Cold Climate Glaciers and Ice Ages Glaciers Glacier

Cold Climate - Glaciers and Ice Ages

Glaciers • Glacier: a large, long-lasting mass of ice, formed on land that moves under the influence of gravity and its own weight • Glaciers form by accumulation and compaction of snow – Packed snow becomes firn – Then refreezes to ice

Formation of Glacial Ice from Snow

Glaciation Types • Alpine glaciation: found in mountainous regions • Continental glaciation: exists where a large part of a continent is covered by glacial ice Cover vast areas

Davidson Glacier near Haines, Alaska An Alpine glacier system

Types of Glaciers Alpine Continental

Alpine Glaciers • Are confined by surrounding mountains Types: • Cirque Glaciers – erode basins in mountainsides • Valley Glaciers – flow into preexisting stream valleys • Icecaps – form on mountaintops

Types of Glaciers– Cirque Glacier Mount Edith Cavell, Jasper National Park, Canada

Types of Glaciers – Valley Glacier Tongas National Forest, Alaska

Types of Glaciers – Icecap and Continental http: //www. nmm. ac. uk/server/show/con. Web. Doc. 1221 Sentinal Range, Antarctica • Antarctica is the broadest high place on Earth, the ice cap is up to 4 km thick and covers the continent • Antarctica is a desert, with only 15 cm (6 inches) of snowfall a year around the South Pole • The lowest recorded temperature is -89. 2 °C. • There is no life in Antarctica except near the coast

Types of Glaciers – Piedmont & Tidewater Piedmont: Originally confined alpine, spread at foot of mountains Source: Jim Wark/Peter Arnold, Inc. Calving

Iceberg Calving – Hubbard Glacier, Wrangell-St. Elias National Park, Alaska Releases fresh water to oceans, CO 2 to atmosphere. More on this later.

A Glacier’s Budget • Budget = Gain – Loss • Gains snow in zone of accumulation • Loses ice in zone of ablation • Budget can be positive (net growth) • Static • or negative (net melting)

A Glacier’s Budget Year round Snow Summer Rain Note that a glacier is a river. Even if the terminus doesn’t advance, still flows

Glacial Flow • Internal deformation – Ice crystals slide past one another • Basal Sliding – Entire glacier slides downhill on a thin film of meltwater at its base. • Glacier always flows toward zone of ablation

Mechanics of Glacial Flow

Erosion by Glaciers • Abrasion – Rocks embedded in glacier’s base make linear scratches and grooves in bedrock • Quarrying – Glacier breaks off and removes large blocks of rock. FROST WEDGING is important

Glacial Abrasion in Bedrock Source: Tom Bean

Glacial Erosion – Roche Moutonée

Glacial Erosion – Roche Moutonée Yosemite NP, Calif

Erosion by Glaciers (cont. ) • Alpine glaciers erode mountain slopes into horseshoe shaped basins called cirques – Melting forms cirque lake (tarn) • Erosion of two or more cirques erodes intervening rock – Horns : pointy peaks made by trios – Arêtes: long serrated ridges by pairs – Cols: passes through the arêtes

Alpine Glacial Erosion

Alpine Glacial Erosion Origin of Hanging Valley

Yosemite Falls

Valley Glaciers • Erode a large quantity of bedrock and sediment • Convert V-shaped stream valleys into Ushaped glacial valleys.

U-Shaped Valley in Tracy Wilderness, Southeastern Alaska

Seawater Flooded U-Shaped Valleys: Fjords Bela Fjord, BC When glaciers melt, sea-level rises

Erosion by Continental Glaciation • Erosional Landforms much larger in scale than alpine glaciers – Whalebacks – huge Roche Moutonée – Huge U-shaped troughs – Finger Lakes, Great Lakes, Puget Sound, Loch Ness were all once stream valleys excavated by Ice Sheets

Erosion of Preglacial Lowlands (Finger Lakes)

Erosion of Lowlands (Great Lakes, Finger Lakes) Superior M i c h i g a n Huron Ontario Erie Source: U. S. Dept. of Interior, USGS Eros Date Center

Glacial Deposits - Drift • Collectively called Glacial Drift • TYPE 1: UNSORTED • Glacial Till: unsorted, unstratified sediments deposited by melting ice. – May contain glacial erratics – Often accumulates in glacier’s channel and at its terminus as a Moraine: – Terminal Moraine: hills of sediment left by a glacier’s retreat. – Terminal Moraines may be reshaped by a later glacial advance into Drumlins: rounded elongated hills perpendicular to their original orientation

Advance & Retreat: Moraines Note moraine, retreat or stationary Stationary Analogy: Escalator

Stationary & Retreat: Moraine at Terminus

Large Granite Erratics

Lateral and Medial Moraines

Wrangell-St. Elias NP, SE AK Lateral and Medial Moraines – Kennicott Glacier

The Origin of Drumlins Glacier retreats, leaving behind a terminal moraine. Later it advances again, and reshapes the moraine into a drumlin.

Drumlins Rochester, NY

Glacial Deposits - Drift • TYPE 2: SORTED • Outwash: sorted stratified sediments deposited by meltwater streams • Loess: wind erosion of drying outwash silt. • Eskers: sinuous meltwater deposits of sand gravel underneath ice

Origin of Eskers

Eskers in Coteau des Prairies, South Dakota

Effects of Glaciation • Change Climate – increase precipitation locally - pluvial lakes • Depress continents & lateral rebound • Drop sea-level: alter coastlines • Form continent-wide Dams – Divert streams – Ohio and Missouri rivers

Formation of Terraces due to Crustal Rebound

Lowered Sea-level - Landbridge

Glacier Distribution 20, 000 ya Approximate Maximum

Lowered Sea-level exposed continental shelf

The Creation of Glacial Lake Missoula Purcell Lobe blocks Clark Fork River

The Draining of Glacial Lake Missoula Repeated many times, last time 13000 kya

Giant Ripples of the Missoula Flooding

Causes of Ice Ages • Plate Tectonics Moves Continents to Poles Raises mountains above snowline • Orbit distance, Axis Tilt and Wobble – Moderates solar radiation north of 65 N – Milankovitch Cycles ~ 100, 000 years – Low summertime radiation 65 N, glaciers expand

Milankovitch Cycles Discussion: cool summers and wet winters Moisture content of air masses 100, 000 years

Warm Wet Winter Cool Summer Cold Dry Winter Hot Summer 41, 000 years

Discussion: Perihelion and Aphelion 25, 700 years

One More Point On This • The orbital affects that Milankovitch suggested as a partial cause for ice ages each have a different period. • They combine at irregular intervals • The average is about 100, 000 years but that is ONLY an average

Earth’s Past Ice Ages (oldest on bottom) • Tertiary- Quaternary cooling – Pleistocene 1. 8 mya • None in Mesozoic • Late Pennsylvanian & Permian Ice covered part “Gondwana” (South Africa, South America, India, Australia, Antarctica) • Ordovician glaciation – Area that is now the Sahara at South Pole • Pre. Cambrian Tillites (Lithified Till) At least three episodes. Interesting examples: – 750 mya ice from poles to tropic “SNOWBALL EARTH” – Oldest 2. 8 bya

Permian Glaciation – Gondwana Tillites Poorly Sorted Unstratified

Cenozoic Cooling

The Late Tertiary and Quaternary oxygen isotope record measured in marine fossil shells Evap. water and CO 2 during glacial time removes 16 O to glacier ice leaving 18 O in oceans for Ca. CO shell 3

Pleistocene Glaciation • • • Since 1. 6 mya – more than 30 advances and retreats In 4 large scale pulses. Latest retreat 10, 000 years ago Laurentide Little ice age 700 to 150 years ago. Sustained warming since 1850

Foraminifera tests - Ice Age Wisconsinan Illinoian > 30 pulses in 4 or so major groups Kansan Nebraskan Evap. water and CO 2 removes 16 O to 18 O in oceans warm Also spiral direction & diversity dep T during glacial glacier ice leaving for Ca. CO 3 shell cold

Continuous Ice Sheet 20 kya then warming Scoured 30 M below sea-level

Cold pulse from about 1300 to 1850 AD (The so-called “Little Ice Age”) Climate has been warming since then. Discussion: Global Warming Worldwide melting, regardless of cause, releases CO 2 and H 2 O and exposes dark land. The atmosphere receives and holds more heat, and temperature rise. Athabaska Glacier, Columbia Icefield, W. Canada

End of Glaciers