline Theory of glaciations Formation of glaciers Movement of glacial ice Glacial erosion, transport and deposition Consequences of glaciations (glacial rebound, sea-level changes, pluvial lakes) Pleistocene Ice Ages The causes of Ice Ages The Theory of Glaciation European farmers broke plows on large rocks. Buried in fine-grained soils, often of enormous size. Unlike local bedrock, they were from hundreds of kilometers away. These rocks became known as erratics. The origin of erratics became a scientific mystery. The Theory of Glaciation Louis Agassiz, a Swiss geologist, observed glaciers. He saw glaciers as agents of landscape change. They carried sand, mud, and huge boulders long distances. They dropped these materials, unsorted, upon melting. He realized glaciers could explain erratic boulders. The Theory of Glaciation Agassiz proposed that an ice age had frozen Europe. Ice sheets covered land. Ice carried and dropped… Erratic boulders, and… Fine-grained unsorted soil. When 1st proposed, Agassiz’s idea was criticized. By the 1850s, many geologists agreed that he was right. Agassiz saw evidence for a North American ice age. Ice Ages Glaciers presently cover ~10% of Earth. During ice ages, coverage expands to ~30%. The most recent ice age ended ~ 11 ka. Covered New York, Montreal, London, Paris. Ice sheets were hundreds to thousands of meters thick. Ice: The Water Mineral Ice is solid water (H2O). Forms when water cools below the freezing point. Natural ice is a mineral; it grows in hexagonal forms. Formation of Glacial Ice Snow is transformed into ice. Delicate flakes accumulate. Snow is buried by later falls. Compression expels air. Burial pressure causes melting and recrystallization. Snow turns into granular firn. Over time, firn melds into interlocking crystals of ice. Formation of Glacial Ice Glacial ice may form… Quickly (tens of years) Slowly (thousands of years). Forming a Glacier Three conditions are necessary to form a glacier: Cold local climate (polar latitudes or high elevation). Snow must be abundant; more snow must fall than melts. Snow must not be removed by avalanches or wind. Forming a Glacier Glacier-sustaining elevation is controlled by latitude. In polar regions, glaciers form at sea level. In equatorial regions, glaciers form above 5 km elevation. This elevation is marked by the “snow line.” Glaciers Thick masses of recrystallized ice… Last all year long. Flow via gravity. Two categories of glaciers: mountain and continental. Mountain Glaciers Flow from high to low elevation in mountain settings. Include a variety of types. Cirque glaciers fill mountain-top bowls. Valley glaciers flow like rivers down valleys. Mountain ice caps cover peaks and ridges. Piedmont glaciers spread out at the end of a valley. Continental Glaciers Vast ice sheets covering large land areas. Ice flows outward from thickest part of sheet. Two major ice sheets remain on Earth: Greenland. Antarctica. Movement of Glacial Ice How do glaciers move? Wet-bottom glaciers – Water flows along base of glacier. Basal sliding – Ice slips over a meltwater / sediment slurry. Dry-bottom glaciers – Cold base is frozen to substrate. Movement is by internal plastic deformation of ice. Movement of Glacial Ice Two types of mechanical behavior: Brittle – Uppermost 60 m. Tension initiates cracking of the ice. Crevasses may open and close with movement. Plastic – Lower than 60 m. Ductile flow occurs in deeper ice. Ice flow heals cracks. Movement of Glacial Ice Rates of flow vary widely (10 to 300 m/yr). Rarely, glaciers may surge (20 to 110 m/day). The rate of flow is controlled by… The severity of slope angle: Steeper = faster. Basal water: Wet-bottom = faster. Location within glacier. Greater velocity in ice center. Friction slows ice at margins. Glacial Advance and Retreat Glaciers behave like bank accounts. Zone of accumulation – Area of net snow addition. Colder temperatures prevent melting. Snow remains across the summer months. Zone of ablation – Area of net ice loss. Zones abut at the equilibrium line. Glacial Advance and Retreat Toe – The leading edge of a glacier. Ice always flows downhill, even during toe retreat. Ice in the Sea In polar regions, glaciers flow out over ocean water. Tidewater glaciers – Valley glaciers entering the sea. Ice shelves – Continental glaciers entering the sea. Sea ice – Non-glacial ice formed of frozen seawater. Ice in the Sea Large areas of the polar seas are covered with ice. Global warming appears to be reducing ice cover. Ice in the Sea Marine glaciers have both grounded and floating ice. Ice debris calves off the edge, forming icebergs. Melting icebergs release dropstones to deep water. Ice in the Sea Floating ice is normally four fifths beneath the waterline. Floating ice exhibits a variety of shapes and sizes. Iceberg – > 6 m above water. Ice shelves yield tabular bergs. Glacial Effects Glaciers are important forces of landscape change. Erosion. Transport. Deposition. Glacial Erosion Glaciers erode substrates in several ways. Glacial incorporation – Rock is surrounded and carried off. Glacial Erosion Glaciers erode substrates in several ways. Plucking – Ice breaks off and removes bedrock fragments. Pressure melts ice against the surface of an obstruction. Entering bedrock cracks, this water refreezes to the glacier. Glacial movement plucks away bedrock chunks. Glacial Erosion Glacial abrasion – A “sandpaper” effect on substrate. Substrate is pulverized to fine “rock flour.” Sand in moving ice abrades and polishes bedrock. Glacial Erosion Erosional features of glaciated valleys. Cirques. Tarns. Aretes. Horns. U-shaped valleys. Hanging valleys. Fjords. Glacial Erosion Cirques – Bowl-shaped basin high on a mountain. Form at the uppermost portion of a glacial valley. Freeze-thaw mass wasting chews into the cirque headwall. After ice melts, the cirque is often filled with a tarn lake. Glacial Erosion Arete – A “knife-edge” ridge. Formed by two cirques that have eroded toward one another. Glacial Erosion Horn – A pointed mountain peak. Formed by three or more cirques that coalesce. Glacial Erosion U-shaped valleys. Glacial erosion creates a distinctive trough. Unlike V-shaped fluvial valleys. Glacial Erosion Hanging valleys. The intersection of a tributary glacier with a trunk glacier. Trunk glacier incises deeper into bedrock. Troughs have different elevations. A waterfall results. Glacial Erosion Fjords. U-shaped glacial troughs flooded by the sea. Accentuated by isostatic rebound. Glacial Sediment Transport Glaciers carry sediment of all sizes – lots of it! Some sediment falls onto the ice from adjacent cliffs. Some sediment is entrained from erosion of the substrate. When glacial ice melts, this material is dropped. Glacial Sediment Transport Glaciers act as large-scale conveyor belts. They pick up, transport, and deposit sediment. Sediment transport is always in one direction (downhill). Debris at the toe of a glacier is called an end moraine. Glacial Deposition Many types of sediment derive from glaciation. Called glacial drift, these include... Glacial till. Erratics. Glacial marine sediments. Glacial outwash. Glacial lake-bed sediment. Loess. Stratified drift is water sorted; unstratified drift isn’t. Glacial Deposition Glacial till – Sediment dropped by glacial ice. Consists of all grain sizes. Aka “boulder clay.” Unmodified by water, hence… Unsorted. Unstratified. Accumulates… Beneath glacial ice. At the toe of a glacier. Along glacial flanks. Glacial Deposition Erratics – Boulders dropped by glacial ice. These rocks are different than the underlying bedrock. Often, they have been carried long distances in ice. Glacial Deposition Loess – Wind-transported silt. Pronounced “luss.” Glaciers produce abundant amounts of fine sediment. Strong winds off ice blows the rock flour away. This sediment settles out near glaciated areas as loess deposits. Sediment Transport on Ice Moraines – Unsorted debris dumped by a glacier. Lateral – Forms along the flank of a valley glacier. Medial – Mid-ice moraine from merging lateral moraines. Depositional Landforms Glacial sediments create distinctive landforms. End moraines and terminal moraines. Recessional moraines. Drumlins. Ground moraine. Kettle lakes. Eskers. Depositional Landforms End moraines form at the stable toe of a glacier. Terminal moraines form at the farthest edge of flow. Recessional moraines form as retreating ice stalls. Depositional Landforms Drumlins – Long, aligned hills of molded lodgment till. Asymmetric form – Steep up-ice; tapered down-ice. Common as swarms aligned parallel to ice-flow direction. Depositional Landforms Ground moraine is till left behind by rapid ice retreat. It fills preexisting topography like a layer of asphalt. Creates a hummocky, mostly flat, land surface. Studded with kettle lakes from stranded ice blocks. Depositional Landforms Eskers are long, sinuous ridges of sand and gravel. They form as meltwater channels within or below ice. Channel sediment is released when the ice melts. Glacial Consequences Subsidence and rebound. Ice sheets depress the lithosphere into the mantle. Slow crustal subsidence follows flow of asthenosphere. After ice melts, the depressed lithosphere rebounds. Continues slowly today. Glacial Consequences Sea level – Ice ages cause sea level to rise and fall. Water is stored on land during an ice age – sea level falls. Deglaciation returns water to the oceans – sea level rises. Sea level was ~ 100 m lower during the Wisconsinan. If ice sheets melted, coastal regions would be flooded. Glacial Consequences Drainages – Glaciation replumbs river systems. Ice and glacial drift block preexisting drainages. After melting, altered river courses remain. Glacial Consequences North America: Glaciation completely changed drainage. Glacial Consequences Gigantic proglacial lakes formed near the ice margin. Glacial Lake Agassiz. Covered a huge area. Existed for 2,700 years. Drained abruptly. Exposed very mud-rich, extremely flat land. Glacial Consequences Climatic changes – Weather patterns were different. The American SW was much wetter. Large lakes occupied today's deserts. Lake Bonneville (now Great Salt Lake). Pleistocene Ice Ages Young (< 1.8 Ma) glacial remnants are abundant. Northern North America. Scandinavia and Europe. Siberia. Landscapes in these regions are clearly glacial. Pleistocene Ice Ages Ice sheets were 2 to 3 km thick in accumulation centers. Near centers, ice scoured bedrock, leaving striations. Ice sheets thinned outward, depositing debris. Pleistocene Life All climate and vegetation belts were shifted southward. The tundra limit was ~ 48 oN. Today, it is above 68 oN. Vegetation evidence is preserved as pollen found in bogs. Pleistocene Life Pleistocene fauna were well adapted. Mammals included now-extinct giants. Giant beaver. Giant ground sloth. Mammoths and mastodons. Modern humans proliferated. Pleistocene Chronology There have been several Pleistocene glacial advances. In North America, four are recognized – youngest to oldest: Wisconsinan. Illinoian. Kansan. Nebraskan. The last two are poorly preserved. Ice ages are separated by interglacials. Pleistocene Chronology Oxygen isotopes suggest twenty or more glaciations. Higher 18O/16O = colder. Lower 18O/16O = warmer. The “original four” ice ages may simply be the largest. Earlier Glaciations Glaciations have occurred before in Earth history. They are indicated by fossil tills and striated bedrock. Pleistocene. Permian. Ordovician. Late Precambrian – Tillites at equatorial latitudes suggest an ice-covered world (snowball Earth). Causes of Glaciation Long-term causes – Set the stage for ice ages. Plate tectonics – Controls factors that influence glaciation. Distribution of continents toward high latitudes. Sea-level flux by mid-ocean-ridge volume changes. Oceanic currents. Atmospheric chemistry. Changes in greenhouse gas concentrations. Carbon dioxide (CO2). Methane (CH4). Causes of Glaciation Short-term causes – Govern advances and retreats. Milankovitch hypothesis – Climate variation over 100-300 Ka predicted by cyclic changes in orbital geometry. The shape of Earth’s orbit varies (~ 100,000 year cyclicity). Tilt of Earth’s axis varies from 22.5o to 24.5o (~41,000 years). Precession – Earth’s axis wobbles like a top (23,000 years). Causes of Glaciation Short-term causes – Govern advances and retreats. Milankovitch hypothesis – Climate variation over 100 to 300 Ka predicted by cyclic changes in orbital geometry. These variations lead to excess warming or cooling. Ice ages may result when cooling effects coincide. Causes of Glaciation Short-term causes – Govern advances and retreats. Changes in albedo (reflectivity). Oceanic thermohaline circulation changes. Biotic modification of atmospheric CO2 concentrations. Pleistocene Model A long-term cooling trend defines the Cenozoic Era. Cessation of warm current flow to the Mediterranean. Development of the circum-Antarctic current. Uplift of the Himalayas altered atmospheric circulation. Closing the Isthmus of Panama. Glacial Reprise? Are we living in an interglacial (will ice return)? Very likely. Interglacials last ~ 10,000 years. It has been ~11,000 years since the last deglaciation. A cool period (1300 to 1850) resulted in the Little Ice Age. Today, a warming trend has caused glaciers to recede. Earth’s climate changes without consulting humanity. • • • • • Geological knowledge is power! (If you need to take only one science course, Physical Geology is the one.) We live on Earth and we need to know how the Earth works. • Environment (clean water, air) • Energy (oil, natural gas, ore deposits) • Hazards (e.g., volcano, earthquakes, tsunami, landslide) • Other aspects (e.g., home, stock, civil and military, law) Time and space scales: Change! Global scale! Sensible science from incomplete information How to think: think independently, open-minded, learn to be more sensible Appreciate the Earth, rocks, water and ice. Glacial Deposition Glacial marine – Sediments from an oceanic glacier. Calving icebergs raft sediments away from the ice. Melting, bergs drop stones into bottom muds. Dropstones… Differ from ambient sediment. Indicate glaciation. Glacial Deposition Glacial outwash – Sediment transported in meltwater. Muds are removed. Sizes are graded and stratified. Grains are abraded and rounded. Outwash is dominated by sand and gravel. Glacial Deposition Glacial lake-bed sediment. Lakes are abundant in glaciated landscapes. Fine rock flour settles out of suspension in deep lakes. Muds display seasonal varve couplets. Finest silt and clay from frozen winter months. Coarser silt and sand from summer months. ...
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  • Fall '07
  • Weislogel
  • Glacier, glacial ice, Glacial Erosion

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