Transcript Why Glaciers Change in Size (2)

Chapter 16: Glaciers and Glaciation
Introduction: The Earth’s Changing
Cover of Snow and Ice (1)
 At any place on the land where more snow
accumulates than is melted during the course of a
year, the snow will gradually grow thicker.
 As the snow piles up, the increasing weight of snow
overlying the basal layers causes them to
recrystallize, forming a solid mass of ice.
 When the accumulating snow and ice become so
thick that the pull of gravity causes the frozen mass
to move, a glacier is born.
Introduction: The Earth’s Changing
Cover of Snow and Ice (2)
 A glacier is a permanent body of ice, consisting
largely of recrystallized snow, that shows evidence
of downslope or outward movement due to the pull
of gravity.
 Glaciers are found in regions where average
temperature is so low that water can exist
throughout the year in a frozen state.
 Most glaciers are found in high altitudes or at high
latitudes.
Figure 16.1A
Figure 16.1d
Figure 16.1 E
Mountain Glaciers and Ice Caps (1)
 The smallest glacier occupies a cirque, a protected
bowl-shaped depression on a mountainside, and is
called a cirque glacier.

It typically is bounded upslope by a steep cliff, or
headwall.
 A growing cirque glacier that spreads outward and
downward along a valley will become a valley
glacier.
Mountain Glaciers and Ice Caps (2)
 Valley glaciers in some coastal mountain ranges at middle to
high latitudes occupy deep glacier-carved valleys whose
lower ends are filled by an arm of the sea. Such a valley is a
fjord, and a glacier that occupies it is a fjord glacier.
 A very large valley glacier may spread out onto gentle terrain
beyond a mountain front where it becomes a piedmont
glacier and forms a broad lobe of ice.
 An ice cap covers a mountain highland or lower-lying land
at high altitude and displays generally radial outward flow.
Figure 16.2
Ice Sheets and Ice Shelves (1)
 An ice sheet is the largest type of glacier on Earth.
 Modern ice sheets, which are found only on
Greenland and Antarctica, include about 95 percent
of the ice in existing glaciers.
 If all the ice in these vast ice sheets were to melt,
their combined volume, close to 24 million km3,
would raise the world sea level by nearly 66 m.
Figure 16.3
Ice Sheets and Ice Shelves (2)
 Antarctica is covered by two large ice sheets that
meet along the Transantarctic Mountains.


The East Antarctic Ice Sheet is the larger one.
The West Antarctic Ice Sheet is the smaller.
 Fed by one or more glaciers on land, an ice shelf is
a thick, nearly flat sheet of floating ice.
Figure 16.4
Temperate Glaciers
 Glaciers can be classified according to their
temperature as well as their size and shape.
 Ice in a temperate glacier is at the pressure
melting point.


The pressure melting point is the temperature at which ice
melts at a particular pressure.
Temperate glacier are restricted mainly to low and middle
latitudes.
Figure 16.5
Figure 16.5A
Figure 16.5B
Polar Glaciers
 Polar glaciers occur at high latitudes and high
altitudes, where the mean annual air temperature is
below freezing, the temperature in a glacier remains
below the pressure melting point, and little or no
seasonal melting occurs.


In summer when air temperature rises above freezing,
solar radiation melts the glacier’s surface snow and ice.
The meltwater percolates downward, where it freezes.
 When changing state from liquid to solid, each gram
of water releases 335 J of heat, which warms the
surrounding ice.
Glaciers and the Snowline
 Glaciers can form only at or above the snowline,
which is the lower limit of perennial snow.
 The snowline is sensitive to local climate, especially
temperature and precipitation.
Figure 16.6
Conversion of Snow to Glacier Ice
 Glacier ice is essentially a very low temperature
metamorphic rock that consists of interlocking
crystals of the mineral ice.
 Newly fallen snow is very porous and has a density
less than a tenth that of water.
 Snow that survives a year or more gradually
increases in density until it is no longer permeable
to air, at which point it becomes glacier ice.
Figure 16.7
Figure 16.8
Why Glaciers Change in Size (1)
 The mass of a glacier is constantly changing as the
weather varies by season and, on longer time scales,
as local and global climates change.
 These ongoing environmental changes cause
fluctuation in the amount of:


Snow added to the glacier surface.
Snow and ice lost by melting and sublimation.
Why Glaciers Change in Size (2)
 Additions to the glacier's ice are collectively called
accumulation.
 Losses are termed ablation.
 The difference between accumulation and ablation is a
measure of the glacier’s mass balance.
 Two zones are generally visible on a glacier at the end of the
summer ablation season:


The accumulation area, the part of the glacier covered by remnants
of the previous winter’s snowfall.
The ablation area, where bare ice and old snow are exposed because
the previous winter’s snow cover has melted away.
Figure 16.9
Why Glaciers Change in Size (3)
 The equilibrium line marks the boundary between
the accumulation area and the ablation area.
 The equilibrium line fluctuates in altitude from year
to year and is higher in warm, dry years than in
cold, wet years.
 If, over a period of years, a glacier’s mass balance is
positive more often than negative, the front, or
terminus, of the glacier advances.
 If negative mass balance predominates the glacier
will retreat.
Figure 16.10
Figure 16.11
Why Glaciers Change in Size (4)
 A lag occurs between a change in accumulation due
to a climate change and the response of the glacier
terminus to that change.
 The length of the lag depends both on the size of the
glacier and the way the ice flows.


The lag is longer for larger glaciers than for small ones.
Temperate glaciers of modest size (like those in the
European Alps) have response lags that range from
several years to a decade or more.
Why Glaciers Change in Size (5)
 Calving is the progressive breaking off of icebergs
from the front of a glacier that terminates in deep
water.

Icebergs produced by calving glaciers constitute an everpresent hazard to ships in subpolar seas.
Figure 16.12
How Glaciers Move (1)
 A glacier moves in two ways, by:
 Internal flow.
 Sliding of the basal ice over underlying rock or sediment.
 When an accumulating mass of snow and ice on a
mountainside reaches a critical thickness, the mass
will begin to deform and flow downslope under the
pull of gravity.
How Glaciers Move (2)
 Under the weight of the overlying snow and ice, ice
crystals are deformed by slow displacement (termed
creep).
 Where a glacier passes over an abrupt change in
slope, the surface ice cracks and form crevasses.
 A crevasse is a deep, gaping fissure in the upper
surface of a glacier.
Figure 16.13
Figure 16.14
How Glaciers Move (3)
 Ice temperature is very important in controlling the
way a glacier moves and its rate of movement.
 Meltwater at the base of a temperate glacier acts as
a lubricant and permits the ice to slide across its
bed.

In some temperate glaciers, such sliding accounts for up
to 90 percent of the total observed movement.
Figure 16.15
How Glaciers Move (4)
 Measurement of the surface velocity across a valley
glacier shows that the uppermost ice in the central
part of the glacier moves faster than ice at the sides.
 In most glaciers, flow velocities range from only a
few centimeters to a few meters a day.
How Glaciers Move (5)
 Some glaciers experience episodes of very unusual behavior
marked by rapid movement and dramatic changes in sizes
and form, called a surge.




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Ice in the accumulation area begins to move rapidly downglacier.
Rates of movement may be as great as 100 times those of nonsurging
glaciers.
The cause of surges is still imperfectly understood.
Over a period of years, steadily increasing amount of water trapped
beneath the ice may lead to widespread separation of the glacier from
its bed.
The escape of the water brings the surge to a halt.
Figure 16.16
Glaciation
 Glaciation is the modification of the land
surface by the action of glacier ice.
 Glaciation involves erosion and the transport
and deposition of sediment.
Small-Scale features of Glacial Erosion
 Small fragments of rock embedded in the basal ice
scrape away at the underlying bedrock forming
glacial striations.
 Larger rock fragments that the ice drags across the
bedrock abrade glacial grooves aligned in the
direction of glacier flow.
 Because striations and grooves are aligned with the
direction of ice flow, geologists use these to
reconstruct the flow paths of former glaciers.
Figure 16.17
Figure 16.18
Landforms of Glaciated Mountains (1)
 Cirques are bowl-like depressions.
 Many cirques are bounded on their downvalley side
by a bedrock threshold that impounds a small lake
(a tarn).
 As cirques on opposite sides of a mountain grow
larger and larger, their headwalls intersect to
produce a sharp-crested ridge called an arête.
Landforms of Glaciated Mountains (2)
 Where three or more cirques have sculptured a
mountain mass, the result can be a high, sharppointed peak (a horn).
 Glacial valleys have a U-shaped cross profile.
 Fjords are glacial valleys flooded by ocean water.
Landforms produced by Ice Caps and
Ice Sheets
 Ice caps and ice sheets produce many of the same
landscape features that smaller glaciers do:
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
Striations.
Moraines.
Fjords.
 They also generate some landforms not usually
produced by small glaciers.

The drumlin is a streamlined hill consisting largely of
glacially deposited sediment and elongated parallel to the
direction of ice flow.
Transport of Sediment by Glaciers
 Unlike a stream, a glacier can carry very large
pieces of rock.
 Where two glaciers join, rocky debris at their
margins merges to form a distinctive, dark colored
medial moraine.
 Much of the load in the basal ice of a glacier
consists of fine sand and silt grains informally
called rock flour.
Glacial Deposits (1)
 Sediments deposited by a glacier or by streams
produced by melting glacier ice are collectively
called glacial drift, or simply drift.
 Ice-laid deposits include:

Till, which is nonsorted drift deposited directly from ice;



Tillite is an ancient till that has been converted to rock.
A glacially deposited rock or rock fragment with a
lithology different from that of the underlying bedrock is
an erratic.
Glacial marine drift is sediment deposited on the
seafloor from ice shelves or icebergs.
Figure 16.23
Glacial Deposits (2)
 There are different types of fill:
 Ground moraine: widespread, relatively smooth-surface topography
with undulating knolls and shallow, closed depression.
 End moraine: a ridge-like accumulation of drift deposited along the
margin of a glacier.



An end moraine deposited at the glacier terminus is a terminal
moraine.
End moraines range in height from a few meters to hundred of meters.
Lateral moraine: a deposit along the side of a valley glacier.

The great thickness of some lateral moraines results from the repeated
accretion of sediment from debris-covered glaciers during successive ice
advances.
Figure 16.24
Stratified Drift (1)
 Some glacial drift is both sorted and stratified.
 This kind of drift is not deposited directly by glacier
ice, but rather by meltwater flowing from the ice.
Stratified Drift (2)
 Outwash is the stratified sediment deposited by
meltwater streams as they flow away from a glacier
margin.
 Such
streams typically have a braided pattern because
of the large sediment load they are moving.
 They deposit outwash to form a broad outwash plain.
 Meltwater streams confined by valley walls build an
outwash body called a valley train.
 Following glacier retreat, a stream’s sediment load is
reduced and the stream cuts down into its outwash
deposits to produce outwash terraces.
Figure 16.25
Figure 16.26
Deposits Associated with Stagnant Ice
(1)
 When ablation greatly reduces a glacier’s thickness
in its terminal zone, ice flow may virtually cease.
 Sediment carried by meltwater flowing over or
beside such stagnant ice is deposited as stratified
drift that slumps and collapses as the supporting ice
slowly melts away.
 Sediment is deposited as contact stratified drift.
Deposits Associated with Stagnant Ice
(2)
 Bodies of ice contact stratified drift have many
distinctive forms and are classified according to
their shape:

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
Kames: small hills of ice-contact stratified drift.
Kettles: basins in drift created by the melting away of a
mass of underlying glacier ice.
Eskers: long sinuous ridges of sand and gravel deposited
by a meltwater stream flowing under or within stagnant
glacier ice.
Periglacial Landscapes And Permafrost
(1)
 Land areas beyond the limit of glaciers where low
temperature and frost action are important factors in
determining landscape characteristics are called
periglacial zones.
 Periglacial conditions are found over more than 25
percent of Earth (circumpolar zones of each
hemisphere and at high altitudes).
Periglacial Landscapes And Permafrost
(2)
 A common feature of periglacial regions is
perennially frozen ground known as permafrost.
 Permafrost is sediment, soil, and even bedrock that
remains continuously below freezing for an
extended time.
 The largest areas of permafrost occur in:
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North America.
Northern Asia.
The high, cold Tibetan Plateau.
Many high mountain ranges.
Figure 16.27
Periglacial Landscapes And Permafrost
(3)
 The southern limit of continuous permafrost in the
Northern Hemisphere lies where the average annual
air temperature is between –5 and –100C (23 and
140F).
 Most permafrost is believed to have originated
during either the last glacial age or earlier glacial
ages.
Periglacial Landscapes And Permafrost
(4)
 The depth to which permafrost extends depends on:
 The average air temperature,
 The rate at which heat flows upward from Earth’s
interior.
 How long the ground has remained continuously frozen.
 The permafrost is 1500 m (4900 ft) deep in Siberia.
 It is 1000 m (3300 ft) deep in the Canadian Arctic.
 It is 600 m (2000 ft) deep in northern Alaska.
Figure 16.28
The Glacial Ages (1)
 The concept of a glacial age with widespread effects
was first proposed in 1837 by Louis Agassiz, a
Swiss scientist.
 Over ten millions of years, the climate slowly grew
cooler as Earth moved into a late Cenozoic glacial
era.
 During the last few million years, the planet has
experienced numerous glacial-interglacial cycles
superimposed on the long term cooling trend.
The Glacial Ages (2)
 About 30,000 years ago, late in the Pleistocene
Epoch, an extensive ice sheet that had formed over
eastern Canada began to spread south toward the
United States and west toward the Rocky
Mountains.
 Simultaneously, another great ice sheet that
originated in the highlands of Scandinavia spread
southward across northwestern Europe.
The Glacial Ages (3)
 The ice sheets in Greenland and Antarctica expanded and
advanced across areas of the surrounding continental shelves
that were exposed by falling sea level.
 Glaciers also developed in the world’s major mountain
ranges (Alps, Andes, Himalaya, and Rockies).
 The area of former glaciation equals about 29 percent of
Earth’s present land area.

Today, by comparison, only about 10 percent of the world’s land area
is covered with glacier ice (84 percent of that lies in the Antarctic
region).
Drainage Diversions and Glacial Lakes
 The growth of ice sheets over the continents caused:
 Disruption of major stream system.
 The Missouri and Ohio rivers to move into new courses
beyond the ice margin.
 The accumulation of water in ice-dammed lakes.
 The creation of large ice-marginal lakes in northern Asia.
Lowering of Sea Level (1)
 The moisture needed to produce and sustain large
facies was derived primarily from the oceans.
 Sea level was lowered in proportion to the volume
of ice on land.
 World sea level fell at least 100 m, thereby causing
large expanses of the shallow continental shelves to
emerge as dry land.
Lowering of Sea Level (2)
 At that time, the Atlantic coast of the United States
south of New York lay as much as 150 km east of its
present position.
 Lowering of sea level joined Britain to France
where the English Channel now lies.
 North America and Asia formed a continuous
landmass across what is now the Bering Strait.
 These and other land connections allowed plants
and animals to migrate.
Figure 16.29
Deformation of the Earth’s Crust
 The weight of the massive ice sheets caused the
crust of the Earth to subside beneath them.
 Because ice (density 0.9 g/cm3) is one-third as
dense as average crustal rock (2.7 g/cm3), an ice
sheet 3 km thick could cause the crust to subside by
as much as 1 km.

The Hudson Bay region of Canada, which 20,000 years
ago lay near the center of the vast Laurentide Ice Sheet, is
still rising as the lithosphere and asthenosphere adjust to
the removal of this ice load.
Earlier Glaciations (1)
 Studies of deep-sea sediments showed that during
the last 800,000 years the length of each glacialinterglacial cycle averaged about 100,000 years.
 In the Pleistocene Epoch, more than 20 glacial ages
are recorded.
Earlier Glaciations (2)
 The glacial record on land is incomplete and marked
by numerous unconformities.

250-300 million years ago, Africa was covered by a vast
continental ice sheet.

Evidence of this ice sheet is spread across parts of southern
Africa, largely in form of striated and grooved bedrock, tillite,
and associated meltwater deposits.
Seafloor Evidence (1)
 The ratio of the amounts of isotopes 18O to
16O
in
layers of calcareous ooze in seafloor sediment cores
also fluctuates, indicating successive changes in
temperature.
 When water evaporates, water containing the light
isotope 16O evaporates more easily than water
containing the heavier 18O.
Seafloor Evidence (2)
 When water evaporates from the ocean and is precipitated on
land to form glaciers, the ice is enriched in the lighter isotope
relative to the ocean water that remains behind.
 The isotope 18O contained in glacier ice also gives a
generalized view of global climatic change.



Warmer temperatures would increase the amount of heavy oxygen
that reaches land.
Cooler temperatures would reduce the amount of heavy oxygen that
reaches land.
The ocean during the Pleistocene became enriched in the heavy
isotope.
Figure 16.30
Pre-Pleistocene Glaciations
 The earliest recorded glaciation dates to about 2.3
billion years ago, in the early Proterozoic.
 Evidence of other glacial episodes has been found
in rocks of the:



Late Proterozoic.
Early Paleozoic.
Late Paleozoic.
 During the late Paleozoic, 50 or more glaciations
are believed to have occurred.
Little Ice Ages
 Short-term fluctuations in average temperature can
occur.
 The “Little Ice Age” was an interval of generally
cool climate starting in the mid-thirteenth century
and lasting until the mid-nineteenth century;
Figure 16.31
What Causes Glacial Ages?
 Two major causes have been identified for
glacial ages.
Shifting continents.
 Astronomical changes.

Glacial Years and Shifting Continents
(1)
 Several different episodes of glacial and interglacial ages,
each lasting tens of millions of years, can be identified in the
geologic record.
 One reasonable explanation seems to be related to important
geographic changes that affected the crust of the planet:
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
The movement of continents.
The large scale uplift of continental crust where continents collide.
The creation of mountain chains where one plate overrides another.
The opening or closing of ocean basins and seaways between moving
landmasses.
Glacial Years and Shifting Continents
(2)
 Where evidence of ancient ice-sheet glaciation is
now found in low latitudes, we are led to infer that
such lands were formerly located in higher latitudes
where large glaciers could be sustained.
 In the late Paleozoic Era, a continental ice sheet
repeatedly covered much of ancient Gondwanaland.
 The absence of widespread glacial deposits in rocks
of Mesozoic age implies that most of the world’s
landmasses had moved away from polar latitudes
and that climate were mild.
Glacial Years and Shifting Continents
(3)
 By the early Cenozoic, slowly shifting
landmasses once again moved into polar
latitudes.

Tectonic movements were beginning to raise
large areas of the western United States and
Central Asia to high altitudes.
Ice Ages and the Astronomical Theory
(1)
 John Croll, in the mid-nineteenth century, and
Milutin Milankovitch, a Serbian astronomer of the
early twentieth century, developed an astronomical
theory to account for ice ages.
 Minor variations in Earth’ orbit around the sun, and
in the tilt of the earth’s axis, cause slight but
important variations in the amount of radiant energy
reaching any given latitude on the planet’s surface.
Ice Ages and the Astronomical Theory
(2)
 Fluctuations of climate on the time scale of glacial
cycles correlate strongly with cyclical variations in
Earth’s tilt and orbital configuration.
 The Earth’s axis traces a cone in space, completing
one full revolution every 26,000 years.
 The axis of Earth’s elliptical orbit is also rotating,
but much more slowly, in the opposite direction
(called procession of the equinoxes).

It completes one full cycle in 23,000 years.
Ice Ages and the Astronomical Theory
(3)
 The tilt of the axis, which is now 23.50, shifts over a
range of about 30 during a span of about 41,000
years (24.50 to 21.50).
 The eccentricity of the orbit changes over periods of
100,000 and 400,000 years.
 At the time of the last glaciation, the tilt was at its
minimum and the eccentricity was at its maximum.
Figure 16B02
Figure 16B03
Ice-Core Archives of Changing Climate
(1)
 Ice cores contains a high resolution record of
changing climate and atmospheric composition
extending far into the past.
 When snow accumulates on a glacier, it compacts
and the air between snow crystals becomes trapped
in bubbles.


Most of what we know about the past composition of the
atmosphere during glacial times has been obtained from
these air bubbles.
From these air bubbles, glaciologists can measure
variations in atmospheric carbon dioxide and methane.
Figure 16.32b
Figure 16.32a
Ice-Core Archives of Changing Climate
(2)
 Ice also contains microparticles (i.e., windblown
dust).



The glacial atmosphere was very dusty compared to that
of interglacial times.
Dust in Greenland ice cores originated in the desert
basins of central Asia.
Dust in the Antarctic Ice Sheet originated in Patagonia.
Atmospheric Composition (1)
 Orbital factors can explain the timing of the glacial-
interglacial cycles.
 The variations in solar radiation reaching Earth’s
surface are too small to account for the average
global temperature changes of 4 to 100C.
 Some of the factors involved are likely to be:



Changes in the chemical composition.
Dustiness of the atmosphere.
Changes in the reflectivity of Earth’s surface.
Figure 16.33
Atmospheric Composition (2)
 During glacial times the atmosphere contained less
carbon dioxide and methane than it does today.

Calculations suggest that the low levels of carbon dioxide
and methane account for nearly half of the total ice-age
temperature lowering.
 The amount of dust in the atmosphere was
unusually high during glacial times.


The sky must have appeared hazy much of the time.
The fine atmospheric dust scattered incoming radiation
back into space, thereby further cooling Earth’s surface.
Atmospheric Composition (3)
 During a glacial age, large areas of land are
progressively covered by snow and glacier ice.

The highly reflective surfaces of snow and ice scatter
incoming radiation back into space, further cooling the
lower atmosphere.
Changes in Ocean Circulation (1)
 The circulation of ocean waters plays an important
role in global climate.
 As warm surface water moving northward into the
North Atlantic evaporates, the remaining water
becomes saltier and cooler.
 Heat released to the atmosphere as the water cools
maintains a relatively mild climate in northwestern
Europe.
Changes in Ocean Circulation (2)
 At the onset of a glaciation:
 The high-latitude ocean and atmosphere cooled.
 Sea ice expanded.
 High latitude evaporation was reduced.
 Cold air masses moved eastward across the North
Atlantic.
 Ice sheet grew on the continents.
 A change in the ocean’s circulation system amplifies
the relatively small climatic effect attributable to
astronomical changes.
Solar Variations, Volcanic Activity, and
Little Ice Ages (1)
 One hypothesis regarding the little Ice Age is based
on the concept that the energy output of the Sun
fluctuates over time.
 Correlations have been proposed between weather
patterns and rhythmic fluctuations in the number of
sunspots appearing on the surface of the Sun.

As of yet, there is no convincing demonstration of this
correlation.
Solar Variations, Volcanic Activity, and
Little Ice Ages (2)
 Large explosive volcanic eruptions can eject huge quantities
of ash into the atmosphere to create a veil of fine dust that
circles the globe, blocking out sunlight.

The dust settles out rather quickly, generally within a few months to a
year.
 Tiny droplets of sulfuric acid, produced by the interaction of
volcanically emitted SO2 gas with oxygen and water vapor,
can scatter the Sun’s rays.

Such droplets remain in the upper atmosphere for several years.
 Volcanic emissions can produce detectable changes of
climate on a decadal time scale.
What Will Happen to Glaciers in a
Warmer World?
 A warming Earth should lead to negative balances
and glacier retreat.
 If the climate warms as much as 3.30C by the end of
the present century, then the snow-line could rise an
average of 500 m or more. This would cause all but
the largest glaciers in the Alps, Cascades, and
Himalayas to disappear.
Figure 16.34
Figure 16.35