Physical Geography by Alan Arbogast Chapter 17

Physical Geographyby Alan Arbogast

Chapter 17

Glacial Geomorphology: Processes & Landforms

Lawrence McGlinnDepartment of GeographyState University of New York - New Paltz

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!

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)

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

Periglacial Landscape(Cross Section)

Ground Ice• Ground Ice – distinct zones of frozen water

within the ground – variable amts of water

• As these areas freeze & thaw, expand & contract, they cause physical weathering

• Ice Wedge – water enters crack in active layer

• Pingo – surface bulges because of ice under pressure below

• Patterned Ground – land broken into polygons as frost pushes coarser material to surface

Ground Ice LandformsIce Wedge Pingo

Patterned Ground