Physical Geography - Version 1 Unit 19 Shaped by Glaciers

Figure Field Trip at Lake Sabrina , Bishop California . Image by Jeremy is used under a license . UNIT 19 SHAPED BY GLACIERS Goals Objectives of this unit Describe the timing and extent of Earth past , going as far back as the early . Describe the important geological events that led up to the and how the orbital variations controlled the timing of those . Explain the differences between continental and alpine glaciation . Describe and identify the various related to alpine glacial erosion , including valleys , horns , hanging valleys , truncated spurs , and . Identify various types of glacial lakes , including , Finger Lakes , and kettle lakes . GEOGRAPHY

GLACIERS A glacier is a body of ice ( decades or more ) that is large enough ( at least tens of meters thick and at least hundreds of meters in extent ) to move under its own weight . About 10 of Earth land surface is currently covered with glacial ice , and although the vast majority of that is in Antarctica and , there are many glaciers in Canada , especially in the mountainous parts of , and Yukon and in the far north . At various times during the past million years , glacial ice has been much more extensive , covering at least 30 of the land surface at times . Glaciers represent the largest repository of freshwater on Earth ( 69 of all freshwater ) and they are highly sensitive to changes in climate . In the current warming climate , glaciers are melting rapidly worldwide , and although some ofthe larger glacial masses will last for centuries more , many smaller glaciers will be gone within decades , and in some cases , within years . Glaciers have long played a role in the geologic history of California . In the past , glaciers were active in several areas of California , leaving behind geologic evidence such as glacial deposits , mountain , and glacial striations . In northern California , evidence from glaciers during the is found at Mount , Volcano , throughout the Mountains , Medicine Lake Volcano , and the Coast ranges . In central California evidence from glaciers can be found in the Sierra Nevada and The White Mountains . The only evidence of glaciation in southern California has been found in the San Mountains . During the cold epoch ( Ice Age ) which lasted between million years ago to years ago , glaciers fluctuated in size . Geologic evidence in the Sierra Nevada suggests at least five major glacial periods McGee , Tahoe , and Recess Peak . Similar evidence of glacier fluctuation is found in other regions of California . In the , the climate began to cool around 1350 , during which time glaciers began to grow . This cool period , referred to as the Little Ice Age , persisted roughly through 1850 , when glaciers are thought to have reached their maximum extent . Since that time , mountain glaciers in California and throughout most ofthe world have shown signs of overall recession during the past century . Glacial Periods in Earth History We are currently in the middle of a glacial period ( although it less intense now than it was years ago ) but this is not the only period of glaciation in Earth history there have been many in the distant past . In general , however , Earth has been warm enough to be for much more of the time than it has been cold enough to be . GEOGRAPHY

4570 Ma 3850 Ma 2500 Ma 540 Ma I 700 Ma ' I Figure The Record Past during Earth History . Image by Steven , Cab , Cambrian Si ran Permian Jurassic Cretaceous Quaternary The oldest known glacial period is the . Based on the evidence of glacial deposits from the area around Lake Huron in and Michigan , it is evident that the Glaciation lasted from approximately to Ma . Because rocks of that age are rare , we know much about the intensity or the global extent of this glaciation . Late in the , for reasons that are not fully understood , the climate cooled dramatically , and Earth was seized by what appears to be its most intense glaciation . The of the Period ( cryo is Latin for icy cold ) are also known as the Snowball Earth because it is hypothesized that the entire planet was frozen even in equatorial regions with ice on the oceans up to thick . A visitor to our planet at that time might not have held out much hope for its inhabitability , although life still survived in the oceans . There were two main glacial periods within the , each lasting for about 20 million years the at around 700 Ma and the at 650 Ma . There is also evidence of some shorter both before and after these . The end ofthe coincides with the evolution of relatively large and complex life forms on Earth . This started during the Period , and then continued with the explosion of life forms in the Cambrian . Some geologists think that the changing environmental conditions of the are what triggered the evolution of large and complex life . There have been three major during the Phanerozoic ( the past 540 million years ) including the ( recorded in rocks of South America and Africa ) the ( named for rocks in southern Africa ) and the . The was the longest of the Phanerozoic , persisting for much of the time that the supercontinent was situated over the South Pole ( to 260 Ma ) It covered large parts of Africa , South America , Australia , and Antarctica . As you might recall from Chapter 10 , this widespread glaciation , across continents that are now far apart , was an important component of Alfred evidence for continental drift . Unlike the , the GEOGRAPHY

, and only affected parts of Earth . During times , for example , what is now North America was near the equator and remained unglaciated . Earth was warm and essentially unglaciated throughout the . Although there may have been some alpine glaciation at this time , there is no longer any record of it . The dinosaurs , which dominated terrestrial habitats during the , did not have to endure icy conditions . A warm climate persisted into the there is evidence that the ( to 60 Ma ) was the warmest part of the Phanerozoic since the Cambrian . Several tectonic events during the contributed to persistent and significant planetary cooling since 50 Ma . For example , the collision of India with Asia and the formation of the Himalayan range and the Tibetan Plateau resulted in a dramatic increase in the rate of weathering and erosion . Higher than normal rates of weathering of rocks with silicate minerals , especially feldspar , consumes carbon dioxide from the atmosphere and therefore reduces the greenhouse effect , resulting in cooling . Global Surface Temperature ( co , 500 HI . Global Surface Temperature ) 60 50 40 30 20 10 Time ( Ma ) Figure The Global Temperature Trend over the Past 65 Ma , the . Image by Steven , BY . At 40 Ma , ongoing plate motion widened the narrow gap between South America and Antarctica , resulting in the opening of the Drake Passage . This allowed for the unrestricted flow of water around Antarctica , the Antarctic Circumpolar Current , which effectively isolated the Southern Ocean from the warmer waters of the Pacific , Atlantic , and Indian Oceans . The region cooled significantly , and by 35 Ma ( glaciers had started to form on Antarctica . GEOGRAPHY

Figure The Antarctic Circumpolar Current Prevents Warm Water from the Rest of Earth Oceans from Traveling to Antarctica . Image by Steven , BY . Global temperatures remained relatively steady during the and early , and the Antarctic glaciation waned during that time . At around 15 Ma , volcanism between central and South America created the connection between North and South America , preventing water from flowing between the Pacific and Atlantic Oceans . This further restricted the transfer of heat from the tropics to the poles , leading to a rejuvenation of the Antarctic glaciation . The expansion ofthat ice sheet increased Earth reflectivity enough to promote a positive feedback loop of further cooling more reflective glacial ice , more cooling , more ice , etc . By the ( Ma ) ice sheets had started to grow in North America and northern Europe . The most intense part of the current glaciation and the coldest climate has been during the past million years ( the last of the ) but if we count Antarctic glaciation , it really extends from the to the , and will likely continue . The has been characterized by significant temperature variations ( through a range of almost ) on time scales of to years , and corresponding expansion and contraction of ice sheets . These variations are attributed to subtle changes in Earth orbital parameters ( cycles ) which are explained in more detail in this unit . Over the past million years , the glaciation cycles have been approximately years . GEOGRAPHY

14 Global temperature based on isotopes of 100 Average global temperature ( ox on I Years before present Figure Forum Oxygen Isotope Record for the past Million Years , Based on Data from Sediments . Image by Steven , BY . At the height of the last glaciation ( Wisconsin Glaciation ) massive ice sheets covered almost all of Canada and much of the northern United States . The massive Ice Sheet covered most of eastern Canada , as far west as the Rockies , and the smaller Cordilleran Ice Sheet covered most of the western region . At various other glacial peaks during the and , the ice extent was similar to this , and in some cases , even more extensive . The combined and Cordilleran Ice Sheets were comparable in volume to the current Antarctic Ice Sheet . Pin It ! Global Ice Viewer ! View this website to access the Global Ice Viewer . NASA has provided imagery to allow us to view glaciers , Iceland , the Arctic and Antarctic regions . GEOGRAPHY

Steven , HOW GLACIERS WORK There are two main types of glaciers . Continental glaciers cover vast areas of land in extreme Polar Regions , including Antarctica and . Alpine glaciers , otherwise known as valley glaciers , originate on mountains , mostly in temperate and Polar Regions , but even in tropical regions if the mountains are high enough . Earth two great continental glaciers , on Antarctica and , comprise about 99 of all of the world glacial ice , and approximately 68 of all of Earth freshwater . The Antarctic Ice Sheet is vastly bigger than the Ice Sheet it contains about 17 times as much ice . If the entire Antarctic Ice Sheet were to melt , sea level would rise by about 80 and most of Earth major cities would be submerged . GEOGRAPHY

South Pole 4000 at 90 Antarctic Ice sheet ' Both drawings profile at Same Scale at 72 1000 Steven , Continental glaciers do not flow because the large areas that they cover are generally flat . Instead , ice flows from the region where it is thickest toward the edges where it is thinner . This means that in the central thickest parts , the ice flows almost vertically down toward the base , while in the peripheral parts , it flows out toward the margins . In continental glaciers like Antarctica and , the thickest parts ( and respectively ) are the areas where the rate of snowfall and therefore of ice accumulation are highest . 4000 Figure Schematic Diagram . Image by Steven , The flow of alpine glaciers is primarily controlled by the slope of the land beneath the ice . In the zone of accumulation , the rate of snowfall is greater than the rate of melting . In other words , not all ofthe snow that falls each winter melts during the following summer , and the ice surface is always covered with snow . In the zone of ablation , more ice melts than accumulates as snow . The equilibrium line marks the boundary between the zones of accumulation ( above ) and ablation ( below ) GEOGRAPHY

equilibrium Above the equilibrium line of a glacier , not all of the winter snow melts in the following summer , so snow gradually accumulates . The snow layer from each year is covered and compacted by subsequent snow , and it is gradually compressed and turned into firn within which the snowflakes lose their delicate shapes and become granules . With more compression , the granules are pushed together , and the air is squeezed out . Eventually , the granules are ' welded together to create glacial ice . Downward percolation of water from melting taking place at the surface contributes to the process of ice formation . SHAPE DENSITY Snowflakes , 90 air , 50 air , Firn , 30 air , Ice , 20 bubbles , Figure Steps in the Process of Formation of Glacial Ice . by Steven , The equilibrium line of a glacier near Whistler , British , is seen below . Below that line , in the zone of ablation , bare ice is exposed because last winter snow has all melted above that line , the ice is still mostly covered with snow from last winter . The position of the equilibrium line changes from year to year as a function of the balance between snow accumulation in the winter and during the summer . More winter snow and less GEOGRAPHY

summer melting favor the advance ofthe equilibrium line ( and ofthe leading edge ) but of these two variables , it is the summer melt that matters most to a budget . Cool summers promote glacial advance and warm summers promote glacial retreat . Figure The Equilibrium Line in 2013 an the Overlord Glacier . Image by Steven , BY . Glaciers move because the surface of the ice is sloped . This generates stress on the ice , which is proportional to the slope and to the depth below the surface . The stresses are quite small near the ice surface but much larger at depth , and also greater in areas where the ice surface is relatively steep . Ice will deform , meaning that it will behave in a plastic manner , at stress levels of around 100 therefore , in the upper 50 to 100 of the ice , flow is not plastic ( the ice is rigid ) while below that depth , ice is plastic and will flow . When the lower ice of a glacier flows , it moves the upper ice along with it , so although it might seem from the stress patterns ( red numbers and red arrows ) that the lower part moves the most , in fact while the lower part deforms ( and flows ) and the upper part does deform at all , the upper part moves the fastest because it is pushed along by the lower ice . The plastic lower ice of a glacier can flow like a very viscous fluid and can therefore flow over irregularities in the base of the ice and around corners . However , the upper rigid ice can not flow in this way , and because it is being carried along by the lower ice , it tends to crack where the lower ice has to flex . This leads to the development of crevasses in areas where the rate of flow of the plastic ice is changing . In the area shown below , for example , the glacier is speeding GEOGRAPHY

up over the steep terrain , and the rigid surface ice has to crack to account for the change in velocity . Figure an a Glacier in Alaska . used under Google Earth reproduction guidelines The base of a glacier can be cold ( below the freezing point of water ) or warm ( above the freezing point ) If it is warm , there will likely be a film of water between the ice and the material underneath , and the ice will be able to slide over that surface . This is known as basal sliding . If the base is cold , the ice will be frozen to the material underneath and it will be stuck unable to slide along its base . In this case , all of the movement of the ice will be by the internal flow . One of the factors that affect the temperature at the base of a glacier is the thickness of the ice . Ice is a good insulator . The slow transfer of heat from Earth interior provides enough heat to warm up the base ofthe ice is thick , but not enough if it is thin and that heat can escape . It is typical for the leading edge of an alpine glacier to be relatively thin , so it is common for that part to be frozen to its base while the rest of the glacier is still sliding . Because the leading edge of the glacier is stuck to its frozen base , while the rest continues to slide , the ice coming from behind has pushed ( or thrust ) itself over top of the part that is stuck fast . Glacial ice always moves downhill , in response to gravity , but the front edge of a glacier is always either melting or calving into water ( shedding icebergs ) If the rate of forwarding motion of the glacier is faster than the rate of ablation ( melting ) the leading edge of the glacier advances ( moves forward ) If the rate of forwarding motion is about the same as the rate of ablation , the leading edge remains stationary , and if the rate of forwarding motion is slower than the rate of ablation , the retreats ( moves backward ) GEOGRAPHY

Glacial Erosion Glaciers are effective agents of erosion , especially in situations where the ice is not frozen to its base and can therefore slide over the bedrock or other sediment . The ice itself is not particularly effective at erosion because it is relatively soft ( hardness at ) instead , it is the rock fragments embedded in the ice and pushed down onto the underlying surfaces that do most of the erosion . A useful analogy would be to compare the effect of a piece of paper being rubbed against a wooden surface , as opposed to a piece of sandpaper that has embedded angular fragments of garnet . The results of glacial erosion are different in areas with continental glaciation versus alpine glaciation . Continental glaciation tends to produce relatively flat bedrock surfaces , especially where the rock beneath is uniform in strength . In areas where there are differences in the strength of rocks , a glacier tends to erode the softer and weaker rock more effectively than the harder and stronger rock . Much of central and eastern Canada , which was completely covered by the huge Ice Sheet at various times during the , has been eroded to a relatively flat surface . In many cases , the existing relief is due to the presence of glacial deposits such as , and ( all discussed below ) rather than to differential erosion ALPINE GLACIERS Alpine glaciers produce very different topography than continental glaciers , and much of the topographic variability of western Canada can be attributed to glacial erosion . In general , glaciers are much wider than rivers of similar length , and since they tend to erode more at their bases than their sides , they produce wide valleys with relatively flat bottoms and steep sides known as valleys . Yosemite National Park was occupied by a large glacier . Glacial systems reached depths of up to feet ( and left their marks in the Yosemite area . The longest glacier in the Yosemite area ran down the Grand Canyon of the River for 60 miles ( 97 ) passing well beyond Valley . Glacier flowed out of Yosemite Valley and into the River Gorge . Lee Glacier carved Lee Canyon and emptied into Lake Russel ( the ice age version of Mono Lake ) Only the highest peaks , such as Mount Dana and Mount , were not covered by glaciers . Retreating glaciers often left recessional that impounded lakes such as the miles ( long Lake Yosemite GEOGRAPHY

Vi . Figure Areal View of Yosemite Valley . used under Google Earth reproduction guidelines valleys and their tributaries provide the basis for a wide range of alpine glacial topographic features , examples of which are visible on the International Space Station view of the Swiss Alps . This area was much more intensely during the last glacial maximum . At that time , the large valley in the lower right was occupied by glacial ice , and all of the other glaciers shown here were longer and much thicker than they are now . But even at the peak ofthe Glaciation , some of the higher peaks and ridges would have been exposed and not directly affected by glacial erosion . A peak that extends above the surrounding glacier is called a nunatak . In these areas , and the areas above the glaciers today , most of the erosion is related to effects . Some ofthe important features visible in the image below are sharp ridges between shaped glacial valleys low points along that constitute passes between glacial valleys horns steep peaks that have been glacially and eroded on three or more sides basins that form at the head of a glacial valley hanging valleys shaped valleys of tributary glaciers that hang above the main valley because the larger valley glacier eroded more deeply into the terrain and truncated spurs ( spurs ) the ends of that have been eroded into steep cliffs by the glacier in the corresponding main valley . GEOGRAPHY

glacial valley , Hanging valley Figure A the International Space Station ofthe Swiss Alps . Image by Steven , BY . Several other glacial erosion features exist at smaller scales . For example , a drumlin is an elongated feature that is streamlined at the end . The image below shows a drumlin , and is larger than most , and is made up almost entirely of rock . made up of glacial sediments are very common in some areas of continental glaciation if . I . Bower Island , a Drumlin in Howe Sound , Canada . Image by Steven , BY . A is another type of elongated erosional feature that has a steep and sometimes jagged end . On a smaller scale still , glacial grooves ( tens of centimeters to meters wide ) and glacial striations ( millimeters to centimeters wide ) are created by fragments GEOGRAPHY

of rock embedded in the ice at the base of a glacier . Glacial striations are very common on rock surfaces eroded by both alpine and continental glaciers . Figure Glacial at Mount Rainier National Park . Image by Walter . Glacial Lakes Lakes are common features in glacial environments . A lake that is confined to a glacial cirque is known as a tarn . are common in areas of alpine glaciation because the ice that forms a cirque typically carves out a depression in bedrock that then fills with water . In some cases , a series of such basins will form , and the resulting lakes are called rock basin lakes or paternoster lakes . Figure Nate the Cirque the Lake Sabrina , a Glacial Tarn . Image by Jeremy is used under a license . GEOGRAPHY

A lake that occupies a glacial valley , but is not confined to a cirque , is known as a finger lake . In some cases , a finger lake is confined by a dam formed by an end moraine , in which case it may be called a moraine lake . In areas of continental glaciation , the crust is depressed by the weight of glacial ice that is up to thick . Basins are formed along the edges of continental glaciers ( except for those that cover entire continents like Antarctica and ) and these basins fill with glacial meltwater . Many such lakes , some ofthem huge , existed at various times along the southern edge of the Ice Sheet . One example is Glacial Lake , which formed within Idaho and Montana , just south of the . border with the United States . During the latter part of the last glaciation ( 30 ka to 15 ka ) the ice holding back Lake retreated enough to allow some ofthe lake water to start flowing out , which escalated into a massive and rapid outflow ( over days to weeks ) during which much ofthe volume of the lake drained along the valley ofthe Columbia River to the Pacific Ocean . It is estimated that this type of flooding happened at least 25 times over that period , and in many cases , the rate of outflow was equivalent to the discharge of all of Earth current rivers combined . The record of these massive floods is preserved in the Channelled of Idaho , Washington , and Oregon . Glacial Deposits Sediments transported and deposited during the are abundant throughout The United States . They are important sources of construction materials and are valuable as reservoirs for groundwater . Because they are almost all unconsolidated , they have significant implications for mass wasting . The Bering Glacier is the largest in North America , and although most of it is in Alaska , it flows from an that extends into southwestern Yukon . The surface of the ice is partially , or in some cases completely , covered with rocky debris that has fallen from surrounding steep rock faces . Muddy rivers are distributed from the glacier in several locations , depositing sediment on land , into Lake , and directly into the ocean . Dirty icebergs are shedding their sediment into the lake . And , not visible in this view , sediments are being moved along beneath the ice . GEOGRAPHY

Figure The Bering Glacier in Southeast Alaska . Image by Steven , The formation and movement of sediments in glacial environments is shown below . There are many types of glacial sediment generally classified by whether they are transported on , within , or beneath the glacial ice . The main types of sediment in a glacial environment are described below . En and su Lateral or medial moraine End glacial sediments Of Englacial sediments terminal moraine . GEOGRAPHY

Supraglacial ( on top of the ice ) and englacial ( within the ice ) sediments that slide off the melting front of a stationary glacier can form a ridge of unsorted sediments called an end moraine . The end moraine that represents the farthest advance of the glacier is a terminal moraine . Sediments transported and deposited by glacial ice are known as till . Subglacial sediment , such as till , is material that has been eroded from the underlying rock by the ice and is moved by the ice . It has a wide range of grain sizes , including a relatively high proportion of silt and clay . The larger ( pebbles to boulders in size ) tend to become partly rounded by abrasion . When a glacier eventually melts , the till is exposed as a sheet of sediment ranging from several centimeters to many meters in thickness . till is normally unbedded . An example is shown in . Supraglacial sediments are primarily derived from eroded material that has fallen onto the ice from rocky slopes above . These sediments form lateral and , where two glaciers meet , medial . Most of this material is deposited on the ground when the ice melts and is therefore called ablation till , a mixture of fine and coarse angular rock fragments , with much less sand , silt , and clay than till . When supraglacial sediments become incorporated into the body of the glacier , they are known as englacial sediments . Massive amounts of water flow on the surface , within , and at the base of a glacier , even in cold areas and even when the glacier is advancing . Depending on its velocity , this water can move sediments of various sizes and most of that material is washed out of the lower end ofthe glacier and deposited as outwash sediments . These sediments accumulate in a wide range of environments in the region ( the area in front of a glacier ) most in fluvial environments , but some in lakes and the ocean . sediments are similar to sediments deposited in normal fluvial environments and are dominated by silt , sand , and gravel . The grains tend to be moderately well rounded , and the sediments have similar sedimentary structures ( bedding , imbrication ) to those formed by glacial streams . A large plain of sediment is called a , otherwise known as an outwash plain , and within that area , deposits can be tens of meters thick . In situations where a glacier is receding , a block of ice might become separated from the main ice sheet and become buried in sediments . When the ice block eventually melts , a depression forms , known as a kettle , and ifthis fills with water , it is known as a kettle lake . GEOGRAPHY

Figure Kettle Lakes in The Peninsula . NASA image by Jesse Allen is in the public domain . A subglacial stream will create its channel within the ice , and sediments that are being transported and deposited by the stream will build up within that channel . When the ice recedes , the sediment will remain to form a long sinuous ridge known as an esker . are most common in areas of continental glaciation . They can be several meters high , tens of meters wide , and tens of kilometers long Figure Esker at , Western Sweden . Image by Hanna . Outwash streams commonly flow into lakes where glaciolacustrine sediments are deposited . These are dominated by and particles and are typically laminated on the millimeter scale . In some cases , develop are series of beds with distinctive summer and winter layers relatively coarse in the summer when melt discharge is high , and finer in the winter when discharge is very low . Icebergs are common in lakes , and most of them contain englacial sediments of various sizes . As the bergs melt , the released sink to the bottom and are incorporated into the glaciolacustrine layers as . GEOGRAPHY

The processes that occur in lakes can also take place where a glacier terminates in the ocean . The sediments deposited there are called glaciomarine sediments UNIT 19 SUMMARY There have been many in Earth distant past , the oldest known starting around Ma . The late Snowball Earth were thought to be sufficiently intense to affect the entire planet . The current glacial period is known as the Glaciation , and while it was much more intense years ago than it is now , we are still in the middle of it . The periodicity of the is related to subtle changes in Earth orbital characteristics , which are exaggerated by a variety of positive feedback processes . The two main types of glaciers are continental glaciers , which cover large parts of continents , and alpine glaciers , which occupy mountainous regions . Ice accumulates at higher elevations , above the equilibrium line , where the snow that falls in winter does not all melt in summer . In continental glaciers , ice flows outward from where it is thickest . In alpine glaciers , ice flows downslope . At depth in the glacier ice , flow is by internal deformation , but glaciers that have liquid water at their base can also flow by basal sliding . Crevasses form in the rigid surface ice in places where the lower plastic ice is changing shape . Glaciers are important agents of erosion . Continental glaciers tend to erode the land surface into flat plains , while alpine glaciers create a wide variety of different forms . The key feature of alpine glacial erosion is the valley . are sharp ridges that form between two valleys , and horns form where a mountain is glacially eroded on at least three sides . Because tributary glaciers do not erode as deeply as glaciers , hanging valleys exist where the two meet . On a smaller scale , both types of glaciers form , and glacial grooves or striations . Glacial deposits are quite varied , as materials are transported and deposited in a variety of different ways in a glacial environment . Sediments that are moved and deposited directly by ice are known as till . sediments are deposited by glacial streams , either forming or large plains known as . Table Table by Steven , BY Table from Foundation , GEOGRAPHY