Chapter 12 Glaciers and Glaciation

  • Glaciers = large, long-lasting mass of ice, formed on land, that moves under its own weight.

    • develops as snow is compacted and recrystallized.

  • Two types of glaciated terrain on Earth’s surface

    • Alpine glaciation = found in mountainous regions

    • continental glaciation = exists where a large part of a continent is covered by glacial ice.

  • Yosemite valley carved by glacier

Distribution of Glaciers

  • found where more snow falls than can be melted during warm months.

  • Glaciers are common even near the equator in the very high mountains of South America and Africa cause of the low temperatures at high altitudes.

  • Glaciation is most extensive in polar regions

  • one-tenth of the land surface on Earth is covered by glaciers

  • Antarctica storing most of Earth's fresh water in the form of ice.

Types of Glaciers 

  • valley glacier = confined to a valley and flows from a higher to a lower elevation.

    • prevalent in areas of alpine glaciation.

  • ice sheet = mass of ice that is not restricted to a valley but covers a large area of land

    • associated with continental glaciation. only Greenland and Antarctica have ice sheets

    • Ice gap = smaller ice sheet

Formation and Growth of Glaciers

  • snowflakes settle by compaction under their own weight, and much of the air between them is driven out.

  • sharp points of the snowflakes destroyed as flakes reconsolidate into granules.

  • firn: compacted mass of granular snow, transitional between snow and glacier ice

  • Glacier ice is texturally similar to the metamorphic rock, quartzite

  • Ablated = glacier ice moves downward and lost

    • due mostly to melting

  • icebergs = moving glacier reaches a body of water, blocks of ice break off

Glacial Budgets

  • postive budget: amount of snow a glacier gains is greater than the ice and water it loses, expands

  • negative budget:

  • advancing glaciers = Glaciers with positive budgets push outward and downward at their edges

  • receding glaciers = grow smaller and their edges melt back

  • zone of accumulation = upper part of a glacier; with a perennial snow cover

  • Zone of ablation = lower part; ice is lost, or ablated, by melting, evaporation, and calving

  • Equilibrium line = boundary between these two altitudinal zones.

    • Its location indicates whether a glacier has a positive or negative budget.

    • migrating upglacier—negative budget,

    • downglacier—positive budget

  • terminus = the lower edge of a glacier

    • receding glaciers—terminus melts back upvalley

  • advancing glacier does not necessarily indicate that the climate is getting colder. It may mean that the climate is getting wetter,

Movement of Valley Glaciers

  • valley glaciers move downslope under influence of gravity. ranging from less than a few millimeters to 15 meters a day.

  • thicker parts of a glacier will flow faster than where it is thinner

  • fastest moving ice to be near the equilibrium line.

  • central portion of a valley glacier moves faster than the sides

  • surface moves faster than the base.

  • basal sliding = the sliding of the glacier as a single body over the underlying rock (base of the pipe has moved downglacier)

  • Plastic flow = movement that occurs within the glacier due to the plastic nature of ice itself. bent pipe

  • The reason the pipe is bent more sharply near the base of the glacier is that pressure from overlying ice results in greater flowage with increasing depth.

  • rigid zone = upper part of the glacier, the pipe has been moved downglacier

Crevasses

  • cracks in ice

    • Being brittle, the ice of the rigid zone is broken by the ten-sional forces.

    • also form along the margins of glaciers in places where the path is curved

Movement of Ice Sheets

  • An ice sheet or ice cap moves like a valley glacier except that it moves downward and outward from a central high area toward the edges of the glacier

  • Most movement of East Antarctic ice sheet is by plastic flow.


Glacial Erosion

  • plucking: pieces of the rock are broken loose and frozen into the base of the moving glacier

  • dragged along by the moving ice. the rock within glaciers grinds away at the underlying rock.

  • The thicker the glacier, the more pressure on the rocks and the more effective the grinding

  • Pebbles and boulders that are dragged along are faceted (given flat surface by abrasion)

  • Bedrock underlying a glacier is polished by fine particles and striated (scratched)

  • Rock Flour = grinding of rock across rock produces this powder.

    • Composed largely of very fine particles of unaltered minerals.

    • Turns water green

Evidence for older Glaciation

  • Glacial Valleys

    • U-shaped valley

  • Thicker a glacier is, more erosive force it exerts and more bedrock is ground away.

  • f valleys = after glaciers disappear, these tributaries.

  • Valley glaciers usually occupy valley carved by streams

  • Truncated spurs = reduces that have triangular facets produced by glacial erosion at lower ends

  • Rock-basin lakes = ice melted into the cracks created by the glacial eroding rocks

  • Cirqfues, horns, and Aretes

    • Cirque = steep-sided, half bowl shaped recess carved into a mountain at head of a valley carved by glacier

      • Also shaped by weathering and erosion of rock walls above surface of ice

      • Frost wedging

    • Horn = sharp peak that remains after cirques have cut back into a mountain

    • Aretes = sharp ridges separate adjacent glacially carved valleys

Erosional Landscapes Associated with Continental Glaciation

  • Ice sheets tend to produce rounded topography.

  • Ice sheet may be thick enough to bury mountain ranges, rounding off ridges and summits.

Glacial Deposition

  • Most of valley glacier’s load comes from rocks broken from the valley walls

    • Most are angular rocks, unsorted, clay sized to boulder sized

  • Till = unsorted and unaltered rock debris carried or deposited by glaciers

  • Erratic = ice transported boulder

Moraines

  • Moraines = the body of unsorted and unaltered debris

  • Later moraines = elongated, low mounds of till that form along sides of valley glacier.

  • Medial Moraine = tributary glaciers come together, adjacent lateral moraines join and carried down glacier as a single long ridge of till

  • End moraine = ridge of till piles up along front edge of ice.

    • Valley glaciers build end moraines that are crescent shaped. Or horseshoe

    • Terminal moraine = the end moraines marking the farthest advance of glacier

    • Recessional moraine = end moraines built while terminus of receding glacier remains temp stationary

  • Ground moraine = fairly thin, extensive layer of till. Ice melts, rock debris that’s been carried deposited.

  • Drumlins = bodies of till shaped into streamlined hills (past continental Glaciation)

Outwash

  • Outwash = material deposited by debris laden meltwater

    • Well shorted, particles not chemically weathered.

  • Esker = Outwash feature of unusual shape associated with former ice sheets and very large valley glaciers

    • Up to 10 m high formed of cross-bedded/well sorted sediment

  • Kettle = ice block melt and depression is formed

  • Streams that drain glaciers tend to be very heavily loaded with sediment

Glacial Lakes and Varves

  • Lakes often occupy depression carved by glacial erosion. lake form between retreating glacier and an earlier end moraines

  • Varve = two layers of sediment representing one year’s despoition

Past Glaciation

  • Theory of Glacial Ages = at times in past, colder climates prevailed during which much more of land surface of Earth was glaciated than at present

  • Less evidence preserved for each successively older glacial episode cause:

    • weathering and erosion ocurred during during warm interglacial periods

    • later ice sheets and valley glaciers overrode many of features of earlier glaciation

Indirect Effects of Past Glaciation

  • Glacial lakes 

  • Pluvial Lakes

    • pluvial lakes = formed in a period of abundant rainfall. Exsited in Utah, Nevada

  • Great salt lake small remnant of much larger lake of Lake Bonneville.

  • Lowering and rising of sea level

    • ice is from ocean water.

    • Sea level worldwide lower than it is today, at least 130 meter lower.

    • Fiord = coastal inlet that is a drowned glacially carved valley.

Evidence for Older Glaciation

  • Tillite = rocks. a lithified till.

    • unsorted rock particles have been consolidated into a sedimentary rock.

  • Snowball earth hypothesis (evidence-tillites)