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Old Tuesday, June 07, 2011
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Originally Posted by pari Ali BNi View Post


salams
i saw ur worh syeda sabahat y u have stop posting???many topics r still left waiting for those...thanks u

was.pari apne to mera 2 klio blood barha dya ha.actually mujhe lag raha tha k koi view he nhi ker raha mere posts to i was disappointed. but now i realised k sub mere post view kerte han.

dont worry pari i will inshallah post all remaining topics.

thanks for your response.
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  #22  
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Work of Glaciers

A glacier is a natural accumulation of land ice showing movement at some time. Many times during Earth's history, great ice sheets waxed and waned over the surface. What caused these periods of glaciation is still not fully understood and no single reason will probably be found.


Causes of glaciation

The onset of a period or stage of glaciation is due to a change in Earth temperature and circulation. It is generally accepted that a global decrease of 4o to 5o C, especially during the summer, and a substantial increase in the amount of snowfall in subarctic and arctic regions is necessary for the onset of a glacial episode.

Several theories have been proposed for such a change in climate -- reductions in solar radiation due to meteorite collisions with the Earth, increased volcanism, the shifting location of continents, and the uplift of vast mountain regions. Milutin Milankovitch proposed one of the most significant theories to account for climate change by variations in Earth orbit. Changes in the eccentricity of earth orbit, the degree of deviation of the orbit from a perfect circular path, is thought to cause the necessary change in insolation to decrease global temperatures.

Recall that the Earth's orbit is elliptical, but over periods of 100,000 years the shape varies. The changes in orbit have been correlated with ocean sediments that record the history of glacial stages. The cyclical nature of warming and cooling correspond well with the estimated dates of glacial and interglacial periods. In addition to the change in orbit, the Earth "wobbles" on its axis which alters the amount of insolation reaching the surface of the Earth.



Anatomy of a Glacier


Whatever the cause, the main reason glacial advances are initiated is that winter accumulation exceeds the summer loss of snow over a long period of time. Snow metamorphoses into glacial ice under the increasing pressure of accumulated layers of snow. It first changes to a granular form called firn, and ultimately into ice. Glacial ice sometimes looks blue because it absorbs all colors of the visible light spectrum except blue, which it transmits and hence its blue appearance. Glacier ice may also appear white because some ice is fractured with pockets of air that indiscriminately scatters the visible light spectrum.

The mass balance of a glacier determines if it will advance across the surface or not. The mass balance is determined by the amount of gain and loss of ice from the glacier. The mass balance is positive when it accumulates more ice than it loses. A glacier has a negative mass balance if it loses more ice than it gains

Glaciers form in the zone of accumulation, the portion of the glacier over which accumulation exceeds ablation. Ablation is the loss of ice (or snow) from the glacier. Ablation includes sublimation, wind erosion, melting, and evaporation. The zone of accumulation for the large continental ice sheets resides at high latitudes. For mountain glaciers, the zone of accumulation is at a high altitude where temperatures are cold prevent complete summertime melt. The zone of ablation is where loss of ice mass is greater than accumulation. The boundary between these two zones is the firn or equilibrium line. If accumulation exceeds ablation the glacier will grow. If ablation exceeds accumulation, the glacier will retreat by melting in place. You can approximate the location of the equilibrium line by examinig an aerial photograph. The glacier looks “dirtier” below the equilibrium line as glacial sediment is exposed on top of the ice. Above the line it is more white because fresh snow usually covers the surface.










to be continued.
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  #23  
Old Wednesday, June 08, 2011
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Types of Glaciers


Glaciers can be found in both polar and more temperate climates. They are the most abundant in the polar regions, where it remains so cold that only a minor amount of water is lost through melting or evaporation. They can also be found in the highest mountains in temperate or even tropical latitudes where temperatures remain cold throughout the year, such as in the Pacific Northwest of the United States and Canada, Alaska, and South America. More snow and ice accumulate during the winter months in these mountain ranges than is lost as meltwater in the summer.

About one-tenth of the land surface on Earth is covered by glaciers today. Over 75 percent of this amount is on Antarctica, and 10 percent is on Greenland. The remainder occurs in mountain regions across the world. If the entire Antarctic ice sheet melted, it would raise the sea level about 60 meters (200 feet) and flood many cities in low-lying coastal areas around the world.

Ice sheets are associated with continental glaciation and cover large areas of a landmass (over tens of thousands of square kilometers). Ice sheets exist in Greenland and Antarctica. Ice caps are similar to ice sheets but are much smaller—they are usually found in the highest part of a mountain range, where the snow accumulation is the greatest. An ice cap can be a source for multiple valley glaciers.

Valley glaciers (or alpine glaciers) are masses of ice that are restricted to high mountain valleys. As they move downslope, they can connect with larger valley glaciers. The majority of alpine glaciation is the result of the repeated advance and retreat of valley glaciers. Valley glaciers are common in the mountain ranges of the United States and Canada.

Piedmont glaciers are the forwardmost extension of valley glaciers and form where the ice emerges at the front of the mountain range. The ice spreads out on the flat terrain to form a wide sheet at the mouth of the valley.




Glacial Erosion


Basal sliding erodes the rock surface underlying a glacier. Meltwater freezes in cracks, and pieces of bedrock are pried loose and incorporated into the ice (similar to frost wedging). These pieces of rock grind against each other and the bedrock underneath. The intensity of the grinding is proportional to the weight and pressure of the glacier. Faceted rocks are those rocks along the base that are worn flat from erosion. The surfaces of the bedrock and the rock fragments are polished just like rocks in a rock tumbler are. Rocks embedded in the ice scratch long, deep grooves in the bedrock called striations, which indicate the direction of ice movement. The constant grinding of rocks in the glacier creates a fine-grained rock powder that turns the meltwater white when released by ablation.
Other erosional features associated with glaciers include increased mass wasting from rapid downcutting of valley sides, frost wedging that creates rockfalls, and avalanches that contribute additional rock and soil material.




Glacial Landforms


The mass of ice at the top of a glacial valley ultimately forms a steep-sided, circular hollow called a cirque. Mass wasting and frost wedging also contribute to the formation of a cirque.
A bergschrund is a crevasse that forms where the glacier separates from the cirque wall and is commonly filled with rock fall debris













An advancing glacier scours out a series of depressions in the underlying bedrock, which later fill with water and become rock-basin lakes, or tarns. Tarns are best developed in softer or highly fractured bedrock. Tarns are less common on smooth, hard bedrock surfaces because it is more difficult for the glacier to “grab hold” and break off pieces of rock.












A horn is a sharply defined peak that has formed from erosional processes along the rim of the cirque. A steep ridge called an arete commonly extends downward from a horn to separate two adjacent glacial valleys.











The most striking glacial erosional features are associated with alpine glaciation. In fact, rugged mountainous areas can be made even more spectacular by glacial action. Alpine glaciers transform V-shaped valleys made by streams into deeper U-shaped valleys called glacial troughs—the ice is too massive to follow the stream bed and pushes right through, scouring out a U shape. The ice also erodes away the ends of any ridges along the valley walls. These eroded ridges are called truncated spurs. The valleys of tributaries can also be truncated, forming hanging valleys that are higher than the main valley and often marked by waterfalls.






u shaped valley





fiords






hanging valley, USA



to be continued.
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Old Thursday, June 09, 2011
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Glacial Deposits

Load. An advancing ice sheet carries an abundance of rock that was plucked from the underlying bedrock; only a small amount is carried on the surface from mass wasting. The rock/sediment load of alpine glaciers, on the other hand, comes mostly from rocks that have fallen onto the glacier from the valley walls. The various unsorted rock debris and sediment that is carried or later deposited by a glacier is called till. Till particles typically range from clay-sized to boulder-sized but can sometimes weigh up to thousands of tons. Boulders that have been carried a considerable distance and then deposited by a glacier are called erratics. Erratics can be a key to determining the direction of movement if the original source of the boulder can be located.

Features left by valley glaciers and ice sheets.

Moraines are deposits of till that are left behind when a glacier recedes or that are carried on top of alpine glaciers.
Lateral moraines consist of rock debris and sediment that have worked loose from the walls beside a valley glacier and have built up in ridges along the sides of the glacier.
Medial moraines are long ridges of till that result when lateral moraines join as two tributary glaciers merge to form a single glacier. As more tributary glaciers join the main body of ice, a series of roughly parallel medial moraines develop on the surface of main glacier.

An extensive pile of till called an end moraine can build up at the front of the glacier and is typically crescent shaped. Two kinds of end moraines are recognized: terminal and recessional moraines. A terminal moraine is the ridge of till that marks the farthest advance of the glacier before it started to recede. A recessional moraine is one that develops at the front of the receding glacier; a series of recessional moraines mark the path of a retreating glacier.
A thin, widespread layer of till deposited across the surface as an ice sheet melts is called a ground moraine.









Ground moraine material can sometimes be reshaped by subsequent glaciers into streamlined hills called drumlins, long, narrow, rounded ridges of till whose long axes parallel the direction the glacier traveled.





Drumlines-alberta.




GLACIO FLUVIAL DEPOSITS


As a glacier melts, till is released from the ice into the flowing water. The sediments deposited by glacial meltwater are called outwash. Since they have been transported by running water, the outwash deposits are braided, sorted, and layered. The broad front of outwash associated with an ice sheet is called an [B]outwash plain[/B

if it is from an alpine glacier it is called a valley train.

Kames are steep-sided mounds of stratified till that were deposited by meltwater in depressions or openings in the ice or as short-lived deltas or fans at the mouths of meltwater streams. The rapid build-up of sediments can bury isolated blocks of ice.
When the ice melts, the resultant depression is called a kettle. Kettle lakes, common in the upper Midwest of the United States, are bodies of water that occupy kettles.

Eskers are long, winding ridges of outwash that were deposited in streams flowing through ice caves and tunnels at the base of the glacier. Generally well sorted and cross-bedded, esker sands and gravels eventually choke off the waterway




.


Glaciofluvial Deposits
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Old Thursday, June 09, 2011
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Smile Syeda Sabahat really a gem

your are really very helpful for the aspirants with geography as optional, keep your efforts up irrespective of a individual comments.
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great work lady...hats Off....plz guide me on geography....especially geo 1...is bajwa enogh for ppr 1 ??
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Originally Posted by Yousafzaii View Post
great work lady...hats Off....plz guide me on geography....especially geo 1...is bajwa enogh for ppr 1 ??

@M.umer and Yousaf zai.thanks for your comments. i will share my knowledge with all of you to easily understand geography for css.

@yousaf i think 1 book is not enough for geo 1 as you can see my notes, i prepare 1 question from several different books to have variety in my ans.to gain good marks.

i will suggest you to buy a physical geography book of graduation level and sarfaraz bajwa book, last but not least css forum. i will try my level best to complete geo.1 notes. then i will start questions,answers from your side.

best of luck to all my geography fellows.
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I think Mian Muhammad Anwar's Physical, human and pakistan geography are best. My friend got 70+ marks in both papers. I ve read all of them , language is also simple.
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Introduction to Mass Wasting

Mass wasting is a natural result of weathering on slopes. Simply put, gravity pulls loose rock and soil downhill.
Mass wasting is the process of erosion whereby rock, soil, and other earth materials move down a slope because of gravitational forces. It proceeds at variable rates of speed and is largely dependent on the water saturation levels and the steepness of the terrain. A destructive, rapid mass-wasting event is called a landslide; if the movement is slow enough that it cannot be seen in motion, it is called creep.

Three kinds of movement are generally recognized: flow, slip, and fall. A mass-wasting event is called a flow if the mass moves downslope like a viscous fluid. If the mass moves as a solid unit along a surface or plane, it is called a slip. A slip that moves along a surface parallel to the slope is called a slide. If the movement occurs along a curved surface where the downward movement of the upper part of the mass leaves a steep scarp (cliff) and the bottom part is pushed outward along a more horizontal plane, it is called a slump. Earth material that free-falls from a steep face or cliff is termed a fall.

Mass‐Wasting Controls


A variety of conditions affect the development of mass wasting in a particular area. Steep slopes, widely varying altitude ranges (relief), the thickness of the loose earth material, planes of weakness parallel to the slopes, frequent freezing and thawing, high water content in the earth material, dry conditions with occasional heavy rainfall, and sparse vegetation are the factors that contribute to the unstable conditions that result in mass wasting. Movements can be triggered by the motion of earthquakes or too much weight added to the upper part of a slope, such as snowpack.

Angle of repose. The angle of repose is the steepest angle at which loose material will remain in place. It is largely dependent on the size, shape, and roughness of the particles. The angle varies from about 25 degrees to about 40 degrees. If the angle is exceeded by additional sedimentation or tilting, a slide or disturbance will result.

Gravity and friction. Rock particles and soil move downslope because of the forces of gravity. The gravity that acts on an object is a combination of the normal force and the shear force. The normal force is perpendicular to the slope the object rests on, and the shear force is parallel to the surface of the slope (Figure 1 ). Steep slopes have high shear forces; the steeper the slope, the greater the chance an object will slide. Friction, such as that from a rough bedrock surface, counteracts shear force. Rough, angular particles maintain steeper slopes than smooth rounded particles do. Water acts as a lubricant and reduces the force of friction, increasing the tendency to slide.
The shear strength is an object's resistance to movement that needs to be overcome in order to make it move. Shear strength is proportional to how solid the mass is, the density of plant roots, the amount of water present, and the roughness of the particles in the mass.

The effects of water. In addition to acting as a lubricant, water increases the weight of a mass of earth material. Water reduces the shear strength by forcing sediment particles apart through pore pressure, which reduces the friction between the particles. Alternatively, smaller amounts of water that don't completely fill the pores are distributed as thin films around the sediment particles, which are attracted to each other through surface tension, increasing the cohesiveness of the mass. Thus, saturated materials are much more likely to flow than a mass that is only a little wet.


Types of Mass Wasting

Rockfalls and rockslides.

Rockfalls occur when pieces of rock break loose from a steep rock face or cliff. These result from the rock face being undercut by rivers or wave action. Frost wedging may also eventually loosen large blocks, causing them to fall. The accumulation of rock debris at the base of a steep slope is called talus.






TALUS


Rockslides usually follow a zone of weakness, such as a bedding plane or foliation plane. Separation of the rock is more likely along these planes because of their reduced shear strength. Water also tends to be channeled along these planes, which increases slippage. Collisions down the slope generally break the rock mass into rubble that eventually comes to rest. If steep slopes are involved, a fast-moving rock avalanche may result. The rockslide or rock avalanche loses energy and speed as it moves across more level terrain.

Debris flows. Debris flows are defined as mass-wasting events in which turbulence occurs throughout the mass. Varieties of these are called earthflows, mudflows, and debris avalanches.

When earth material moves down a hillside as a fluidlike mass, it is called an earthflow. These flows typically occur in humid areas on steep slopes with thick, clay-rich soil that becomes saturated with water during storms. The earthflow usually leaves a steep scarp behind where it separated from the hillside. Earthflows can be fast (a few hours) or slow (a few months). Velocities range from 1 millimeter per day to meters per day. Intermittent activity can continue for years as the earthflow continues to settle and stabilize. Earthflows typically have rounded, hilly fronts. A common trigger for an earthflow is the undercutting of the slope by erosion from wave action or rivers or by construction projects.









MUD FLOW


A mudflow is a liquidy mass of soil, rock debris, and water that moves quickly down a well-defined channel. Generally viscous and muddy colored, it can be powerful enough to move large automobiles and buildings. Mudflows occur most often in mountainous semiarid environments with sparse vegetation and are triggered by heavy rainfall that saturates the loose soil and sediment. They are also the natural result of volcanic ash build-ups on flanks of volcanoes and of forest fires that have exposed the soil to rapid erosion. A mudflow originating on a volcanic slope is called a lahar.

A variety of earthflow called solifluction is the flow of watersaturated earth material over an impermeable surface such as permafrost. It occurs frequently in bitterly cold regions such as in Alaska or Canada. Springtime temperatures thaw only the first few feet of the frozen ground (the active layer), which becomes saturated quickly and slowly flows over the ever-frozen permafrost below. Solifluction can occur on even the gentlest of slopes. Not forceful enough to break apart the surface vegetation, the migrating material drags it along like a wrinkled green rug. The soil finally settles on level ground at the base.





SOLIFLUCTION


The deadliest variety of debris flow is the debris avalanche, a rapidly churning mass of rock debris, soil, water, and air that races down very steep slopes. It has been theorized that trapped air may increase the speed of an avalanche by acting as a cushion between the debris and the underlying surface.

Creep. A slow, gradual movement of soil or regolith downhill over time is called creep. Velocities are typically less than a centimeter per year. Freezing and thawing contribute to soil creep by progressively moving soil particles down the hill. Creep is manifested at the surface by such things as tilted utility poles that become more out of alignment every year. Vegetation helps reduce the rate of soil creep.


Slump. Earth material that has moved as a unit along a curved surface is called slump. A slumped mass of sediment is typically clay rich. Slump usually results when the geometrical stability of a slope is compromised by the undercutting of its base, such as by wave action, a meandering river, or construction.





SLUMP


Prevention of Mass Wasting


Proper design during construction projects can eliminate the potential for increased mass wasting. Human activities such as undercutting the base of the slope, adding weight to the upper part of the slope by building large structures, removing vegetation, and saturating the ground with water increase the risks of mass wasting. Engineering solutions include barriers and retaining walls, drainage pipes, terracing the slope to reduce the steepness of the cuts, and immediate revegetation. Rockfalls can be controlled or eliminated by the use of rock bolts, cables, and screens and by cutting back slopes to lesser gradients.
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i hope ur doing good....i jst bought sarfaraz's book that have mentioned to me...so now i have bajwa and sarfaraz...wud it be enough for geo 1..?? as seniors are of the view that one must have to study the books of foreign authors like deblij and cho cheng etc....
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