Cummings 2016 GEO1111 Lecture 17 Glaciers

8/19/2019 Cummings 2016 GEO1111 Lecture 17 Glaciers

1/102

The cryosphere

1


2/102

The present volumeof ice on Earth

accounts for about

2% of water in any

form and contains

~80% of Earth’sfreshwater.

Bindschadler, 2004 2


3/102

Cryosphere: All frozen water on Earth. Includes glaciers, the main

component of the cryosphere, in addition to sea ice, lake and river ice,

permafrost, and seasonal snow cover.

Hydrosphere

Geosphere Atmosphere

Cryosphere Biosphere

3


4/102

Water is the only substance on the Earth’s surface that occurs

naturally in all three phases—solid (ice), liquid and gas (water

vapour).

4

Ice

Liquid

(NOTE: You can’t see the

water vapour…the

clouds are not water

vapour; they consist of

ice crystals and liquiddroplets)

Gas


5/102

Glaciers are the most obvious and important part of the cryosphere.

At present, they cover about 11% of the Earth’s surface.

Antarctic Ice Sheet

Greenland Ice Sheethttp://www.grida.no/graphicslib/detail/the-cryosphere-world-map_e290

Alpine glaciers

5


6/102

An additional 15% of the land mass is underlain by permafrost .

Permafrost is permanently frozen ground (i.e., temperature always <

0°C). It should not be confused with ground ice—ice that

commonly (but not invariably) occurs as layers and in pore spaces inpermanently frozen ground. Permafrost

http://www.grida.no/graphicslib/detail/the-cryosphere-world-map_e290

6


7/102

Permafrost depths range from metres to 100s of metres.

7

( m )


8/102

In contrast to glaciers and permafrost, the extent of snow cover fluctuates

dramatically on a seasonal basis. About 50% of the land mass in the Northern

Hemisphere becomes snow-covered in winter. Snow is less widespread in the

southern hemisphere because continents are located farther north, closer to

the equator. Only New Zealand, Antarctica and mountainous regions in the

Andes and SE Australia receive appreciable snow.

With the exception of the snow near the tops of mountain glaciers and on the higher parts of the Greenlandand Antarctic Ice Sheets, the snow that blankets the continents in winter melts almost entirely in summer. 8


9/102

January



Hemisphere becomes snow covered in winter. Snow is less widespread in the




With the exception of the snow near the tops of mountain glaciers and on the higher parts of the Greenlandand Antarctic Ice Sheets, the snow that blankets the continents in winter melts almost entirely in summer. 9


10/102

February

With the exception of the snow near the tops of mountain glaciers and on the higher parts of the Greenlandand Antarctic Ice Sheets, the snow that blankets the continents in winter melts almost entirely in summer.







10


11/102

March








11


12/102

April








12


13/102

May








13


14/102

June








14


15/102

July








15


16/102

August








16


17/102

September








17


18/102

October








18


19/102

November








19


20/102

December








20


21/102

The white, seasonal blanket of snow that spreads over the land

surface has a direct parallel in high-latitude oceans, namely sea ice.

Sea ice

http://www.grida.no/graphicslib/detail/the-cryosphere-world-map_e290

21


22/102

Most sea ice is seasonal. However, some remains year round,

especially in the Arctic.

Because it forms by freezing of

seawater, melting sea ice does

not contribute to sea level rise.

22


23/102

Most sea ice is seasonal. However, some remains year round,

especially in the Arctic.

23


24/102


25/102

Sea ice is thin. It is generally only several meters thick (average ~3

m). Permanent sea ice can reach thickness of 6 m.

25


26/102

Glaciers account for most of the volume of the cryosphere and

they are an important, dynamic component of Earth’s climate

system. The rest of this lecture will focus on them.

26


27/102

Viewed from space, the striking thing about glaciers, especially

where snow-covered (i.e., not bare ice), is the amount of light they

reflect. In other words, they have a high albedo relative to the

ocean and continents.

Albedo (% reflected light)

0

50

100

27

Albedo: the amount of light reflected

back into space off a given surface,

measured in percent.


28/102

ASIDE. The high albedo and considerable size of the Antarctic Ice

Sheet contributes to pronounced earthshine during our winters. 28


29/102

Glacier: A naturally accumulated ice mass that deforms

(flows) under its own weight.

Brodzikowski & Loon, 1991

29


30/102

Force Normal stress (σ) Shear stress (τ)


31/102

Shear

stress

Water and air are considered to be “normal” fluids (i.e.,

Newtonian fluids). They deform instantaneously with applied

shear and their viscosities (viscosity = the slope of the lines in

the graph below) remain the same as shear stress is increased.


32/102


33/102

Shear

stress

Rate of deformation

Newtonian fluid






34/102

Shear

stress

Rate of deformation






35/102

Shear

stress

Rate of deformation






36/102

Shear

stress

Rate of deformation






37/102

Shear

stress

Rate of deformation

Some fluids are non-Newtonian in that they exhibit shear

thinning behavior: their viscosities decrease as shear stress is

increased.

Wall paint


38/102

Shear

stress

Rate of deformation



increased.

Wall paint


39/102

Shear

stress

Rate of deformation



increased.

Wall paint


40/102

Shear

stress

Rate of deformation



increased, and vice versa. This behaviour helps prevent drips.

Wall paint

h h h h b h h k


41/102

Shear

stress

Rate of deformation

Others are non-Newtonian in that they exhibit shear thickening

behavior: their viscosities increase as shear stress is increased.

Corn starch & water (“ooblek”)


42/102

What about something like ice?

Solid? Fluid?

Some substances like ice are hard to describe. They can behave


43/102

Shear

stress

Rate of deformation


either as a solid or a fluid; they are said to be viscoelastic. Ice will

flow like a fluid, given enough time. It exhibits considerable shear

thinning behavior; as such, it only flows in earnest when shear

stress is sufficiently high stresses (e.g., an ice cube will sublimatelong before it flows, but ice that is >30 m thick will flow). If

stressed quickly, crystal dislocations will not have time to

propagate, and the ice will behave elastically or fracture like a

solid.


44/102



45/102

Shear

stress

Rate of deformation

y







solid.



46/102

Shear

stress

Rate of deformation

y







solid.



47/102

Shear

stress

Rate of deformation

y







solid.


48/102

For simplicity, sometimes ice is modelled as a plastic

material—one that will flow, but only once a yield stress has

been breached. A critical thickness of ~30 m is typically

assumed to provide the requisite yield stress.

Shear

stress

Rate of deformation

Yield

stress


49/102

There are two main types of glaciers, ice sheets, which are not

confined by topography, and alpine glaciers, which are.

49

Antarctic Ice Sheet

Greenland Ice Sheethttp://www.grida.no/graphicslib/detail/the-cryosphere-world-map_e290

Alpine glaciers


50/102

Broecker & Denton, 1992

There are two ice sheets today, one in Antarctica and one in

Greenland.

50


51/102

They are thick —over 4 km in places.

4 km3 km2 km

1 km

GreenlandAntarctica

51


52/102

Ice sheets are subdivided into domes—elevated “recharge”

areas from which ice flows from more or less radially.

52


53/102

Lecture 2: The cryo sphere

Types of glaciers

ice domes

53

Flow in the centre of ice sheets is slow and unconfined. At the extremities, however,


54/102

flow becomes focused into ice streams that form between areas of slower moving

ice. These ice streams flow one to several orders of magnitude faster than flow in

the central part of the ice sheet —over 1 km per year in places. In Antarctica, most

of the ice in the ice sheet is discharged to the ocean through ice streams.

54

Where an ice stream meets the ocean, it starts to float, thins out


55/102

Where an ice stream meets the ocean, it starts to float, thins out

considerably, and forms an ice shelf from which icebergs break off, a

process known as “calving”. Ice shelves are generally 100 to 1000 m thick,

much thicker than seasonal sea ice.

55

h l d dl d l


56/102

image: John Shaw

Jan 21,

2002

Larson B Ice Shelf, Antarctica

Ice shelves can disintegrate rapidly during massive calving events.

56

I h l di i idl d i i l i


57/102

March 5,

2002

Larson B Ice Shelf, Antarctica

Ice shelves can disintegrate rapidly during massive calving events.

57

In terms of total volume, alpine glaciers are insignificant relative to ice sheets.


58/102

image: John Shaw

Swiss Alps

They contain 0.3% of the volume of ice on Earth. Perhaps not surprisingly, they

have been much less significant than ice sheets in the geological past in terms

of their influence on the ocean, geosphere, atmosphere and biosphere. As such,

when I use the word “glacier” in the course, you can take it as being

synonymous with “ice sheet” unless noted otherwise.

58

T diti i d f l i t f Wh t th ?


59/102

Two conditions are required for glaciers to form. What are they?

59

T diti i d f l i t f Wh t th ?


60/102

Two conditions are required for glaciers to form. What are they?

1. Snowfall

2. Preservation of the snowfall over summer

60

Wh th diti t?


61/102

Where are these conditions met?

61

Wh th diti t?


62/102


-High latitudes (i.e., near the poles)

Solar radiation

per m2

highest

lowest

62



63/102



ASIDE: For a similar reason, you tend

become sunburned in the middle of

the day, not at the end of it.

No threat here

63



64/102



-High altitudes

Mt Kilimanjaro, Tanzania 64

3° S of equator

Although composed of a volatile substance* (water) as opposed to refractory


65/102

* Volatile: A substance that evaporates (i.e., boils) at a relatively low temperature (e.g., room temperature).

Examples include water, carbon dioxide, methane and ammonia. The opposite of volatile is refractory.

** Refractory : A substance that evaporates (i.e., boils) at a relatively high temperature (e.g., hundreds of degrees

C). Examples include the metals and silicate minerals that make up the geosphere. 65


substances** like the rock in the geosphere, glacier ice can essentially be

thought of as metamorphic rock . Why?



66/102

Although composed of a volatile substance (water) as opposed to refractory

substances** like the rock in the geosphere, glacier ice can essentially be

thought of as metamorphic rock . Why? Glaciers form by recrystallization of

snow flake crystals, just like a metamorphic rock forms by recrystallization of

an igneous, sedimentary or metamorphic rock precursor.

* Volatile: A substance that evaporates (i.e., boils) at a relatively low temperature (e.g., room temperature).

Examples include water, carbon dioxide, methane and ammonia. The opposite of volatile is refractory.

** Refractory : A substance that evaporates (i.e., boils) at a relatively high temperature (e.g., hundreds of degrees

C). Examples include the metals and silicate minerals that make up the geosphere. 66

Generation of glacier iceFresh snow


67/102

involves (1) burial

metamorphism of snow

and, in temperate regions,

(2) melting (summer) and

refreezing (winter) of snow.

Fresh snowDensity ~0.05 g/cm3

Very high porosity ~95%

FirnDensity ~0.4-0.83 g/cm3

Pore spaces stillconnected

(Firn = snow that has

survived a summer melt

season.)

Glacier iceDensity ~0.83-0.9 g/cm3

Pore spaces

disconnected; air

bubbles form

B ur i al d e p t h

67

Ice sheets tend to have an inner zone of mass gain referred to as


68/102

Ice sheets tend to have an inner zone of mass gain referred to as

the accumulation zone (accumulation > ablation) and an outer

zone of mass loss referred to as the ablation zone (accumulation

< ablation). The line separating the two zones in the equilibrium

line (accumulation = ablation)

Why is this so? Because the

atmosphere is heated from

the ground up, snow tends to

preserve in the elevated

central portions of ice sheets,where it is colder. These

areas of net mass gain are

referred to as accumulation

zones. Closer to sea level, air

temperatures are warmer,

which can lead to meltingand mass loss; if the glacier is

marine-terminating, calving

will occur. This zone is

referred to as the ablation

zone. (Ablation = any form of

mass loss.) 68

Accumulation = all mass gained from precipitation

Ablation = sum of all forms of mass loss (melting, sublimation, calving)

---- For a glacier to exit, accumulation must exceed ablation ----


69/102

Mass balance gradient

Accumulation

Ablation

Equilibrium line

69

Maintenance of accumulation and ablation zones drives glacier motion


70/102

Mass balance gradient

Accumulation

Ablation

Equilibrium line

70

g

Glacier advance and recession


71/102

Glaciers require a positive mass balance to

form and be maintained (i.e., to get a glacier,

accumulation must exceed ablation).

Glacier advance

Accumulation > ablation

Glacier retreat

Ablation > accumulation

ASIDE: There is a common misconception

that glaciers stop flowing when their fronts

recede. This is false. The ice within a

receding glacier continues to flow forward,

down the gravitational gradient.

For example, the glacier pictured in this

slide continued to flow to the right (as

indicated by red arrows) as its front

receded between 2001 and 2005. Helheim Glacier, Greenland

2001

2003

2005

71


72/102

There are three mechanisms by which a glacier moves:

a) ice deformation,

b) basal sliding, and

c) subsole deformation

72(a) Ice deformation (b) Basal sliding (c) Subsole deformation(i.e., deformation of sediment beneath glacier)


73/102


74/102

There are three mechanisms by which a glacier moves:

a) ice deformation,

b) basal sliding, and

c) subsole deformation

74

If the base of a glacier is above the

pressure-melting point (i.e., it is a “warm-

based glacier” aka “ wet-based glacier”),then the glacier can move by all three

processes—basal slip can occur,

subglacial sediments can deform, and the

ice will deform internally.

(a) Ice deformation (b) Basal sliding (c) Subsole deformation(i.e., deformation of sediment beneath glacier)

Unlike cold-based glaciers, warm-based glaciers are at their pressure-melting

point. A process known as regelation can occur beneath these glaciers,


75/102

p p g g ,

whereby pressure fluctuations (e.g., around obstacts) causes ice to melt

(upflow of obstacle; high pressure) then refreeze (lee of obstact; low pressure).

75

Regelation (gif) demonstrated using a thin wire

Regelation in the lee of bedrock obstacles can cause blocks of bedrock to be

plucked from these locations. Once the plucked chunks of rock are


76/102

plucked from these locations. Once the plucked chunks of rock are

incorporated in the ice, they can then “abrade” the bedrock in a sandpaper

type fashion. This generates mm-wide scratches (“striations”) on the bedrock,

and it produces mud-sized sediment. Subglacial erosion occurs by these two

processes, plucking and abrasion.

2. Abrasion1. Plucking

High pressure (melting)Low pressure

(freezing & plucking)

76

Erosion of bedrock by glaciers

Plucking


77/102

Any evidence of abrasion and/or plucking here?

77Plucked bedrockStriations

Whi h did l i fl ?


78/102

Any evidence of abrasion and/or plucking here?

78

Ice flow

Plucked bedrockStriations

Which way did glacier flow?


79/102

This outcrop is striated. Do striations run left right, or up down?

79

Stratification in rock

Striations on rock

Because ice is extremely competent* it can transport just about


80/102

Because ice is extremely competent , it can transport just about

anything, from the smallest clay particle up to slabs of

bedrock the size of Marion Hall (and bigger).

The Big Rock (Okotoks Erratic)

Alberta

80*Competence = a measure of flow strength, as gaged by the size of the

largest particle it can carry.

In ice sheets this debris is typically frozen up into the base of the


81/102

In ice sheets, this debris is typically frozen up into the base of the

glacier and gets transported downflow until it melts out or is

plastered (“lodged”) back onto the substrate. (The surface of

ice sheets, unlike alpine glaciers, tends to be debris-free;rather, all the action happens at the base.)

Ice

Till

81


82/102

82

Most non-glacial sedimentary rocks

Unlike wind and water, which can only transport relatively fine sediment, glaciers

tend not to sort their sediment load at all. As such, glacial deposits, which are


83/102

g p

referred to as till, tend to be diamictons (i.e., poorly sorted, massive mixes of mud,

sand and gravel)*. Clasts within tills are commonly angular; some may be striated.

How would you describe this?

83

*This is not always the case, however. “Till” is a genetic term, not a descriptive one: tills are defined solely based on their origin

(they must have been deposited directly from glacier ice with little to no subsequent reworking by water or other agent). A till may

be composed of any mix of grain sizes so long as it satisfies the above definition. That being said, most tills are diamictons, and

people (me included) commonly think of diamicton when they think of till.

Diamicton

Unlike wind and water, which can only transport relatively fine sediment, glaciers

tend not to sort their sediment load at all. As such, glacial deposits, which are


84/102

g p

referred to as till, tend to be diamictons (i.e., poorly sorted, massive mixes of mud,

sand and gravel)*. Clasts within tills are commonly angular; some may be striated.

How would you interpret this?

Striated clast

84

Till

*This is not always the case, however. “Till” is a genetic term, not a descriptive one: tills are defined solely based on their origin

(they must have been deposited directly from glacier ice with little to no subsequent reworking by water or other agent). A till may

be composed of any mix of grain sizes so long as it satisfies the above definition. That being said, most tills are diamictons, and

people (me included) commonly think of diamicton when they think of till.

Glacial landforms


85/102

85

Drumlins: Streamlined hills generated beneath flowing glacier ice.

Glacial landformsOak Ridges Moraine


86/102

86

Moraine: Mound of sediment (commonly mix of till and well sorted sand

and gravel), typically generated at the edge (end or sides) of a glacier.

Oak Ridges Moraine

Glacial landforms


87/102

87

Esker: Elongate ridge of sand and gravel generated by meltwater

flowing along base of glacier. Eskers commonly link to large fan-

shaped sand and gravel bodies (outwash) generated where the

stream expands and decelerates at ice front.

Close-up

Esker

Outwash


88/102

Esker deposition


89/102

Esker deposition


90/102

Esker deposition


91/102

esker

X

18 000 14C yr BP


92/102

18,000 14C yr BP

14 000 14C yr BP


93/102

14,000 14C yr BP

13 000 14C yr BP


94/102

13,000 14C yr BP

12 000 14C yr BP


95/102

12,000 14C yr BP

11 000 14C yr BP


96/102

11,000 14C yr BP

10 000 14C yr BP


97/102

10,000 14C yr BP

9 000 14C yr BP


98/102

9,000 14C yr BP

8 000 14C yr BP


99/102

8,000 C yr BP

7 000 14C yr BP


100/102

7,000 C yr BP

7 000 14C yr BP

Evidence of glaciation?

-Striations

-Landforms


101/102

7,000 C yr BPLandforms-Till and dispersal trains within

-Isostatic rebound

-Disrupted drainage

Questions


102/102

1. What is a glacier? (slide 29)

2. What two things are required for a glacier to form? (slide 59-64)

3. Where do glaciers form? (slide 61-64)

4. Why is glacier ice described as viscoelastic? (slides 29-48)

5. What is albedo? (slide 27)

6. How does sea ice differ from an ice shelf? (slides 21-25 and 55-57)

7. How does formation of sea ice contribute to oceanic circulation? (slide 24)

8. What are the two main types of glaciers? (slide 49)

9. What is the mass balance of a glacier? (slides 68-71)10. How do cold-based glaciers differ from warm-based ones? (slides 72-74)

11. By which three mechanisms can glaciers move? (slides 72-74)

12. What is the difference between till and diamicton? (slides 83-84)

13. What evidence exists that Canada was (almost entirely) covered by glacier

ice during the last glaciation? (slide 101)

Date post:	07-Jul-2018
Category:	Documents
Upload:	cady-morris
View:	214 times
Download:	0 times

Cummings 2016 GEO1111 Lecture 17 Glaciers

Documents