Chapter 12

In general, valley glaciers around the world have been receding during the past century.

-Glaciers at glacier national park in montana and mount kilamanjaro in africa are receding at a rate that will lead them to disappear in the years to come.

The headward erosional processes that enlarge a cirque also help create the sharp peaks and ridges characteristics of glaciated mtns ranges.

A horn is the sharp peak that remains

Under the influence of gravity, glacier ice moves downward and is eventually ablated, lost.

Ablation is due mostly to melting. If moving glacier reaches a body of water, blocks of ice break off (calve) and float free as icebergs.

Cirques, horns, aretes. : A cirque is a steep sides half bowl shaped recess carved into a men at the head of a valley carved by a glacier, topographic, a larger percent of snow accumulates that eventually converts to glacier ice and spills over the threshold as the valley glacier starts downward,

Aslo shaped by weathering and erosion of the rock walls above the surface of the ice. Frost wedging and avalanches break up the rock and steepen the slopes above the glacier. Rock may fall into a creevasse that develops where the glacier is pulling away from the cirque.

Velocity also varies within the glacier itself. The central portion of a valley glacier moves faster than the side(as water does in a stream) and the surface moves faster than the base.

How ice moves within a valley glacier has been demonstrated by studies in which holes are drilled through the glacier ice and flexible pipes inserted. Changes in the shape and position of the pipes are measured periodically.

Glacial Budgets: Over a period of time, the amount of snow a glacier gains is greater than the amount of ice and water is loses, the glaciers budgeet is positive and it expands.

If the oppositve occurs, the glacier decreases in volume and is said to have a negative budget.

In the rigid zone, or upper part of the glacier, the pipe has been move down glacier. and has remained unbent.

In the rigid zone, grains of ice do not move relative of their neighbors.

Outwash: In a zone of ablation, large quantities of meltwater usually run over, beneath, and away from the ice. the marieral deposited by the debris laden meltwater is called outwash.

Outwash can be distinguished from the unlayered and unsorted deposits of till. Its well sorted and the particles are not chemically weathered, its an excellent source of aggregate for building roadways and for mixing with cement to make concrete.

Erosional landscapes associated with continental glaciation: The rock underneath the ice sheet is erodes in much the same ways as the rock beneath a valley glacier;

Rounded knobs are common grooved and started. An ice sheet may be thick enough to bury mtn ranges. -northwestern canada=shows us that erosional effects that used to cover na.

Movement of valley glaciers- valley glaciers move downslope under the influence of gravity at a variable rate. Glacier will flow faster if its somewhere that is steeper. Also the thicker parts of a glaciers will flow faster than where it is thinner. The upper part of a glacier tends to be steeper than at lower levels. If a glacier has an even gradient, the glacier will be thickest near its equilibrium.

So for locally steeper stretches we expect the fastest moving ice to be near the equilibrium line. Below this line, the glacier usually becomes progressively thinner and slower. -Glaciers in temperate climates-where the temps of the glacier is at or near the melting point for ice- tend to move faster than those in colder regions-where the ice temperature stays well below freezing.

Multiple til deposits and other glacial feature indicate several episodes of glaciation during the late Cenozoic era. During each of these episodes, large ice sheets covered most of northern Europe and northern NA and glaciation in the mtn areas of the world was much more extensive than at present. At the peak of glaciation, about a third of earths land surface was glaciated. warmer climates prevailed durning interglacial episodes.

The glacial ages also affect regions never covered by ice. Because of wetter climate in the past, large lakes formed in now arid regions of the sea. sea level was considerably lower. Glacial ages also occured in the more distant geologic past, as indicated by late palezoic and precambrain tillites.

The upper part of a glacier is called the zone of accumulation and is the part of the glacier with a perennial snow cover.

The lower part is called the zone of ablation and there ice is lost(ablated) by melting, evaporation, and calving.

The lower portion of the pipe is bent in a down glacier direction, the bent pipe indicates plastic flow; movement that occurs within the glacier due to the plastic or deformable nature of the ice itself.

The reason the pipe is bent more sharply near the base of the glacier is that pressure form overlying ice results in greater flow age with increasing depth.

Past glaciaition- early hypothesis: Agassiz and Buckland-concluded a great glacier covered most or all of eurpope

The theory of glacial ages- states that at times in the past, colder climates prevailed during which more of the land surface of earth was glaciated than at the present. climate change for the past ice age began 3 million years ago-tertiary period.Antarctica has been glaciated for 14 million years.

glacially carved valleys are usually

U shaped

The grinding of rock across rock produces a powder called rock flour. -Composed of very fine silt and clay sized particles of unaltered minerals(pulverized from chemically unweathered bedrock).

WHen meltwater washed rock flour from a glacier, the streams draining the glacier appear milky and lakes into which glacial meltwater flow often appear a milky green color.

Distribution of glaciers- glaciers occur in temperate as well as polar climates. They are found where more snow falls during the cold time of year than can be melted during warmer months.

Washington has more glaciers than any other states aside from Alaska, because of the extensively glacial mountains of western washington. these mtns have warmer winters but much more precipitation in the higher elevation than do the rocky mtns. -glaciers are common even near the equator in the very high mtns of south america and africa because of the low temps at high altitudes.

the boundary between the zone of accumulation and the zone of ablation of a glacier is called the

equl line.

Which is not a type of moraine medial end terminal recessional ground esker

esker

Continental glaciaiton

exists where a large part of a continent is covered by glacial ice.

during the ice ages much of Nevada utah, and eastern cali was covered by

huge lakes

Which is not a type of glacier? -valley glacier -ice sheet -ice cap -sea ice

ice cap

A glacier

is a large, long lasting mass of ice, formed on land that moves under its own weight. It develops as snow is compacted and recrystallized. Glaciers can develop anywhere, over a period of time, more snow accumulates than melts away or is otherwise lost.

Alpine Glaciaiton

is found in mtn regions

AS meltwater builds thick deposits of outwash alongside and in front of retreating glacier, locks of stagnant ice may be surrounded and buried by sediment. When the ice block final melts, a depression called a kelttle forms.

many lakes in the upper middle west are kettle lakes. -a kame is a low mound or irregular ridge formed of outwash deposits on a stagnating glacier. -When ice melts(where sediment has accumulated in the glaciers surface) the sediment remain as irregular, moundlike hills. the irregular and bumpy landscape of hills and depressions associated with many moraines is knowns as kame and kettle topography.

recently, geologist have been drilling through ice sheets for clues about

past climates

Glacial lakes and varves: lakes often occupy depression carved by glacial erosion but can also form behind damns built by glacial deposition. A lake forms between a retreating glacier and n early end moraine.

silt and clay- 2 characteristic (bottom in thin layer) that indicate a glacier lake. Two layers of sediment representing one years deposition in a lake are called a valve-light colored layer consists of silt and clay. think of tree rings.

in a receding glacier

the equl line moves to a lower elevation

Types of glaciers: Whether or not a glacier is restricted to a valley- is the basis for classifying glaciers by form

-A valley glacier is a glacier that is confined to a valley and flows from a higher to a lower elevation. Like streams, small valley glaciers may be tributaries to a larger trunk system. Vally glaciers are prevalent in areas of alpine glaciation. Most glacier in the us and canada, being in mountains are of the valley type.

AN actively flowing glacier brings debris to its terminus. if the terminus remains stationary for a few years or advances, a distinct end moraine, a ridge of till, piles up along the front edge of the ice.

-Valley glacier=end moraines that are crescent shaped or smoetimes horseshoe shaped. -end moraine of an ice sheet takes a similar but longer/irregular form than that of a valley glacier.

Glacial Eroision- whereever basal sliding takes place, te rock beneath the glacier is abraded and modified. As melt water works into cracks in bedrock and refreezes, piece of rock are broken loose and froze into the base of the moving glacier, a process knowns as plucking.

-While being dragged along by moving ice, the rock within the glacier grinds away at the underlying rock. The thicker the glacier, the more pressure on the rock and the more effective the grinding and crushing. Peebles and boulders that are dragged along are faceted which means they are given a flat surface by abrasion. Bedroock underlying a glacier is polished by fine particles and striated (scratched) by sharp edged larger particles. Striations and grooves on bedrock indicate the direction of ice movement.

An outwash feature of unusual shape associated with former ice sheets and some very large valley glaciers is an esker, a long sinuous ridge of water deposited sediment

-eskers deposited in tunnels within or under glaciers, where meltwater loaded with sediment flows under and out of the ice.

-If the terminus of an ice sheet is on land, it will advance or retreate in response to a positive or negative budget, just as for a valley glacier. -If the terminus is at the continents shoreine(like in antarcitc and greenland ice sheets) a positive budget results in a greater volume of icebergs calcing into the sea

...

A glacier deposits unsorted rock debris or till, which contrast sharply with the sorted and layered deposits of glacial outwash. TIll forms various types of moraines. Fine silt and clay may settle as varves in alake in front of a glacier, each pair of layers representing a years accumulation.

...

Frost wedging works on the rock exposed above the glacier, steeping n cutting back the side walls of the valley. sharp ridges called aretes separate adjacent glacially sides.

...

Glaciers- where they are, how they form and move.

...

Present only___ % of the land surface of the earth is covered by glaciers

10

the last episodes of extensive glaciation in NA as it its peak about ____ years ago

18000

Summary- a glacier is a large, long lasting mass of ice that forms on land and movies under its own weight. a glacier can form wherever more snow accumulates than is lost. Ice sheets and valley glaciers are the most common type types of glaciers. Glaciers move downward from where the most snow accumulates toward where the most ice is ablated.

A glacier moves by both basal sliding and internal flow. The upper portion of a glacier tends to remains rigid and is carried along by ice flow beneath it. Glaciers advance and recent in response to changes in climate. A receding glacier has a negative and an advancing one has a positive budget. A glacier budget for the year can be determined by noting the relative position of the equilibrium line, which separates the zone of accumulation/ablation.

Snow recrystallizes into firn, which eventually becomes converted to glacier ice. Glacier ice is lost(ablated) by melting, breaking off of icebergs and direct evaporation of the ice into the air. A glacier erods by plucking and the grinding action of the rock it carries. The grinding produces rock flour and faceted and polished rock fragments. Bedrock over which a glacier moves is generally polished, stretched, an grooved.

A mtn area showing the erosional effects of alpine glaciation possesses relatively straight valleys with U shaped cross profiles. The floor of a glacial valley usually has a cirque at its head and descends as a series of rock steps. Small rock basin lakes are commonly found along the steps and in cirques. A hanging valley indicates that a smaller tributary joined the main glacier. A horn is apeak between separate cirques. Aretes usually separate adjacent glacial valleys.

Although flowage in ice sheets is away from central highs where snow accumulates, movement is not uniform. most of the flowage in the east antarctic ice sheet takes place in ice streams, where the movement is considerably faster than in adjoining ice, which is froze to its bed.

AN ice stram is often heavily crevasses, and its boundaries are determinable by the transition from crevassed to crevass free ice. -south pole: neith the thickest part nor the center of the east antarctic ice sheet- the ice sheet is 2700 m thick. the thickest part of the east antarctic ice sheet is 4776 meters thick. Most of the movement of the east antarctic ice sheet is by the means of plastic flow. It has been though that most of the ice sheet is frozen to the underlying rocks and basal sliding takes place only locally. -Scientists have shown that an ice sheets grows not only by snow fall accumulation at its surface but also by accumulation of ice at its base.

There are two types of glaciated terrain on the Earth's surface: alpine and continental glaciation.

Alpine glaciation is found on mountainous regions. Continental glaciation exists where a large part of a continent is covered by glacial ice.

Advancing or receding glaciers are significant indicators of climatic change.

An advancing glacier does not mean the climate is getting colder-it could mean that the climate is getting wetter, more precipitation is falling during the winters months or the summers are cloudier. -its estimated that a worldwide decrease in teh mean annual temp about 5 C could bring about a new ice age. Conversely global climate waming can significantly reduce the size and number of glaciers.

Movement of Ice Sheets: An ice sheet or ice cap moves like a valley glacier except that it moves downward and outward form a central high area toward the edges of glacier.

Antarctica has two ice sheets: the West Antarctic Ice Sheet is separated by the transartic mtns from the much larger east antarctic ice sheet. The two ice sheets join in the low areas between mtn ranges. Both are nearly completely within a zone of accumulation because so little melting takes place (ablation is largely calving ice bergs) and because occasional snowfalls nourish their high central part.s Much of the base of the west antarctic ice sheet is on bedrock that is below sea level. at least one active volcano underlies the WAIS(resulting in depression in the ice sheet). Where mtn rangers are higher than ice sheets, the ice flows between mtns as valley glaciers, known as outlet glaciers.

Glaciation is most extensive in polar regions, where little melting takes place at any time of the year. Currently, 1/10 of the land surface on earth is covered by glaciers by glaciers-[compare with about one third during the peak of the glacial ice is on ages].

Approx 85% of the present day glacier ice is on the antarctic continent, covering an area larger than the combined areas of western europe and the us. 10% is in Greenland. -All the remaining glaciers of the world amount to only about 5 % of the worlds freshwater ice. -->This means that Antarctica is in fact storing most of earths fresh water in the form of ice. --> Note: if all of the ice in Antarctica would melt, it would rise over top of the statue of liberty (213 ft).

-the compacted mass of granular snow, transitional between glacier ice and snow is called firn.

Firn is analogous to a sedimentary rock such as a weakly cemented sandstone. Firn becomes more deeply buried as more snow accumulates. More air is expelled, the remaining pore space is greatly reduced, and granules forced together recrystallizes into the tight, interlocking mosaic of glacier ice. The recrystallizaiton process involves little or no melting and is comparable to metamorphism. Glacier ice is texturally similar to metamorphic rock, quartzite.

The terminus is the lower edge of a glacier. Its position reflects the glaciers budget.

For a valley glacier, a positive budget results in the terminus moving down valley. In a receding glacier, the terminus melts back upvalley. Because glacial ice moves slowing, migration of the terminus tends to lag several years behind a change in the budget .

Crevasses also form along the margin of glaciers in places where he path is curved because ice(like water) flows faster towards the outside of the curve.

For glaciers in temperate climate, a crevasse should be no deeper than about 40 meters the usual thickness of the rigid zone.

Indirect Effects of Past Glaciation continental ice sheets wasted away leaving rives that were once nothing but a trinkle were huge in glacial ages. other rivers were blocked bu the ice sheet or clogged by debris.

Glacial Lakes- retreating laurentide ice sheet was a dam for an enormous lake- glacial lake aggassiz. Canadas lake winnipeg is the largest remnat of lake aggassiz; glacial lake- glacial lake missoula(ice sheet advanced into montana)

Erosional Landscaes associated with Alpine Glaciation:

Glacial Valleys, Cirques, Horns, Aretes

A similar but smaller body is an ice cap.

Ice caps (& valley glaciers as well) are found in a few mtn highland in iceland and on islands in the arctic ocean, off canada, russia, and scandinavia.

an ice sheet is a mass of ice that is not restricted to a valley but covers a large area of land.

Ice sheets are associated with continental glaciation. Only two places on earth now have ice sheets- greenland and antarctica.

Tributary glaciers come together, the adjacent lateral moraines join and are carried down glacier as a single long ridge of til known as a medial moraine.

In a large trunk glacier that has formed form many tributaries, the numerous medical moraines give the glacier the appearance from the air of a multilane highway.

Formation and growth of glaciers: Snow converts to glacier ice in somewhat the same way that sediment turns into a sedimentary.

In a short time time, the snowflakes settle by compaction under their own weight. In warmer climates, partial thawing and refreezing result in a coarse granuels- the "corn snow" of spring skiing. In colder climates where little or no melting takes place, the snowflakes will recrystallize into fine granules

Glaciers with positive budgets push outward and downward at their edges- they are called advancing glaciers. -Those with negative budgets grow smaller and their edged melt back- receiving glaciers.

In receding glacier, the rate of flow of ice is insufficient to replace all of the ice lost in the lower part of the glacier. -If the amount of snow retained by the glacier equals the amount of ice and water lost, the glacier has a balanced budget and is neither advancing nor receding.

Some areas of past continental glaciation, there are bodies of till shaped into streamlined hills called drumlins.

Its shaped like an inverted spoon aligned parallel to the direction of ice movement of the former glacier. Its gentler end points in the down glacier direction.

As ice melts, rock debris that has been carried by a glacier is deposited to form a ground moraine- a fairly thin, extensive layer or blanket of till.

Large areas that were once covered by an ice sheet now have the generally rolling surface characteristic of ground moraine deposits.

Direct effects of past glaciation in north america- moving ice abrade vast areas of northern and eastern canada during the growth of the NA ice sheet-soil/sediment was scraped off, and underlying crystalline bedrock was scoured-many lake basins were gouged out of the bedrock.

Largest ice sheet-laurentide ice sheet. TIl deposited=hudson bay and great plains-most deposited as ground moraines with outwash deposits which equals great soil for agriculture. drumlins-ontarios, new england, NY(long island is made from terminal//recessional moraines). most states an provinces that were glaciated have thousands of lakes. mini-land of 10000 lakes; the finger lake, great lakes, ND,MN,Manitoba and saskatchemwas were covered by ice dammed lakes- largest lake agassiz. cirques and other features typical of valley glacier can be found in regions that currently have no glaciers- Appalachians and white mtns.

Moraines - when till occurs as a body of unsorted and unaltered debris either on a glacier, the body is regarded as one of several types of morianes.

Lateral moraies- are elongate, low mound of till that form along the dies of a valley glacier. rock fall debris form the steep cliffs that border valley glaciers accumulates alnog the edges of ice to form lateral moraines.

Pluvial lakes- lakes formed in the period of abundant rainfall,like in the western us during glacial ages; utah, nevada, cali. Great salt lake is a remant of a larger body of fresh water called bonnevile. even death valley- was once occupied by deep lake during th epleitocnet.

Lowering and Rising of Sea Level- the water was borrowed from the ocean. stream channels have been charted in the present continental shelves- today continuation of todays major rivers and streams. Teeth and bone have been found on the Atlantic continental shelf. -A foird is a coastal inlet that is drowned glacially carved valley. common:AK, British Columbia, Chile, New Zealand, Norway. THeser are evidence that valleys eroded by past glaciers were later partly submerged by rising sea level. --Crustal Rebound: the weight of an ice sheet seen thousands meters thick depressed the crust of a person depressing a mattress. A land surface bearing the weight of a continent ice sheet may be depressed several hundred meters-Once the glacier is gone, the land begins to rebound. great lakes is still doing this.

Not all glacier associated with erosion is caused by glaciers.

Mass wasting takes place on steep slopes. -frost wedging breaks up bedrock ridges and cliffs above a glacier(leads to rockfalls). -snow avalanches- bring down loose rock. If rocks collect in the zone of accumulation, may be incorporated into the body of the glacier. If rocks fall in the zone of ablation-they will ride on te glacial ice surface.

The effects of glaciers on our lives today: Much of the fertile soil of the northern great plains in the us developed on the loose debris transported by glaciers that moved from northern California.

More examples: Thick blankets of sediment left in the Midwest tore vast amounts of groundwater. -Great Lakes and thousands of lakes in minnesota.

Glacial Deposition: rock frags scraped and plucked from the underlying bedrock and carried along at the base of the ice melt up most of the load carried by an ice sheet but only part of a valley glaciers.

Most rocks carried away form glaciers are angular. -debris is unsorted and clay sized to boulder and unlayered rock debris carried or deposited by a glacier is called till. -Glaciers can carry just about anything. An erratic is an ice transported boulder that has not been served from underlying bedrock.

Evidence for older glaciation: Evidence of older glaciation comes from rocks called till it-is lithified till. some places, tillite layers overlie surfaces of older rock that have been polished and started.

Oldest glaciaiton-ontario-around 2.3 billion years ago. Snowball Earth hypothesis-worlds oceans were frozen-eveidence includes tillites that were deposited near the equator. paleozoic glaciation provides strong support for plate tectonics-tillites indicated that south Africa, Australia, antarctica, and south america were joined.

Glacial carved valley are easy to see. A U shaped valley is characteristic of glacial erosion. V shaped=stream erosion.

Thicker glacier=more erosive force it exerts on the underlying valley floor, and the more bedrock is ground away. -A large trunk glacier erodes downward more rapidly and carves a deeper valley than do the smaller tributary glaciers that going it. After the glaciers disappear, these tributaries remain as hanging walls high above the main valley.

WHen the base pipe has moved down glacier, it indicated basal sliding,which is the sliding of the glacier as a single body over the underlying rock. A thin film of meltwater tht develops along the base from the pressure of the overlying glacier facilitates basal sliding.

Think of a larger bar of wet soap sliding down an inclined board.

The boundary between these two altitudinal zones of a glacier is an irregular line called the equilibrium line-> sometimes called the snow line or firn line, which marks the highest point at which the glaciers winter snow cover is lost during a melt season.

This line may shift up or down from year to year, depending on whether there has been more accumulation or more ablation. Its location indicates therefore whether a glacier has a positive or negative budget. An equilibrium line migrating upglacier over a period of years is a sign of negative budget, where an equilibrium migrating down glacier indicates that the glacier has a positive budget. If the line remains the same year after year, the glacier has a balanced budget.

Valley glaciers straighten the curves formed by running water because the mass of ice of a glacier is too sluggish and inflexible to move easily-the process of carving the sides of its valley, a glacier erodes or truncates the lower ends of ridges that extended to the valley-->

Truncated spurs are ridges that have triangular facets produced by glacial erosion.

Glaciers smooth/straighten valley walls, ice action leaves the surface of the underlying bedrock carved into a series of steps due to the variable resistance of bedrock of glacial erosion.

Water seeps into cracks in the bedrock, freeze there, and enlarge fracturs or makes new ones. Rock free into the base of the glacier grinds and loosens more pieces- after the ice has melted back, a chain of rock basin lakes(tarns) may occupy the depression carved out of the weaker rocks. series of small lakes-paternoster lakes. -areas where bedrock is more resistant to erosion stand out after glaciation as rounded knobs and elongated parallel to the direction of glacier flow.(also knowns as roches mtns)

Crevasses- along its length, a valley glacier moves at different rates in responses to changes in the steepness of the underlying rock. Typically, a valley glacier rides over a series of rock steps.

Where the glacier passes over a steep part of the valley floor, it moves faster. The upper rigid zone of ice, cannot stretch to move as rapidly as the underlying plastic flowing ice. Being brittle the ice of the rigid zone is broken by the tensional forces:Open fissures, or crevasses, develop

Glacial Action: North American National parks.

Yosemite Valley in California: Unlike stream carved valleys, yosemite valley is straight for long stretches. Its sides are steep and the valley floor is flat (it is U shaped rather than the characteristic V shape of a stream carved valley). The sediment beneath the vegetation in the valley floor is poorly sorted debris, unlike the sorted sediment deposited by a stream. -All of these things are evidence that Yosemite park was formed by glaciers.

2 special kinds of end moraines: Terminal moraine and a recessional moraine

a terminal moraine is the end moraine marking the farthest advance of a glacier. A recessional moraine is an end moraine is built while the terminus of a receding glacier remains temporarily stationary. A single recending glacier can build several recessional moraines.

How fast does the central part of a valley glaciers move compared to the sides of the glacier

faster