I ghiacciai e l’opera del ghiaccio

Presentazione sul tema: "I ghiacciai e l’opera del ghiaccio"— Transcript della presentazione:

1 I ghiacciai e l’opera del ghiaccio
Cap. 16 Press et al., Capire la Terra Cap. 31 Strahler, Geografia fisica

2 I ghiacciai Da neve a ghiaccio
Gli strati nevosi, soggetti a processi di costipamento costituiscono il nevato. Press et al., Capire la Terra

3 I ghiacciai Limite delle nevi permanenti
Lupia Palmieri, Parotto, Osservare e capire la Terra © Zanichelli editore 2010

4 BILANCIO DI MASSA GLACIALE
L’ablazione può avvenire per: Fusione Calving Sublimazione Erosione eolica

5 A trim line, also written as trimline, is a clear line on the side of a valley formed by a glacier. The line marks the most recent highest extent of the glacier. The line may be visible due to changes in color to the rock or to changes in vegetation on either side of the line. 15.1 Taku and Norris Glaciers, Juneau Icefield, Alaska (Aerial) Photograph by Peter L. Kresan The advancing terminus of the Taku Glacier (upper center) and the retreating terminus of the Norris Glacier (bottom) are shown in this aerial view. The convex profile of the terminus and lack of a trimline along the margins of the Taku Glacier are indicative of an advancing glacier. In fact, the glacier is plowing down forest on both sides of this advancing lobe. The concave profile of the terminus, the clearly visible trimline, and proglacial lake of the Norris Glacier are characteristic of a retreating glacier.

6 15.10 Athabasca Glacier, Columbia Icefield, Canada Photograph by Peter L. Kresan
The terminus of the Athabasca Glacier in 1985 can be compared to its position, marked by the monument, in Since 1945, the glacier has retreated 2950 ft (900 m). Total recession of the ice front from 1870 to 1881 is approximately 1 mi (1.6 km), about 50 ft (15 m) per year. The Athabasca Glacier is 3.7 mi (6 km) long. Meltwater from the glacier flows into the Sunwapta and Athabasca rivers and eventually into the Arctic Ocean via the Mackenzie River.

7 Ghiacciai vallivi e calotte glaciali

8 I ghiacciai vallivi I ghiacciai vallivi, o alpini, si originano ad elevate altitudini e defluiscono lungo valli scavate nelle rocce in posto Lupia Palmieri, Parotto, Osservare e capire la Terra © Zanichelli editore 2010

9 I ghiacciai vallivi: morfologia

10 Strahler, Geografia fisica. Ed. Piccin

11 15.18 Taku Glacier Terminus, Taku Inlet, Juneau, Alaska (aerial) Photograph by Peter L. Kresan
This aerial view (taken in the summer) shows the advancing terminus of the Taku Glacier as it enters Taku inlet near Juneau, Alaska. The convex (bulging) profile of the terminus and lack of a trimline along the margins of the Taku Glacier are indicative of an advancing glacier. In fact, the glacier is plowing down forest on both sides of this advancing lobe. In the distance, where the glacier bends to the left, the surface of the glacier becomes lighter and smoother as the rough, crevassed surface becomes covered with snow. This change in the surface of the glacier marks the snowline, above which snowfall from the previous winter does not melt entirely. Largest in the Juneau Icefield of southeastern Alaska, the Taku Glacier is a temperate, maritime glacier. It has advanced more than 6.8 kilometers since 1890 and 1.6 kilometers since Studies by the Juneau Icefields Research Program show that the Taku Glacier has a positive mass balance of meter per year for the period This positive accumulation of snow on the glacier accounts for its growth through the end of the 20th Century, even though many other glaciers have been retreating (Pelto & Miller, 1990).

12 Forme superficiali La superficie del ghiacciaio si comporta come un corpo rigido, i movimenti differenziali dei vari strati che lo compongono determinano la frantumazione del ghiaccio in blocchi dando origine ai crepacci, che spesso si incrociano isolando blocchi e formando profonde fenditure, i seracchi.

13 Movimento di un ghiacciaio vallivo

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15 I ghiacciai continentali o inlandsis
15.12 North Pole, Arctic Ocean Photograph by Yar Petryszyn Yamal, a Russian nuclear icebreaker, makes it to the North Pole, Arctic Ocean, in July. 15.13 North Pole, Arctic Ocean Photograph by Yar Petryszyn A break in sea ice at the North Pole exposes open water in the Arctic Ocean.

16 15.14 Flow of Sea Ice in Arctic Ocean Photograph by Yar Petryszyn
Flow of sea ice in Arctic Ocean mimics the movement of Earth's crustal plates. Note the oblique spreading centers connected by transpressional transform faults.

17 15.15 Mount Munson, Antarctic Photograph by Paul Fitzgerald
View east-northeast across the Queen Maud Mountains from Mount Munson, Antarctica. Mount Munson, Antarctic

18 Movimento di un ghiacciaio continentale

19 Prima della glaciazione:
Rilievi dalle forme morbide, valli a V In cosa si differenziano le valli glaciali dalle valli fluviali? Durante la glaciazione: Ghiacciai tributari e valle glaciale principale

20 Quando il ghiaccio scompare…emergono rilievi dalle forme aspre, circhi, valli sospese con cascate, valli a U Anche i ghiacciai tributari scavano dei truogoli ad U, ma questi hanno una sezione trasversale di ampiezza più ridotta ed il loro fondo è situato più in alto rispetto a quello delle valli principali, per cui sono chiamati truogoli sospesi.

21 Le valli glaciali sono tipicamente a U poiché il ghiaccio erode sia il fondovalle che i versanti.
Esse hanno un profilo ondulato perché il ghiacciaio tende ad accentuare le irregolarità. Spesso le valli fluviali si sovrappongono, al ritiro del ghiaccio, alle preesistenti valli glaciali. Dove due o più ghiacciai confluiscono si possono formare valli sospese.

22 15.21 Glacial Carved Valley of Glen Coe, Scotland Photograph by Peter L. Kresan
This is a view to the west up Glen Coe, a classic example of a U-shaped valley. In the foreground is a glacial polished surface of 380 million year old andesitic volcanic rock. About 3 million years ago a deterioration of climate led to the build up of glaciers and ice sheets in Scotland. The last glaciation began about 27,000 years ago, reached its maximum about 18,000 years ago, and melted away by about 10,000 years ago, leaving this landscape.

23 15.4 Half Dome and Tenaya Canyon, Yosemite, California Photograph by Peter L. Kresan
From Glacier Point in Yosemite, one can easily imagine Tenaya Canyon partially filled with ice 15,000 years ago, just like the Great Gorge in Alaska is today (see Slide 15.3). Immediately to the left of this view, Tenaya and other canyons join to form Yosemite Valley. The U-shape profile of Tenaya Canyon is characteristic of glacial carved valleys. Till and outwash deposits level out the floor of the valley.

24 15.3 The Great Gorge, Ruth Glacier, Alaska Range (Aerial) Photograph by Peter L. Kresan
About 15 mi up the Ruth Glacier from the location shown in Slide 15.2 is the Great Gorge. This is a wonderful analog to what Tenaya Canyon and Yosemite Valley must have looked like during the last ice age (see Slide 15.4). Work reported in Earth magazine (March 1993) indicates that the Great Gorge is the deepest gorge in North America-3800 ft (1160 m) deep. The vertical relief from the top of Mt. Dickey (upper right) to the bedrock floor of the gorge is 9000 ft (2740 m) .

25 L’evoluzione di una valle glaciale
Il ghiaccio ha una densità tale che, quando galleggia, da 3/4 a 9/10 della sua massa si trovano sotto il livello del mare. Quindi, un ghiacciaio può erodere ben sotto il livello del mare!

26 15.22 Sognefjord near Aurland, Norway Photograph by Peter L. Kresan
This is a view of the Aurland arm of the Sognefjord, the world’s longest fjord, in the heart of Fjordland, southwestern Norway. Glaciers retreated from this region about 9,000 to 10,000 years ago, leaving a dramatic landscape of fjords, waterfalls, and rounded mountain tops and plateaus. Fjords typically mark the location of faults or fracture systems in the bedrock. Fjords open to the ocean. Their dimensions are difficult to imagine. Glacial sediments and water partially fill the fjord to depths of hundreds of meters. One section of the Sogne Fjord has sides that tower 1,220 meters above the water level and extend another 1,220 meters below. To reach open ocean from the location of this image, you would travel close to 200 kilometers in a jet boat.

27 15.9 McCarty Ford, Kenai Peninsula, Alaska (Aerial) Photograph by Peter L. Kresan
The Harding Icefield on the Kenai Peninsula of Alaska feeds the McCarty Tidewater Glacier, which cut this 23-mi (37-km) -long fjord. Since 1860, the McCarty Glacier has retreated 17 mi (27 km) to its present position at the head of the fjord. Most tidewater glaciers in Alaska have undergone rapid retreat during the twentieth century.

28 Azione geomorfologica: forme di erosione
I ghiacciai esercitano un’azione erosiva potente ed efficace sulle rocce del fondo e sui versanti. Essa si esplica attraverso due processi: La rimozione dei frammenti già disgregati L’estrazione di detriti dai versanti e dal fondo. I detriti sono trasportati nella massa di ghiaccio o sulla sua superficie. L’azione erosiva sul fondo è detta esarazione. Le rocce vengono levigate e striate solo sul lato rivolto verso monte, sono così asimmetriche e vengono dette rocce montonate.

29 15.7 Glacial Striations and Polish, Athabasca Glacier, Columbia Icefield, Canada Photograph by Peter L. Kresan Glacier-striated and -polished bedrock is exposed beneath a mantle of till at the terminus of the Athabasca Glacier in the Rocky Mountains, Canada. This photo was taken on top of a bedrock hill, probably a roche moutonné, that has just become exposed as the glacier retreats (see Slide 15.10).

30 15.20 Exposed Bedrock along the Flank of the Mendenhall Glacier, Juneau, Alaska Photograph by Peter L. Kresan This exposure of ice-polished bedrock near the terminus of the Mendenhall Glacier is within a trimline which parallels the glacier’s flank. The trimline is barren of vegetation and exposes bedrock and glacial till. A trimline forms during a period of recent glacial ice retreat. Over a period of decades, the trimline will become progressively more obscure as vegetation expands into this disturbed area. So in contrast to the Taku Glacier, the Mendenhall is retreating. Additional evidence for its retreat is found beyond the glacial terminus behind the person in the upper right, where thirteen distinct recessional moraines mark different stages of the retreat. In 1750, during the Little Ice Age, the Mendenhall Glacier extended 4 kilometers farther down the valley.

31 Massi erratici 15.6 Glacial Erratic, South Bubble, Acadia, Maine Photograph by Peter L. Kresan The speckled white color of this glacial erratic does not match the pink granitic rock of the South Bubble in Acadia National Park, near Bar Harbor, Maine. The closest outcrop of bedrock matching the boulder is many miles to the north.

32 Forme di deposito

33 15.2 Ruth Glacier, Alaska Range (Aerial) Photograph by Peter L. Kresan
This low aerial oblique view down the Ruth Glacier in Alaska shows medial and lateral moraines. The Ruth Glacier is a valley or piedmont glacier formed by the confluence of many smaller glaciers.

34 15.19 Lateral Moraine, Tasman Glacier, Mount Cook National Park, New Zealand Photograph by Peter L. Kresan This long lateral moraine along the west flank of the Tasman Glacier was formed when the glacier was much thicker during its last advance. Starting in the 1890s, the ice began its retreat and the rock debris, plastered along the mountain flank, collapsed to form this ridge.

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36 I cambiamenti climatici
La temperatura atmosferica media annua degli ultimi anni è variata da condizioni di clima più freddo (periodi glaciali) a condizioni di clima più caldo (periodi interglaciali) Lupia Palmieri, Parotto, Saraceni, Strumia, Scienze integrate © Zanichelli editore 2010 36 36

37 I cambiamenti climatici
L’andamento della concentrazione di anidride carbonica nell’atmosfera da anni fa a oggi Lupia Palmieri, Parotto, Saraceni, Strumia, Scienze integrate © Zanichelli editore 2010 37 37

38 15.16 Ice Lab at the Department of Geosciences, University of Arizona Photograph by Peter L. Kresan
Sublimation of a 100,000-year-old Vostok ice core from Antarctica in the Ice Lab at the Department of Geosciences, University of Arizona. Trapped gases within the ice, such as carbon dioxide, are collected and studied to reconstruct past levels of the gases in Earth's atmosphere. 15.17 Lemon Glacier, Juneau Icefield, Alaska Photograph by Peter L. Kresan Snow accumulation study on the Lemon Glacier, Juneau Icefield, Alaska. The thin bluish ice layers mark past summers' melt surfaces.

39 I cambiamenti climatici
Le due fotografie, scattate dallo stesso punto di osservazione nel 1910 e nel 2001, mostrano di quanto si è ridotto il Ghiacciaio di Tschierva, nelle Alpi svizzere Lupia Palmieri, Parotto, Saraceni, Strumia, Scienze integrate © Zanichelli editore 2010 39 39

40 Variazioni rapporto isotopico O16/O18
18O is two neutrons heavier than 16O and causes the water molecule in which it occurs to be heavier by that amount. The addition of more energy is required to vaporize H218O than H216O, and H218O liberates more energy when it condenses. In addition, H216O tends to diffuse more rapidly. The 18O/16O ratio provides a record of ancient water temperature. Water 10 to 15 degrees Celsius (18 to 27 degrees Fahrenheit) cooler than present represents glaciation. Precipitation and therefore glacial ice contain water with a low 18O content. Since large amounts of 16O water are being stored as glacial ice, the 18O content of oceanic water is high. Water up to 5 degrees Celsius (9 °F) warmer than today represents an interglacial, when the 18O content of oceanic water is lower. A plot of ancient water temperature over time indicates that climate has varied cyclically, with large cycles and harmonics, or smaller cycles, superimposed on the large ones. This technique has been especially valuable for identifying glacial maxima and minima in the Pleistocene.

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46 Glaciazioni europee Modello riferibile al versante settentrionale delle Alpi  Pleistocene inferiore interglaciazione Donau-Günz ( anni fa) glaciazione Günz ( anni fa) Pleistocene medio interglaciazione Günz-Mindel ( anni fa) glaciazione Mindel ( anni fa) interglaciazione Mindel-Riss ( anni fa) glaciazione Riss ( anni fa) Pleistocene superiore interglaciazione Riss-Würm ( anni fa) glaciazione Würm ( anni fa)

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48 Il sollevamento isostatico post-glaciale