What Condition Is Most Necessary to Build a Glacier?

EENS 1110	Physical Geology
Tulane University	Prof. Stephen A. Nelson
Glaciers and Glaciation

Glaciers establish much of the Earth that makes upwardly the cryosphere, the part of the Earth that remains below the freezing point of water. Well-nigh glacial ice today is found in the polar regions, above the Arctic and Antarctic Circles. While glaciers are of relatively small importance today, covering but about ten% of the surface, evidence exists that the Earth's climate has undergone fluctuations in the past, and that the amount of the Globe's surface covered past glaciers has been equally much as 30% in the past. In fact, much of the topography in the northern function of Northward America, too as in the high mountain regions of the westward, owe their form to erosional and depositional processes of glaciers. The latest glaciation ended only ten,000 years ago.    The World has experienced numerous glaciations, the almost contempo during the Pleistocene Epoch between one.8 meg years ago and eleven,000 years ago. Other episodes occurred in the Permian, Ordovician, and Late Precambrian.

Definition of a glacier

A glacier is a permanent (on a homo time calibration, because nothing on the Globe is really permanent) body of ice, consisting largely of recrystallized snow, that shows evidence of downslope or outward movement due to the pull of gravity.

Types of Glaciers

(note: images of these features are shown in your textbook and will be shown in class.)

Mountain Glaciers - Relatively small glaciers which occur at higher elevations in mountainous regions.

• Smallest of these occupy hollows or basin-shaped depressions on sides of mountains ( cirque glaciers ).

• As cirque glaciers grow larger they may spread into valleys and flow downwardly the valleys as valley glaciers . Paths these valley glaciers have are controlled past existing topography.

• If a valley glacier extends down to body of water level, information technology may cleave a narrow valley into the coastline. These are chosen fjord glaciers , and the narrow valleys they carve and later go filled with seawater after the water ice has melted are fjords .

• If a valley glacier extends down a valley and then covers a gentle slope beyond the mountain range, information technology is called a piedmont glacier .

• If all of the valleys in a mountain range become filled with glaciers, and the glaciers encompass so entire mountain range, they are called ice caps .

Ice Sheets (Continental glaciers) - are the largest types of glaciers on Earth. They encompass large areas of the land surface, including mountain areas. Modern ice sheets encompass Greenland and Antarctica. These two ice sheets comprise about 95% of all glacial water ice currently on World. They have an estimated volume of about 24 million km ³ . If melted, they comprise enough h2o to raise sea level well-nigh 66m (216 ft.). This would cause serious problems for coastal cities (50.A., NY, Washington DC, New Orleans, Miami, SF etc). The Greenland ice sheet is in some places over 3000 one thousand (9800 ft) thick and the weight of ice has depressed much of the crust of Greenland beneath sea level. Antarctica is covered by two large ice sheets that run into in the cardinal role along the Transantarctic Mountains. These are the just truly polar water ice sail on earth (Northward Pole lies in an ocean covered by thin layer of ice).

Water ice Shelves - Ice shelves are sheets of water ice floating on water and attached to land. They commonly occupy littoral embayments, may extend hundreds of km from land and accomplish thicknesses of 1000 g.

Glaciers tin likewise be classified by their internal temperature.

• Temperate glaciers - Ice in a temperate glacier is at a temperature near its melting signal.

• Polar glaciers - Water ice in a polar glacier always maintains a temperature well below its melting point.

The Formation of Glacial Ice

Three conditions are necessary to form a glacier: (one) Cold local climate (polar latitudes or high height). (2) snow must be arable; more snow must fall than melts, and (3) snow must non be removed by avalanches or wind.

Glaciers can merely form at latitudes or elevations above the snowline , which is the peak to a higher place which snow can form and remain nowadays year round. The snowline, at present, lies at sea level in polar latitudes and rises upwards to 6000 grand in tropical areas. Glaciers course in these areas if the snow becomes compacted, forcing out the air between the snowflakes. As compaction occurs, the weight of the overlying snow causes the snowfall to recrystallize and increase its grain-size, until information technology increases its density and becomes a solid block of ice.  A glacier is actually a metamorphic rock.

Changes in Glacier Size

A glacier tin can change its size past Accumulation , which occurs by addition of snowfall, compaction and recrystallization, and Ablation , the loss of mass resulting from melting, usually at lower altitude, where temperatures may rise above freezing bespeak in summer. Thus, depending on the balance betwixt accumulation and ablation during a full season, the glacier can advance or retreat (encounter figure 22.nine in your text volume).

Movement of Glaciers

Glaciers move to lower elevations under the strength of gravity past two different processes:

• Internal Menstruation - called creep, results from deformation of the ice crystal construction - the crystals slide over each other like deck of cards. This type of movement is the only type that occurs in polar glaciers, but information technology likewise occurs in temperate glaciers.

• Basal sliding - meltwater at base of glacier reduces friction by lubricating the surface and allowing the glacier to slide across its bed. Polar glaciers are usually frozen to their bed and are thus likewise cold for this machinery to occur.

The upper portions of glaciers are brittle, when the lower portion deforms by internal flow, the upper portions may fracture to form large cracks called crevasses. Crevasses occur where the lower portion of a glacier flows over sudden change in topography (encounter figure 22.half dozen in your text).

The velocity of glacial ice changes throughout the glacier. The velocity is low next to the base of the glacier and where it is contact with valley walls. The velocity increases toward the center and upper parts of the glacier (see figure 22.viii in your text).

Glaciation

Glaciation : is the modification of the land surface past the activeness of glaciers. Glaciations have occurred so recently in N. America and Europe, that weathering, mass wasting, and stream erosion have not had time to modify the landscape. Thus, show of glacial erosion and deposition are yet present. Since glaciers movement, they tin can choice up and ship rocks and thus erode. Since they transport material and can melt, they can also deposit material. Glaciated landscapes are the result of both glacial erosion and glacial degradation.

Glacial Erosion - Glaciers erode in several ways.:

• Abrasion – Rock fragments carried by the glacier scrape against rock causing abrasion, like sandpaper.
• Plucking – Water ice breaks off and removes bedrock fragments
• Ice melts by pressure level against the up-ice side of an obstruction. Entering cracks in bedrock, this water re-freezes to the ice. Glacial motility plucks away bedrock chunks (come across figure 22.13 and 22.fifteen in your text).
  

Small scale erosional features  (notation: most of this material will exist presented as slides in class)

• Glacial striations - long parallel scratches and grooves that are produced at the lesser of temperate glaciers by rocks embedded in the ice scraping against the rock underlying the glacier (see figure 22.12 in your text).

• Glacial polish - rock that has a smooth surface produced as a result of fined grained material embedded in the glacier interim similar sandpaper on the underlying surface (run across effigy 22.12 in your text).

Landforms produced by mountain glaciers (see effigy 22.14 in your text)

• Cirques - bowl shaped depressions that occur at the heads of mountain glaciers that result form a combination of frost wedging, glacial plucking, and abrasion. Sometimes small lakes, chosen tarns occur in the bottom of cirque.

• Glacial Valleys - Valleys that once contained glacial ice get eroded into a "U" shape in cantankerous section. Stream erosion, on the other hand, produces valleys that are "V" shaped in cantankerous section.

• Ar�tes - If two side by side valleys are filled with glacial water ice, the ridges between the valleys can be carved into a sharp knife-border ridge, called an ar�te.

• Horns - Where three or more cirques are carved out of a mountain, they tin produce a abrupt peak chosen a horn.

• Hanging Valleys - When a glacier occupying a smaller tributary valley meets the larger valley, the tributary glacier usually does not have the ability to erode its base of operations to the floor of the primary valley. Thus, when the glacial ice melts the flooring of the tributary valley hangs above the floor of the chief valley and is called a hanging valley. Waterfalls generally occur where the hanging valley meets the chief valley.
  

• Fjords - Fjords are narrow inlets along the seacoast that were once occupied by a valley glacier, chosen a fjord glacier.

Landforms produced by Ice Caps and Ice Sheets

• Abrasional features - The same pocket-sized-scale abrasional features such as striations and glacial polish can occur beneath water ice caps and ice sheets, specially in temperate environments.

• Streamlined forms - The state surface beneath a moving continental water ice sheet can be molded into smooth elongated forms called drumlins (encounter figure 22.20 in your text).  Other elongated hills carved into bedrock by plucking and abrasion are called roche mountomées (encounter effigy 22.15 in your text).

Glacial Deposition and Deposits Since glaciers are solid they tin transport all sizes of sediment, from huge firm-sized boulders to fine-grained clay sized material. The glacier tin bear this material on its surface or embedded within information technology. Thus, sediment transportation in a glacier is very much different than that in a stream. Thus, sediments deposited directly from melting of a glacial tin can range from very poorly sorted to ameliorate sorted, depending on how much water transport takes place after the water ice melts. All sediment deposited as a result of glacial erosion is chosen Glacial Drift . Ice Laid Deposits Till - nonsorted glacial drift deposited directly from ice. Till consists of a random mixture of unlike sized fragments of angular rocks in a matrix of fine grained, sand- to dirt-sized fragments that were produced by abrasion within the glacier. This fine-grained material is often chosen stone flour considering it is actually basis upwardly rock. A till that has undergone diagenesis and has turned into a rock is called a tillite. Erratics - a glacially deposited rock or fragment that at present rests on a surface made of different rock. Erratics are oftentimes found many kilometers from their source, and by mapping the distribution pattern of erratics geologists tin can often determine the flow directions of the water ice that carried them to their present locations. Moraines - are deposits of till that have a form different from the underlying bedrock. Depending on where it formed in relation to the glacier moraines tin be: Ground Moraines - these are deposited beneath the glacier and upshot in a hummocky topography with lots of enclosed small basins. End Moraines and Terminal Moraines are deposited at the depression pinnacle finish of a glacier equally the ice retreats due to ablation (melting) (see illustration on page 820-21 in your text).  It is notable that Long Island, NY and function of Cape Cod, MA are terminal moraines from the terminal glaciation (come across figure 22.nineteen in your text).. Lateral Moraines are deposits of till that were deposited along the sides of mountain glaciers. Medial Moraines - When two valley glaciers meet to form a larger glacier, the rock droppings forth the sides of both glaciers merge to grade a medial moraine (see figures 22.17). These black streaks in an active glacier, likewise equally the deposits left backside after the ice melts are chosen medial moraines.

Glacial Marine drift - Glaciers that achieve the oceans or even lakes, may calve off into large icebergs which so bladder on the water surface until they cook. Upon melting, the rock debris that they contain becomes immediately deposited on the bounding main floor or lake bed as an unsorted chaotic deposit. Sometimes single large rock fragments fall out on the floor of the water body, and these are called dropstones .

Stratified Drift - Glacial drift tin can be picked upward and moved past meltwater streams which tin then deposit that fabric as stratified migrate.

• Outwash Plains - Streams running off the stop of a melting glacier are commonly high-strung with sediment and course braided streams, which deposit poorly sorted stratified sediment in an outwash plain. These deposits are ofttimes referred to every bit outwash.
• Kettle Lakes (called kettle holes in your text) - If depressions form underneath a glacier and remain later on the glacier is melted then h2o filling these depressions become small lakes where fine-grained sediment is deposited. The land of Minnesota is called the land of x g lakes, near of which are kettle lakes.
• Kames and Kame Terraces - Streams and lakes forming on top of or on the sides of stagnant water ice may deposit stratified sediment on top of the glacier. When the glacier melts these deposits are set down on the ground surface and get kames.
• Eskers - Eskers are long sinuous ridges of sediment deposited by streams than ran under or inside a glacier. The sediment deposited past these streams becomes an esker after the water ice has melted (see figure 22.21 in your text).
• Meltwater Lakes -  Depressions created by glacial erosion and deposition collect water released past melting glaciers. Sediment that collects in the bottom on the lakes is oft finely layered with coarser grained layers forming during times of warmer temperatures and fine grained layers formed during cold times when no new sediment is entering the lake.  Such finely layered strata are termed varves .

Other Consequences of Glaciation

Ice Loading and Glacial Rebound

The weight of glacial ice sheets depress the lithosphere into the mantle causing the chaff to subside. Afterwards the ice melts, the depressed lithosphere rebounds. The rebound process is withal taking place today (see figures 22.23 in your text).

Sea Level Changes

• During glacial periods much ocean water was tied up in glaciers so body of water level was lower.
• during interglacial periods ocean level was higher due to melting of the water ice.
  

Water ice Dams, Drainage Reversals, and Lakes

When glacial ice forms, it can block existing drainages causing the formation of new lakes and forcing streams to find new pathways that develop into new drainage networks. Once the ice melts, the new drainage network become well established and the old drainage networks are frequently abandoned.

Such a change in drainage networks took identify as a result of the last ice age in North America (see figure 22.24 in your text book).  Prior to glaciation, streams in the northern U.S. and Canada tuckered to the northeast into what is at present Hudson Bay and just the southern part of the U.S. tuckered into the Mississippi River system.   Because the glacial ice retreated toward the northward, the Mississippi drainage organization became the major drainage organisation for much of the U.S.

During the Pleistocene Epoch, large lakes formed both equally effect of water ice dams and melting of glaciers.  Examples include the Bully Lakes of the northern U.Due south., and a now much reduced lake, Lake Agassiz the formed from northern Minnesota, into the Canadian provinces of Manitoba, Saskatchewan and Ontario.   Every bit ice melted, lakes were also formed in the western U.S. at large distances from the glacial source.   For example in the Bowl and Range Province, basins were filled with large lakes formed by internal drainage.  One of these lakes. Lake Bonneville, covered much of western Utah, eventually draining and evaporating leaving the remnant chosen the Keen Table salt Lake.

Glacial Ages

The final glaciation concluded about 11,000 years ago. But the period between xi,000 years agone and ii one thousand thousand years agone (the Pleistocene epoch) was a time of many glacial and interglacial ages.

Based on evidence from glacial deposits and glacial erosion features geologists have been able to certificate at least 4 glaciations during the Pleistocene, two of which are poorly documented. Just recent studies of abyssal sediments and dating of these deposits propose that in that location were at least xxx glaciations that occurred during the Pleistocene. This evidence comes from studies of fossils institute in deep-sea sediment cores, and what they tell us about ocean surface temperatures in the past. The results come from studies of the isotopes of oxygen.

• Oxygen has 2 major isotopes, ¹⁸O, which is considered heavy, and ^sixteenO, which is considered low-cal. Both of these isotopes are stable and non-radiogenic, then their ratio is abiding through time.
• Because ^sixteenO is lighter, information technology is preferentially evaporated with bounding main water from the oceans, and thus gets concentrated in the water that eventually falls on the continents as pelting or snowfall. Because of this, ^eighteenO gets concentrated in bounding main water.
• During constant climatic conditions the ¹⁶O lost to evaporation returns to the oceans by pelting and streams, so that the ratio of ¹⁸O to ¹⁶O (¹⁸O / ¹⁶O) is constant.
• Simply, during a glaciation, some of the ^xviO gets tied up in glacial ice and does not return to the oceans. Thus during glaciations the ^xviiiO / ¹⁶O ratio of sea water increases.
• During an interglaciation, on the other mitt, the ^sixteenO that was tied up in glacial ice returns to the oceans causing a decrease in the ¹⁸O / ¹⁶O ratio of seawater.

Thus, we wait that during glaciations the eighteenO / 16O ratio in seawater volition exist high, and during interglaciations the 18O / xviO ratio in seawater volition exist low. Since organisms that live in the oceans extract Oxygen from seawater to form their carbonate (CO3 -2) shells, measuring the 18O / 16O ratio in the shells of dead organisms gives a record of past ocean temperatures. The record for the past two one thousand thousand years is shown hither and in effigy 22.30 in your text. The data suggests nearly 30 glaciations separated by interglaciations during the past 2 million years.

During the concluding 1 meg years information technology appears that each glacial - interglacial cycle has lasted virtually 100,000 years, but earlier cycles were about twoscore,000 years long.

Other periods of glaciation are known from the geologic record, mainly from preserved glacial striations and tillites (consolidated till). The primeval recognized glaciation occurred about 2.3 billion years ago, but at least l other glaciations are recognized to have occurred during the Paleozoic era.

Causes of Glacial Ages

In order to sympathize what causes these cycles of glacial - interglacial episodes we demand a much meliorate understanding of what causes global climate changes. Considering human history is so brusque compared to the time scales on which global climate change occurs, nosotros do non completely understand the causes. Still, nosotros tin advise a few reasons why climates fluctuate.

• Long term variations in climate (tens of millions of years) on a single continent are likely caused past drifting continents. If a continent drifts toward the equator, the climate will become warmer. If the continent drifts toward the poles, glaciations can occur on that continent.
• Short-term variations in climate are probable controlled by the amount of solar radiation reaching the Earth. Amid these are astronomical factors and atmospheric factors.
• Astronomical Factors -
• Variation in the eccentricity of the World'due south orbit around the sun has periods of about 400,000 years and 100,000 years.
• Variation in the tilt of the Earth'south axis has a period of well-nigh 41,000 years.
• Variation in the mode the Earth wobbles on its axis, chosen precession, has a period of well-nigh 23,000 years.
• The combined effects of these astronomical variations results in periodicities (called Milankovitch Cycles) similar to those observed for glacial - interglacial cycles (meet figure 22.46 in your exam book)
  

• Atmospheric Factors- the composition of the World's atmosphere tin be gleaned from air bubbling trapped in water ice in the polar ice sheets. Studying drill core samples of such glacial water ice and their contained air bubbling reveals the following:
• During past glaciations, the amount of CO_two and methane, both greenhouse gasses that tend to crusade global warming, were lower than during interglacial episodes.
  

• During past glaciations, the corporeality of dust in the atmosphere was higher than during interglacial periods, thus more estrus was likely reflected from the Earth'southward atmosphere dorsum into space.
• The trouble in unraveling what this ways comes from not being able to understand if depression greenhouse gas concentration and loftier dust content in the atmosphere caused the ice ages or if these atmospheric condition were caused past the ice ages.

• Changes in Oceanic Circulation - small changes in bounding main circulation tin can dilate modest changes in temperature variation produced past astronomical factors.
  

• Other factors
  
• The free energy output from the sun may fluctuate.
• Large explosive volcanic eruptions can add pregnant quantities of dust to the atmosphere reflecting solar radiation and resulting in global cooling.
• Impacts of big asteroids with the Earth can cause all-encompassing amounts of grit and soot to exist placed in the temper. Dust and soot would block incoming solar radiations and cause a cooling outcome for as long as the dust remains in the temper.

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Examples of questions on this material that could be asked on an exam.

1. Describe the unlike types of mountain glaciers.
2. How does glacial ice cause erosion?
3. Describe the erosional features produced by glaciers.
4. What are the characteristics of sediment deposited straight from glaciers?  Describe the various types of ice laid deposits.
5. How does glaciation modify the landscape in terms of drainage networks and lakes?
6. During the Pleistocene Epoch, how many glacial/interglacial episodes are suspected and what is the show for this?
7. What factors can cause long term and short term variations in climate that may be responsible for the glacial/interglacial cycles.
8. Why is Minnesota called the Land of Ten Thousand Lakes?

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