Lecture Outlines

Physical Geology, 15/e

© McGraw-Hill Education. Permission required for reproduction or display.

Plummer, Carlson & Hammersley

Glaciers & Glaciation

Physical Geology 15/e, Chapter 12

© McGraw-Hill Education. Permission required for reproduction or display.

Glacier – a large, long-lasting mass of ice, formed onland, that moves downhillunder its own weight

• Glaciated Terranes

• Alpine – found inmountainous regions

• Continental – largeparts of continentscovered by glacialice

~ 70% of the world’s supply

of fresh water is locked up inglacial ice

© McGraw-Hill Education. Permission required for reproduction or display.

What is a Glacier?

Types of Glaciers

Develop as snow is compacted andrecrystallized, first into firn and then glacialiceCan only form where more snow accumulatesduring the winter than melts away during thespring and summer

• alpine glaciation occurs in mountainousregions as valley glaciers

• continental glaciation covers large landmasses in Earth’s polar regions in the formof ice sheets

• glaciation occurs in areas cold enough toallow accumulated snow to persist fromyear to year

© McGraw-Hill Education. Permission required for reproduction or display.

What is a Glacier?

Distribution of Glaciers• Most extensive in polar climates but can occur anywhere

there is more snow falling than melting during the year.• In the USA, Alaska has the most glaciers followed by

Washington State• Approximately 10% of Earth’s surface is covered by

glaciers.• Approximately 85% of all glacial ice is in Antarctica.• If all the ice on Antarctica were to melt sea level would

rice approximately 65 meter (213 feet) flooding theworlds coastal cities

© McGraw-Hill Education. Permission required for reproduction or display.

Glaciers-Where They Are,How They Form and Move

Formation and Growth ofGlaciers•Snowfall•Compaction of the snow removes air•Snowflakes recrystallize into granules•Firn – transitional between granular snowand glacial ice•Glacial Ice – formed once the firn is furthercompacted and more air removed and has acrystallize texture similar to the metamorphicrock quartzite•Gravity causes the glacier to movedownslope•Ablation – loss of the glacier due tomelting, evaporation or calving of icebergs

© McGraw-Hill Education. Permission required for reproduction or display.

Glaciers-Where They Are,How They Form and Move

Formation and Growth of Glaciers

Glacial Budgets• Advancing glacier – gains more snow than it loses, has a positive

budget

• Receding glacier – loses more snow than it gains, has a negativebudget• zone of accumulation – snow added• zone of ablation – melting and calving of icebergs• equilibrium line– separates accumulation and ablation zones, will

advance or retreat depending on climate

© McGraw-Hill Education. Permission required for reproduction or display.

Glaciers-Where They Are,How They Form and Move

Movement of Valley Glaciers• Move downslope due to gravity• Basal sliding –sliding of the

glacier over the underlying rock• Plastic flow – movement that

occurs within the glacial ice dueto its plastic nature

• Rigid zone – upper part of theglacier that moves rigidly downslope• Crevasses – fractures formed in

the upper rigid zone duringglacier flow

• Glacier flow is fastest at the topcenter of a glacier and slowestalong its margins due to friction

© McGraw-Hill Education. Permission required for reproduction or display.

Glaciers-Where They Are,How They Form and Move

Movement of Ice Sheets• Move downslope and outward from a

central high area due to gravity• Basal sliding, plastic flow and rigid

zone movement similar to alpineglaciers

• Antarctica – two ice sheets separated bythe Transantarctic Mountains (WestAntarctic Ice Sheet and the EastAntarctic Ice Sheet)

• Outlet glaciers – mountainvalley glaciers that occur wherethe mountains are higher thanthe ice sheet

• Ice streams- flow zones that aremuch faster than adjoining ice

© McGraw-Hill Education. Permission required for reproduction or display.

Glaciers-Where They Are,How They Form and Move

Glaciers erode underlying rock byplucking of rock fragments andabrasion as they are dragged along

• basal abrasion polishes andstriates the underlying rocksurface and produces abundantfine rock powder known asrock flour

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Erosion

Erosional Landscapes Associatedwith Alpine Glaciation

• Glacial Valleys

• U-shaped valleys

• Hanging valleys – smallertributary glacial valleys leftstranded above more quicklyeroded central valleys

• Truncated Spurs – ridges thathave triangular facets

• Cirques, Horns and Aretes

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Erosion

Erosional LandscapesAssociated with AlpineGlaciation

•Cirques – steep-sided,half-bowl-shaped recessescarved into mountains at theheads of glacial valleys

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Erosion

Erosional Landscapes Associated with Alpine Glaciation

• Arêtes – sharp ridges separating glacial valleys

• Horns – sharp peaks remaining after cirques have cut back into amountain on 3+ sides

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Erosion

Erosional Landscapes Associatedwith Continental Glaciation

• Rounded topography is morecommon

• Weight and thickness of continentalice sheets produce morepronounced effects

• Rounded knobs

• Grooved or striated rock(several meters deep andkilometers long)

• Thick enough to bury mountainsrounding off ridges and summits

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Erosion

Till – general name for unsorted, unlayered glacial sediment

• deposits of till left behind at the sides and end of a glacier arecalled lateral, medial and end moraines, respectively

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Deposition

Moraines

• Lateral Moraines - elongate, lowmounds of till along sides ofvalley glaciers

• Medial moraines – lateralmoraines trapped between adjacentice streams

• End moraines – ridges of till piledup along the front end of a glacier

• Recessional moraines –successive end moraines leftbehind by a retreating glacier

• Ground Moraine –thin layer of till atthe base of the glacier

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Deposition

Outwash – sediment deposited by large amounts ofmeltwater flowing over, beneath and away from the ice atthe end of a glacier

• sediment-laden streams emerging from ends ofglaciers have braided channel drainage patterns

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Deposition

Outwash

• Eskers – sinuous ridge

• Kettles – glacial depression

• Kames – low glacial mound

Glacial Lakes and Varves -annual sediment deposition in glaciallakes produces varves, which can becounted like tree rings

© McGraw-Hill Education. Permission required for reproduction or display.

Glacial Deposition

In the early 1800s, past extensive glaciation of Europe was firsthypothesized

• hypothesis initially considered outrageous, but further observations byLouis Agassiz (initially a major opponent of the hypothesis) in SwissAlps found much supporting evidence

• Agassiz traveled widely in Europe and North America, finding moreand more supporting evidence, eventually leading to the theory ofglacial ages

Theory of glacial ages states that at times in the past, colder climatesprevailed during which much more of the land surface of Earthwas glaciated than at present

• most recent glacial age was at its peak only ~18,000 years ago• Earth has undergone episodic changes in climate during the last 2-3

million years (Tertiary Period and later Pleistocene epoch)

© McGraw-Hill Education. Permission required for reproduction or display.

Past Glaciation

Direct Effects of PastGlaciationLarge-scale glaciation of NorthAmerica during the most recent iceage produced the following effects:

• most of the soil andsedimentary rocks were scrapedoff underlying crystalline rockin northern and eastern Canada,and lake basins were gougedout of the bedrock

• extensive sets of recessionalmoraines were left behind byretreating ice sheets in theupper Midwestern U.S. andCanada

© McGraw-Hill Education. Permission required for reproduction or display.

Past Glaciation

Indirect Effects of PastGlaciation

• Glacial Lakes

• Lake Agassiz

• Lake Missoula

• Pluvial Lakes (formed in a periodof abundant rainfall) existed inclosed basins in Utah, Nevada andeastern California

• Lake Bonneville

• Lake Missoula

© McGraw-Hill Education. Permission required for reproduction or display.

Past Glaciation

Indirect Effects of Past Glaciation

• Lowering and Rising of Sea level

• fiords - coastal inlets formed by drowning of glaciallycarved valleys by rising sea level

• Crustal Rebound

• Great Lakes region continues to rebound as crust adjustto removal of the last ice sheet

© McGraw-Hill Education. Permission required for reproduction or display.

Past Glaciation

Evidence for Older Glaciation• Tillites – lithified glacial till, have

distinctive textures that suggestemplacement of sediments by glaciers• Paleozoic era in portions of the

southern continents indicate that theselandmasses were once joined

• Snowball Earth hypothesis – LatePrecambrian glaciation when theoceans were frozen over.

• Oldest glaciation evidence dates backto 2.3 billion years ago

• strong evidence supporting theory ofplate tectonics

© McGraw-Hill Education. Permission required for reproduction or display.

Past Glaciation

End of Chapter 12

© McGraw-Hill Education. Permission required for reproduction or display.