Glaciers and Climate Change Chapter 13 Geology Today Barbara W. Murck Brian J. Skinner N. Lindsley-Griffin, 1999 Mount Fairweather, Glacier Bay, Alaska

Glaciers and Climate ChangeGlaciers and Climate Change

Chapter 13

Geology Today

Barbara W. Murck

Brian J. Skinner

N. Lindsley-Griffin, 1999

Mount Fairweather, Glacier Bay, Alaska

GlaciersGlaciersGlaciers are permanent bodies of ice (recrystallized snow) that show evidence of movement due to gravity.

N. Lindsley-Griffin, 1999Fig. 14.11, p. 410

Types of GlaciersTypes of GlaciersIce sheets are continent-sized glaciers that overwhelm nearly all the land within their margins.


Antarctica, one of two present-day ice sheets (the other is Greenland)

Types of GlaciersTypes of GlaciersIce caps cover mountain highlands, or low-lying land at high latitudes

N. Lindsley-Griffin, 1999Ice caps in Iceland (Fig. 14.11, p. 410)

Vatnajokull

Types of GlaciersTypes of Glaciers

Types of glaciers are determined by their size and location -

Confined to a valley?

Spread over mountain tops?

Cover a continent?


Types of GlaciersTypes of GlaciersValley glaciers flow down valleys. Pressure of ice at higher elevations pushes them down below snowline.

N. Lindsley-Griffin, 1999 Denali National Park, Alaska (Fig. 14.11, p. 411)

Types of GlaciersTypes of Glaciers Piedmont glaciers form large lobes where valley glaciers come together and flow onto lowlands.


Gorner Glacier, Swiss Alps (Fig. 14.11, p. 410)

Types of GlaciersTypes of GlaciersFjord glaciers occupy fjords: glacier-carved troughs in bedrock that fill with seawater as the glacier retreats

N. Lindsley-Griffin, 1999Southwestern Greenland (Fig. 14.11, p. 411)

Types of GlaciersTypes of GlaciersCirque glaciers are confined to cirques: bowl-shaped depressions where snow and ice accumulate on mountains.

N. Lindsley-Griffin, 1999 Denali National Park, Alaska (Fig. 14.11, p. 410)

How Glaciers FormHow Glaciers Form

1) Fresh snow is fluffy and porous. The delicate crystal points evaporate; their vapor fills pore spaces.

2) The ice crystals gradually become smaller, rounder, denser.

3) Successive snowfalls bury and compact the ice crystals until they recrystallize into a metamorphic rock - glacier ice


Alpine or valley glaciers form on high mountains at all latitudes, wherever snow remains all year.

Snow granular ice glacial ice (fused, massive)

Glacier Budgets Glacier Budgets Positive budget - glacier grows

Negative budget - glacier shrinks

Accumulation zone - snow builds up to form glacier ice at the head

Zone of ablation - snow melts and evaporates at the terminus

Lutgens & Tarbuck; N. Lindsley-Griffin, 1999 See Fig. 14.14, p. 414

Glacier will advance if more ice is added to head than is removed at terminus

Glacier will retreat if more ice is removed at terminus than is added at head.


Glacier Budgets Glacier Budgets

Glacier Movement

Glacier Movement

Ice crystals move by internal creep.

Stress imposed by weight of overlying ice aligns the crystal axes.

Internal cleavage planes slip past each other like a deck of playing cards.


Glaciers flow by internal creep at the center, away from the sides.

Flow is slower along sides and base, where they are abrading.

Shallow ice is brittle, it cracks to form crevasses under tensional stress.


Fig. 14.14, p. 414

Glacier Movement

Glacier Movement

Basal sliding, in which the glacier slides along its bed, occurs when meltwater lubricates the base of the glacier.

Basal sliding may be one cause of glacial surges - very rapid advances.


Fig. 14.14, p. 414

Glacier Movement

Glacier Movement

Glacial LandscapesGlacial Landscapes

The landscapes that result depend on the type of glaciation.

Ice sheets and ice caps override nearly everything in their reach - they smooth out the landscape.

Valley or alpine glaciers carve valleys deeper and wider, and leave sharp ridges and peaks between the valleys.


Glacial ErosionGlacial Erosion Ice abrades on the upstream side; plucks on the downstream side.

Lutgens & Tarbuck; N. Lindsley-Griffin, 1999Ice flow on uneven bedrock surface

Glacial Erosion Glacial ErosionAbrasion smooths rock surfaces to form glacial polish; scrapes surfaces to make grooves, striations

N. Lindsley-Griffin, 1999Glacial polish, Sierra Nevada Range, California

Striated cobbles, Peyto Glacier, Alberta

Erosion by Pleistocene ice sheets:

Scooped out Great Lakes

and the Finger Lakes of New York

U.S.G.S., N. Lindsley-Griffin, 1999

Produced the smooth, scoured topography of the Canadian Shield

Erosion by Pleistocene ice sheets:


Glacial ErosionGlacial Erosion Landscapes shaped by valley glaciers

= Alpine glaciers

Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999

V shape

U shape

CirqueHorn TarnArete

Hanging Valley

U shape

Steep, straight valley walls

Cirque

Aretes

Crevasse zone

Cirque

Arete

Icefall


Glacial ErosionGlacial Erosion Alpine glaciation is signaled by:

glacial horns and aretes

glacial grooves and striations

U-shaped valleys


Pilot Peak, WY/MT

The Matterhorn, Switzerland

U-shaped valleys carved by glaciers have broad, flat floors and steep walls.


Glacial ErosionGlacial Erosion

Beartooth Range, Montana

Glacial ErosionGlacial ErosionYosemite Valley was carved by a glacier. The famous waterfalls are streams that flow down hanging valleys and fall to the valley floor.

N. Lindsley-Griffin, 1999 Yosemite National Park, California

Glacial erosion may indicate flow direction

Striations -- parallel grooves and scratches gouged into bedrock by rock fragments embedded in the glacier


Roche moutonee -a glacially carved rock knob that is smooth on the upstream side, steep and rough on

the downstream side.

Ice Flow


Glacial DepositsGlacial Deposits

The stone walls of New England, immortalized by poet Robert Frost, were built by European settlers clearing glacial boulders from their fields. (The surface layer is till with little or no soil.)

Mt. Chocorua, White Mountains, NHN. Lindsley-Griffin, 1999

“Stone walls do not good neighbors make...”

Glacial DepositsGlacial DepositsTill - a heterogeneous mixture of

crushed rock deposited by a glacier.

Poorly sorted: boulders, cobbles, pebbles, sand, silt, rock flour

No layering; may be angular or rounded; striated or grooved.

Deposited far from source; may rest on striated surface.


Pavement outcrop and till, central Maine

Glacial DepositsGlacial DepositsMoraines - ridges or piles of debris deposited along glacier edges.


Lateral moraine

Terminal moraine

Moraine

Medial moraine

Glacial DepositsGlacial

DepositsTerminal moraines form at the ends of glaciers as they retreat.


Terminal moraine left behind by retreat of Lobuche Glacier

(Fig. 14.17D, p. 418)

Lateral moraine


Deposits

Lateral moraines are deposited along the sides of valley glaciers.


Grand Plateau Glacier, St. Elias Mts., Glacier Bay National Park, Alaska (Fig. 14.17, p. 418)


Deposits

Lateral moraines are deposited along the sides of valley glaciers.




Deposits

Medial moraines form where two valley glaciers merge, joining their lateral moraines into a stripe of debris in the middle of the glacier.




Deposits

Medial moraines form where two valley glaciers merge, joining their lateral moraines into a stripe of debris in the middle of the glacier.



Glacial DepositsGlacial DepositsBesides moraines, retreating glaciers leave behind kames and kettles, drumlins, eskers, outwash plains


Kame

DrumlinOutwash Plain

EskerKettle

Braided stream


Deposits


Drumlin - streamlined, elongate hill of glacially deposited sediment, parallel to ice flow. Blunt end is upstream, tapered end points in direction of ice flow.

Drumlin field, Alaska

Glacial DepositsGlacial DepositsEsker - ridge of sand and gravel deposited by a subglacial stream.


Kettle-Moraine State Park, Wisconsin (Fig. 14.17B, p. 418)

Glacial erratics, isolated boulders deposited by glaciers, are different than the underlying bedrock.



Denali National Park, Alaska (Fig. 14.17 A, p. 418)

Kame and kettle topography, Alaska

Kame - mound of stratified drift deposited by water under or within glacial ice

Kettle - depression formed when a buried ice block melted after the glacier retreatedU.S.G.S.; N. Lindsley-

Griffin, 1999



Glacial Outwash - stratified sediments deposited by pools or streams of glacial meltwater.

Sorted by size; layered. Look like other alluvial or lacustrine deposits

Glacial outwash, Vermont

Glacial Deposits Glacial Deposits

Varves tend to form in glacial meltwater lakes with seasonal fluctuations in sediment supply and wintertime freezing.

Each varve consists of: one light sand-silt layer (deposited in summer when streams are active)

one dark clay layer (deposited in winter when lake is frozen and quiet)

1 varve = 1 year



Glacial outbursts occur when an active volcano erupts under an ice cap or sheet.

Lava melts ice, meltwater forms large pool under ice cap.

Explosive eruption of volcano splits glacier open, releases water in a catastrophic flood.

Vatnajokull volcano, Iceland - 1996 (Frontispiece p. 397)


Periglacial LandformsPeriglacial LandformsIce wedges form in regions of permafrost when surface meltwater seeps into open cracks in the ground and freezes.

Wedges grow wider each season as more meltwater flows in during the summer and freezes in winter.

N. Lindsley-Griffin, 1999 Fig. 14.18, p. 420

Periglacial LandformsPeriglacial Landforms

Individual ice wedges join together to form polygonal patterns.

After hundreds of seasons patterned ground results.


Patterned ground, Alaska

(Fig. 14.18, p. 420)

Pluvial lakes are formed by increased rainfall in outlying regions adjoining large ice sheets.

In the Western U.S., during the cooler and wetter climate of the late Pleistocene, Lake Bonneville and Lake Lahontan were the two largest pluvial lakes.

N. Lindsley-Griffin, 1999; Lutgens & Tarbuck, J.R. Griffin , 1999



Lake terraces (wave-cut benches) formed by wave action when Lake Lahontan was at high water levels during the Pleistocene.

N. Lindsley-Griffin, 1999Pluvial lake terraces, Nevada

Three major factors: Tectonic plate motion

Long term cyclical variations in solar

radiation (Milankovitch cycles)

Changes in Earth’s atmosphere

Causes of Climate Change

Causes of Climate Change


Houghton Mifflin 1998; N. Lindsley-Griffin, 1999

Plate motions: Continents at high latitudes favor growth of ice sheets :

Ice sheets on Gondwana when located overSouth Pole 276 m.y.a.

Ice sheets on Eurasiaand North Americaduring Pleistocene

Evidence of Gondwanalandice sheets found onsouthern continents today

Causes of Climate ChangeCauses of Climate Change

Temperatures normally cycle from warm to cold and back again. Glaciation occurs when global temperatures drop a few degrees and remain low long enough for ice sheets to form.


Causes of Climate ChangeCauses of Climate Change

Global Climate ChangeGlobal Climate Change

Astronomic basis for climatic cycles (Milankovich cycles):1) variations in Earth’s orbital distance from the sun, 2) tilt of Earth’s axis varies slightly, 3) Earth’s axis wobbles slowly like a spinning top

All 3 act on different time scales that combine in a complicated way to alter amount of solar energy reaching Earth’s surface


Lutgens & Tarbuck, J.R. Griffin , 1999

Maximum extent of glaciation in the Northern Hemisphere during the ice age

Pleistocene Ice AgePleistocene Ice Age

Tarbuck & Lutgens, J.R. Griffin , 1999

North America Coastline during maximum Pleistocene glaciation

Location of coast line if all the present ice sheets melt


W. Wayne, J.R. Griffin, N. Lindsley-Griffin, 1999

Northern Midwest

Blue line = Maximum extent of latest period of glaciation (Wisconsin)

Red = Maximum extent of earlier Pleistocene glaciation


W. Wayne, J.R. Griffin, N. Lindsley-Griffin, 1999

Blue line = Maximum extent of latest period of glaciation (Wisconsin) in Nebraska

Red = Maximum extent of earlier Pleistocene glaciation in Nebraska


NE Conservation & Survey, J.R. Griffin , 1999 Light green area -Glaciated area

In Alaska, loess is forming today

Loess, wind-deposited silt, is common near glaciers because of abundant rock flour

Loess, Indian Cave State Park, NE Thick Pleistocene loess, AK

NE Conservation & Survey, J.R. Griffin , 1999 Olive green - Area covered by Loess

Documents

Glaciers and Climate Change Chapter 13 Geology Today Barbara W. Murck Brian J. Skinner N. Lindsley-Griffin, 1999 Mount Fairweather, Glacier Bay, Alaska