Geol 33 Environmental Geomorphology
J Bret Bennington
A glacier is a permanent body of ice that forms over land from the compaction and cystallization of snow. As the ice mass reaches a critical thickness it begins to deform and flow under its own weight.
Formation of glacial ice
Glacial ice begins as snow, which has high porosity and low density (.07 - .18 g/cc). Snow that survives the summer season undergoes a progressive transformation into more compact granules and finally into ice crystals. As the snow becomes buried under successive seasons of new snow it compacts and the crystals melt at grain-grain contacts. Melt pore water flows into empty spaces and re-freezes. The material near the surface of a glacier has a density from .4 to .8 g/cc and is known as firn or neve. This transformation can take from days to years, depending on the frequency and severity of freeze-thaw cycles. With further compaction and recrystallization the firn reaches a density of .8 - .9 g/cc and becomes ice. This final process can take years in temperate glaciers or hundreds to thousands of years in polar glaciers.
Glaciers will only form is sufficient precipitation occurs. During summer months, snow on the surface melts or ablates, resulting in a residual layer of dust. This gives glacial ice a banded internal structure, with each band representing a year's worth of snow accumulation.
Glaciers move because ice under pressure behaves plastically and flows in response to gravity, either downslope or outward from a region of accumulation.
The relationship between how much new snow is accumulated vs how much old snow / firn / ice melts away annually is called the net mass balance of the glacier. The areal extent of a glacier can be divided into a zone of accumulation (mass balance is positive) and a zone of ablation (mass balance is negative) The boundary between the two zones is called the equilibrium line. The equilibrium line is often synonymous with the firn line, which is a visible boundary on the glacier surface between new white snow above and old, grey ice below.
If the mass balance is positive, more ice accumulates than melts, and the glacier will flow forward.
If the mass balance is negative, more ice melts than accumulates, and the glacier will melt back.
Note that the two types of motion are fundamentally different.
How do glaciers move?
Glaciers move due to a combination of internal flow in the solid ice and sliding along the base of the glacier.
Internal flow: Individual ice crystals respond to pressure and gravity by deforming to realign parallel to the direction of flow and then by sliding past one another (creep).
Crevasses: The ice is prevented from cracking during creep by the high confining pressures deep in the glacier. However, near the surface of the glacier, where there is less pressure, the ice can behave as a brittle rather than a ductile solid and crack. Glaciers travelling over abrupt changes in slope often show large cracks in the upper 150 feet or so of ice. These large cracks are called crevasses.
Basal sliding: Whereas polar glaciers are frozen to the ground beneath them and so can only move by internal flow, temperate glaciers are usually warm enough at the base to melt and form a layer of water between the ice and the ground. This basal melting occurs for two reasons.
The meltwater at the base of the glacier acts as a lubricant and allows the ice to slide along its bed. Glacial movement due to sliding can be as high as a few meters per day, compared to only a few centimeters a day for internal ice flow.
Rarely, a temperate glacier will surge downslope, moving at speeds up to 16 meters per day. This results in a jumbled mass of ice in the ablation area and a chaos of crevasses along the glacier. Although surging is not completely understood, it is probably caused by a build-up of water pressure at the base of the glacier that seperates the glacier from its bed and causes the ice mass to hydroplane.
Ice flow velocities vary between different types of glaciers and within glaciers. Generally, temperate alpine glaciers have flow velocities of about 1 m or less per day. Polar glaciers flow much more slowely, moving only a few meters per year. Where glaciers descend steep slopes and where outlet glaciers discharge from large accumulation areas, velocities up to 30 m / day have been observed.
Types of Glaciers
Glaciers come in a variety of sizes and forms. Many are found on mountains, and are called alpine glaciers. Alpine glaciers can be found in most modern mountain chains.
Alpine glaciers include:
Valley glaciers: flow downslope along valleys from a high altitude snowfield. These are essentially ╬rivers of iceÔ.
Cirque glacier: a small glacier confined to a bowl shaped valley-head.
Fjord or Tidewater glacier: a valley glacier that empties into a flooded valley that has been deepened by glacial carving. Fjord glaciers rapidly lose ice at their terminus due to the constant breaking away of large chunks of ice to form icebergs. This process is called calving.
Piedmont glacier: a broad, lobate glacier that forms when the valley glacier leaves the valley.
Ice caps are larger regions of ice that completely cover broad regions of landscape.
Continental glaciers are glaciers that cover continent-sized land masses under thousands of feet of ice. These glaciers are also called ice sheets. The south polar continent of Antarctica is almost completely covered by ice sheets. Except along its margins, Greenland is covered by an ice sheet that is up to 1 3/4 miles thick near the center.
Greenland and Antarctica contain the only modern continental ice sheets. Their combined mass includes 95% of all of the ice on the earth. If these ice sheets were to melt, the volume of water generated would be about 5 million cubic miles, enough to raise global sea level about 200 feet above its present level.
Continental glaciers often include ice shelves at their margins in contact with the sea. An ice shelf is a thick, nearly flat sheet of partially floating ice, often lying within the confines of a bay. The largest modern ice shelf is the Ross ice shelf of Antarctica.
Glaciers can also be classified on the basis of their internal temperatures. Temperate glaciers exist in regions of the world where the temperature is above freezing for part of the year. These glaciers tend to have internal temperatures near the pressure-melting point (this is simply the temperature at which ice melts at a given pressure). As the surface of the glacier starts to melt in the summertime, the heat of melting is transferred downward through the ice.
Polar glaciers exist in the polar regions where mean annual temperatures are below freezing year-round. Here, little or no melting occurs so that the internal temperature of the glacier remains below the pressure-melting point.
Temperate glaciers appear smooth and rounded at their terminal ends due to melting, while polar glaciers terminate in abrupt ice cliffs.
The effect of glaciation on the landscape
Abrasion ¸ pure ice is an ineffective agent of abrasion. Glacial ice abrades bedrock because it contains embedded fragments of rock that are dragged along the base of the glacier. Ground bedrock forms rock flour, fine silt- and clay-sized particles of rock that themselves act as an abrasive to polish bedrock surfaces.
Plucking / Quarrying ¸ the freezing of glacial ice to loose bedrock, which is then pulled with the ice as the glacier moves.
Glacial features produced by abrasion and plucking
Glacial striations, grooves, and polishing: as glaciers move they scrape at the bedrock below them with sand, cobbles, and boulders embedded in the basal ice.
Striations and grooves made by rocks indicate the orientation of ice flow, while the fine sediments abraid and polish the bedrock beneath the glacier.
Chatter marks are indentations left in bedrock by the plucking action of the glacial ice. Chatter marks may indicate the direction of ice flow, but the "horns" of the chattermarks way point both up-flow and down-flow.
Erratics (wanderers): these are large boulders carried and left behind by the melting ice. Often they consist of rock types found hundreds of miles from where they come to rest. Erratics themselves often show the effects of being pushed and tumbled by the flowing ice, with rounded and polished surfaces and striations.
Landforms of glaciated mountain ranges
The magnificently rugged topography of most of the worlds high mountain ranges is a direct result of the actions of alpine glaciers sculpting the rock into steep peaks and wide, deep valleys.
Cirques: steep, bowl-shaped valley heads carved out by the frost wedging, plucking, and abrasion of glacial ice. Cirques often have a bedrock lip on their downslope side that can hold-in a small lake or tarn.
Where two cirques on opposite sides of a mountain meet they form a sharp, steep ridge called an arete. Where three of more cirques are backed-up against one another they produce a steep, high sculpted peak called a horn. The most famous horn is probably the Matterhorn in the Swiss Alps, although many of the peaks surrounding Mt Everest in the Himalaya are also horns.
Glacial valleys: Valleys that have been carved by glaciers have a broad, deep, U-shape in cross section. Often, the floor of the main valley will have been carved to a depth below that of the tributary valleys. These smaller valleys form hanging valleys, and the streams that flow through them tumble to the floor of the main valley as waterfalls or cascades.
Yosemite valley in the Sierra mountains of California is a famous example of a glacial valley. The Cascade Range of Oregon and Washington derives its name from its abundance of hanging valleys.
Landforms of ice caps and continental ice sheets:
Until about 10,000 years ago, huge ice sheets existed in northern Europe and North America, extending as far south as southern Ohio and Long Island. These ice sheets created a variety of distinctive features in the landscape that are familiar to us today.
Drumlins: These are elongate, streamlined hills consisting of glacial sediments or carved bedrock. As the ice flows over and around the rock or sediment, it carves it into the form that offers the least amount of resistence to the moving ice.
Drumlins are very common over the generally flat landscapes of northern New York, Ohio, and Michigan, indicating that these areas were covered in glacial ice in the not-to-distant past. As with striations, the orientation of the drumlins indicates the direction of ice flow.
Glacial features produced by deposition of sediment
Most of the sediment carried by a glacier is concentrated at its base and margins, where the ice is in contact with the land surface.
Glacial sediment is generally unsorted, consisting of a mixture of many different sizes of clasts, from boulders to fine sand and silt (called rock flour). Rock flour sediments are different from stream carried sand and silt in that the individual particles are angular due to crushing and grinding, rather than rounded due to abrasion and tumbling.
The sediment carried by a glacier can be deposited directly by the ice, or it can by deposited by flowing water running beneath and away from the melting glacier. The general term for sediments of glacial origin is drift.
Ice lain deposits
Ice deposited, unsorted glacial sediment is called till. Lodgement till consists of glacial debris that has melted out of the basal ice and plastered over the ground surface by the weight and motion of the overriding ice sheet. Lodgement tills sometimes show an oriented fabric and are usually more compacted than other glacial deposits. Ablation till is rock debris released by melting at the margin of a glacier. It generally has a random fabric and is less compacted than glacial tills.
Moraines: The major deposits of till are produced at the terminus and at the margins of the glacier where ice is continually melting and being replaced.
Moraines can form either due to sediment being bulldozed into position or by continuous melting of a stable glacier terminus.
Lateral moraines are produced along the margins of flowing valley glaciers.
End moraines are produced along the margins of larger glaciers. An end moraine that marks the farthest extent of the glacier terminus is called a terminal moraine. End moraines behind the terminal moraine produced by pauses in the retreat of a glacier are called recessional moraines.
Ground moraines are blankets of glacial sediments up to 30 feet thick deposited by receeding glaciers as they melt along their bases. Ground moraines can consist of lodgement till or ablation till.
Water lain deposits
As a glacier melts in the region of its terminus, the flowing water derived from the glacier carries sediments away from the melting ice and deposits them near the foot of the glacier as outwash. These water lain deposits are distinquished from till in that they are stratified deposits of glacial drift.
Glaciers that are stagnant or that are retreating due to rapid melting at the terminus deposit large amounts of stratified drift to form a number of distinctive landforms.
Kames are piles of stratified drift that accumulate in small ponds and lakes on the glacier. As the glacier melts these kames are left strewn about the landscape as small, irregular hills.
Kettles are small ponds within the stratified drift that form as blocks of glacial ice are left stranded within the accumulating outwash.
Eskers ¸ Rivers flowing within or beneath the glacial ice accumulate sediment in their channels. When the glacial finally melts, these channels form sinuous hills of stratified drift called eskers.
Glaciolacutrine deposits ¸ sediments deposited in lakes along the margin of a glacier. Fine-grained deposits that settle out in the deeper water of a glacial lake or kettle pond often form varves, which are rhythmites composed of silt/clay couplets. A single varve is deposited annually because during summer months both silt and clay are delivered by meltwater and wind to the lake, but in the winter the lake freezes over and inflowing meltwater streams have reduced discharge and carry only very fine clay sediments.
Glacioeolian deposits ¸ sand, silt, and clay released by glacial ablation can be picked up and transported by wind, forming dunes and thick deposits of loess. Loess deposits hundreds of meters thick occur in parts of the Mississippi Valley and in China.