Landforms Geography

Glaciers

Glacial Geomorphology

• Development of a glacier

• Types of glaciers

• Glacial landforms

• History of glaciers

• What causes glaciation?

• Impact of global climate change on glaciers

• Periglacial processes and landscapes

Development of a Glacier

• Glacier – slowly moving mass of dense ice formed by gradual thickening, compaction, and refreezing of snow & water over time

• After summer melt, some snow left over

• With weight and partial melting, snow turns to Firn, crunchy transition from snow to ice

• Further compaction, ice crystals align, become dense glacial ice which flows slowly downslope

• At least 40-m thick to become glacier

Glacial Mass Budget

• Glacial input : Snow

• Glacial output : ice, meltwater or water vapor

• Zone of Accumulation – top of glacier where temps are cooler - input > output

• Zone of Ablation – lower part of glacier where temps are higher – output > input

• Equilibrium line – point on glacier where input = output

Glacial Formation

Glacial Mass Budget

Glacial Mass Budget

Glacial Movement

• Glaciers move through internal deformation

• Interior of glacier like malleable plastic

Glacial Movement

Glacier Types

• Mountain Glaciers– Ice Cap – Continuous sheet of ice covering entire

landscape– Ice Field – Buries all but tallest mountains – can be

very thick– Alpine Glacier – Flows down valleys away from

high country– Cirque - Bowl-shaped depression on mountain flank

due to glacial erosion – snow source

Alaskan Glaciers

Hubbard Glacier

Continental Glaciers

• Huge ice masses covering a large part of a continent or large island – also called ice sheets

• More than 3000 m deep in places

• Covers most of Antarctica and Greenland

• Weight of ice presses lithosphere down into asthenosphere, called isostatic depression

Continental Glaciers

Glacial Landforms

• Rock & debris picked up by glaciers, transported in direction of movement & deposited

• Glacial erosion:– Glacial Abrasion – scratch and gouge bedrock– Glacial Striations – caused by glacial abrasion– Glacial Grooves – deep striations– Glacial Plucking – boulders ripped from ground by

glacier – deposited by retreating glacier, called Glacial Erratics

Glacial Erosional Landforms• Roche Moutonnée – rounded hill, gradual on

side toward direction from which glacier comes

Glacial Striations Glacial Erratic

Alpine Erosional Landforms

• Glacial Erosion:– Cirque – bowl-like feature on mountain flanks– Tarn – small lake in bottom of cirque– Arête – narrow, steep ridges between cirques– Horn – mountain with 3 or more arêtes at summit– Glacial Trough – u-shape valley eroded by glacier– Hanging Valley – side trough above main trough –

possible waterfall

Alpine Erosional Landforms

Cirque

Horn

“Matterhorn”

Glacial Trough

Glacial Depositional Landforms (Till)

• Glacial Till – sediment directly deposited by glacier – many particle sizes

• Moraine – winding ridge formed by till at the front or side of glacier – Moraine types:– Lateral – along former edges of glacier– Terminal – along front of former glacier– Recessional – formed as glacier recedes– Medial – between 2 glaciers– Ground – irregular deposition as glacier recedes

Glacial Depositional Landforms (Till)

Glacial Depositional Landforms (outwash)

• Glacial Outwash – sediments deposited by water out & under a glacier as it melts – forms Outwash Plain, flat feature in front of former glacier

• Kame – large mound deposited near glacier front

• Esker – winding ridge from water flowing in tunnel through ice under glacier

• Kettle Lake – big ice block fallen off glacier front is buried by outwash, melts later forming lake

Glacial Depositional Landforms

Glacial Depositional Landforms

History of Glaciation• As early as 2.3 B years ago, ice covered much of

Earth, and off and on since then

• Most important Ice Age was Pleistocene Epoch, 1.8 M years ago till 10K years ago

• Glacial – period when glaciers expand from poles – cooler temps, lower sea level,

• Interglacial – period when glaciers recede: warmer temps, higher sea level

Pleistocene Glaciations

• Named for southern extent of ice sheet in North America– Nebraskan – 1 million yrs ago– Kansan – 625 K yrs ago– Illinoisan – 300 K yrs ago– Wisconsin – 35 K to 10 K yrs ago

• Laurentide Ice Sheet – eastern North America

• Cordilleran Ice Sheet – western North America

Maximum Extent of Pleistocene Glaciation

30% of earth’s surface covered by ice sheets (Only 11% coverage today)

Oxygen Isotopes

Evidence of More Glaciations?

• Ice core samples suggest more than the known 4 glaciations – show more cool, glacial periods

• Oxygen isotopes O-16 & O-18 both in water, but O-18 evaporates more in warmer climate, so ratio of O-16 to O-18 in ice cores can indicate relative warmth of climates over 1 million yrs ago!

Great Lakes

Causes of Glaciation• Summer temp (melting) is key to glaciation

• Possible Factors:1. Variations in solar radiation (dust, sunspots…)

2. Reduced carbon dioxide (escaping heat)

3. Increased volcanic activity (reflective dust)

4. Variations in Earth-Sun geometry (axial tilt, shape of orbit, rotation)

5. Plate Tectonics

Milankovitch Theory

• Dominant theory of causes of glaciation, based on Earth-Sun geometry:

• Orbital eccentricity – strongly elliptical orbit puts Earth farthest from Sun in summer, cooling it

• Tilt obliquity – Earth’s tilt varies from 22.1º to 24.5º - less tilt means lower angle Sun and less insolation at poles, thus cooler summers

• Orbital precession – wobbles of Earth’s axis - North Pole may point toward Sun at farthest point of orbit, creating a cool summer

Milankovitch TheoryOrbital Eccentricity Axial Tilt Orbital Precession

When three factors coincide, high probability of glaciation

Glacial Geomorphology: Processes and Landforms

Climate Change and Glaciers

• Since mid-1800s glaciers have been receding, both alpine and continental

• Alps, Parts of Andes, Mt. Kilimanjaro melting

• Thousands of sq miles of Antarctica & Greenland ice sheet lost over last 30 years due to warming

• Melting area of Greenland has increased rapidly since early 1990s

Climate Change and Glaciers

Climate Change and Glaciers

Periglacial Processes and Landscapes

• In near-glacial environments – constant freeze/thaw cycle effects on landscape

• Permafrost – ground that is permanently frozen– Continuous – poleward of -7ºC mean annual isotherm

– all surfaces frozen exp under water – avg 400 m thick, up to 1000 m thick

– Discontinuous – poleward of -1ºC mean annual isotherm – thinner than continuous, esp. on south facing slopes

Extent of Permafrost

Permafrost Processes

• Active Layer – soil that melts & refreezes daily or seasonally – as thin as 10 cm in continuous permafrost, up to 2 m thick in discontinous

• Dramatic warming in arctic is making active layer much thicker & releasing tons of CO2

• Talik – body of unfrozen ground within permafrost, e.g. under a lake, important for movement of groundwater