chapter 16: glaciers and glaciation. introduction: the earth’s changing cover of snow and ice (1)...

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.


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


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


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.


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 ever-

present 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.


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


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


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.


Some glaciers experience episodes of very unusual behavior

marked by rapid movement and dramatic changes in sizes and form, called a surge. 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, sharp-pointed 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: 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: 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).


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: North America. Northern Asia. The high, cold Tibetan Plateau. Many high mountain ranges.

Figure 16.27


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.


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.


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 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 glacial-interglacial 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: 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.


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.


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.


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.


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


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.


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

chapter 16: glaciers and glaciation. introduction: the earth’s changing cover of snow and ice (1)...

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