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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.
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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.
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.
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.
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.
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.
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.
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.
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.
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.
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 ever-present hazard to ships in subpolar seas.
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.
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.
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. • 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.
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.
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.
