Glaciers

A. Glacier Formation

What is a glacier? A glacier is an accumulation of ice and snow that flows over land, formed in regions where snow accumulates faster than it melts.

1. Properties of Ice

   • Crystal Structure & Density:
     Glacier ice is composed of hexagonal crystal structures that vary in size and density depending on the compaction and temperature. As snow densifies into glacier ice, its porosity decreases, and its density increases from ~0.3 g/cm³ (fresh snow) to ~0.9 g/cm³ (glacial ice).

     • Key Research: Studies on how crystal deformation affects glacier dynamics, particularly in polar ice sheets (e.g., Antarctica, Greenland).

2. Formation of Glacier Ice

   • Transition from snow to ice occurs over years/decades through pressure-driven compaction. Snow transforms into névé, firn, and finally ice due to recrystallization.

   • Glacial Ice Crystallization - Over time, firn transforms into solid ice as air pockets are squeezed out, and the ice crystals grow larger, sometimes reaching the size of a fist.

   • Firn is an intermediate stage of glacial ice, where snow has been compressed into dense ice that is about two-thirds as dense as water.

   • Over time, firn  transforms into solid ice as air pockets are squeezed out, and the ice crystals grow larger, sometimes reaching the size of a fist.
     
     

   • Research Areas: Snow metamorphism and firn densification processes are critical for understanding glacier growth and shrinkage.

3. Glacial Budget/Mass Balance

   • The balance between accumulation (snowfall) and ablation (melting, sublimation). The equilibrium line altitude (ELA) marks where these processes balance.

   • Glacier Mass Balance - Glaciers grow in the accumulation zone (where snow adds more mass than melts) and shrink in the ablation zone (where melting or calving exceeds accumulation).

   • Current Issues: Global warming is shifting the ELA upward, contributing to glacier retreat worldwide. Key studies focus on glaciers in the Himalayas, Andes, and Alps.

4. Glacial Flow

   • Ice flows through internal deformation (creep) and basal sliding, influenced by temperature, slope, and water at the bed.

   • Findings: Enhanced basal sliding due to meltwater at glacier beds has accelerated flow in temperate and polar glaciers.

B. Types of Glaciers & Their Geographic Distributions

1. Valley/Alpine Glaciers

   • Found in mountainous regions (e.g., the Himalayas, Rockies). Types include cirque, hanging, and piedmont glaciers.

   • Alpine glaciers are glaciers that form in mountainous regions and flow down valleys, acting like frozen rivers of ice.

2. Ice Sheet/Continental Glaciers

   • Massive glaciers covering Greenland and Antarctica. Features include ice streams and ice shelves.

   • The Greenland Ice Sheet spans about 1.7 million square kilometers, covering 80% of Greenland. Its complete melting would raise sea levels by about 7.4 meters.

   • Key Research: Ice shelves act as buttresses, slowing down glacier flow. Recent studies emphasize their vulnerability to ocean warming.

3. Cirques

• Cirques are bowl-shaped depressions carved by glaciers, often forming tarn-filled lakes after glaciers retreat.

• Glaciers flow in the direction their surface slopes, enabling them to move uphill in certain terrains.

C. Features in Glacial Ice

1. Crevasses, Ogives, Icefalls

   • Crevasses form from differential flow; ogives appear as wave-like ridges, and icefalls occur where glaciers flow over steep terrain.

   • Research: Studies on crevasse propagation help understand glacier calving and instability.

2. Ice Shelf Processes

   • Ice shelves like those in Antarctica are vulnerable to warming and play a critical role in stabilizing ice sheets.

   • Key Findings: Research on the Larsen B and Thwaites Glacier ice shelves shows rapid disintegration due to warming.

     3. Glacial Ice Caps and Sheets

• Ice caps are smaller than ice sheets, usually less than 50,000 square kilometers, and form in polar and subpolar mountain regions, flowing in all directions.

•  Icefields are similar to ice caps but are generally smaller and more influenced by the underlying terrain. They don’t form a dome shape like ice caps.

D. Formation of Landscape Features by Glaciers

1. Erosional Features

• Glaciers carve cirques, arêtes, horns, U-shaped valleys, and Roche moutonnée.

• Research: Modeling glacier erosion rates and sediment transport in glacial landscapes.

2. Glacial Erosion - Nunataks, Aretes, and Horns

• Nunataks are rocky islands that are surrounded by flowing glacier ice. As the glaciers surrounding a nunatak come together, a medial moraine composed of rockfall from the nunatak often marks their confluence.

• An arête is a thin, jagged crest that separates—or that once separated—two adjacent glaciers. These rugged ridgelines often look like serrated knives or saw blades, with steep sides and a sharp crest. The low points on the serated surface are known as cols. Cols act as spillways for the ice and occur where glacier action has eroded the rock sufficiently to overtop it.

• Horns are pointed peaks that are bounded on at least three sides by glaciers. They typically have flat faces that give them a somewhat pyramidal shape and sharp, distinct edges.

4. Depositional Features

• Moraines (end, lateral, medial), drumlins, eskers, and erratics are formed by deposited glacial debris.

1. Lateral moraines 

• are ridges of debris left by melting glaciers, marking the boundary between the accumulation and ablation zones.

• They remain on the landscape after glaciers retreat and often connect with terminal moraines.

2. Medial moraines 

• form where two glaciers meet, consisting of debris from rockfalls at the convergence point.

• They are thin and typically not preserved after the glaciers retreat.

3. Terminal and recessional moraines 

• If a glacier remains stationary, a large terminal moraine forms, but if it retreats quickly, smaller recessional moraines are created as the terminus moves.

• mark the farthest extent of a glacier at a given time, formed by debris transported to the glacier’s edge and left behind as the ice melts.

• Studies: Emphasis on mapping moraines to reconstruct past glacial extents.

4. Drumlins 

• elongated hills of sediment shaped by glacier flow, often found in large fields with thousands of them.

• Their classic shape is a hill that’s highest at the up-glacier end and tapers toward the down-glacier end, and they help scientists determine past glacier flow directions.

5. Eskers 

• Meandering ridges of sediment, called eskers, form in water channels beneath or within glacier ice.

• As water slows, it drops sediment, creating winding ridges that follow the path of the channels.

6. Lakes

• Tarns, proglacial lakes, and kettle lakes form from glacial processes.

• Current Focus: Research on glacial lake outburst floods (GLOFs), which pose significant hazards.

E. Periglacial Processes & Landforms

• Permafrost: Permanently frozen ground found in polar and alpine regions. Warming permafrost releases methane, a potent greenhouse gas.

• Pingos: Ice-cored hills formed by permafrost processes.

• Studies: Impacts of permafrost thaw on infrastructure and ecosystems.

F. Sea Ice

• Formation includes frazil ice (small crystals), pancake ice (circular floes), and pressure ridges.

• Key Issues: Decline in Arctic sea ice extent due to climate change, with significant implications for albedo and ocean circulation.

G. Glacial Hydrology

• Surface Melt and Lakes: Seasonal melt influences glacier flow; surface lakes contribute to hydrofracturing.

• Subglacial Lakes: Over 400 discovered beneath Antarctica; they hold clues to ancient ecosystems.

• Research: Studies on how subglacial hydrology affects glacier stability and dynamics.

H. Global Connections of Glaciation

1. Atmosphere:

   • Melting glaciers alter albedo, releasing greenhouse gases (e.g., CO₂, CH₄) stored in ice.

2. Oceans:

   • Sea-level rise from melting ice sheets. Key studies on ice-ocean interactions (e.g., Thwaites Glacier).

3. Lithosphere:

   • Glacial isostatic adjustment (e.g., Earth’s crust rebounds as glaciers retreat).

4. Planetary Influence:

   • Milankovitch cycles explain ice age cycles through orbital changes.

I. History of Ice on Earth

1. Snowball Earth:

   • Global glaciation during the Neoproterozoic (~700 million years ago).

   • Research: Focus on geological evidence like dropstones and glacial deposits.

2. Pleistocene Glaciation:

   • Laurentide Ice Sheet retreat studied for insights into past climate changes.

3. Recent Cryosphere Changes:

   • Rapid disintegration of glaciers like Larsen B and Thwaites.

J. Sedimentary Sequences in Glacial Environments

• Varves (annual sediment layers) and till (unsorted glacial debris) provide records of past glacier activity.

K. Methods of Studying Glaciers

1. Techniques:

   • Altimetry, radar, optical imagery, and gravimetry to monitor glacier changes.

2. Ice Cores:

   • Contain records of past climates through trapped gases and isotopes.

L. Glacial Hazards

• Ice Avalanches: Triggered by icefalls or warming conditions.

• Glacial Lake Outburst Floods (GLOFs): Sudden draining of proglacial lakes, causing catastrophic flooding.

1. Definition of a Glacier
   A glacier is an accumulation of ice and snow that flows over land, formed in regions where snow accumulates faster than it melts.
   
   

2. Glacial Movement
   Glaciers move slowly downhill under their own weight, and the flow is driven by gravity.
   
   

3. Alpine Glaciers
   Alpine glaciers are glaciers that form in mountainous regions and flow down valleys, acting like frozen rivers of ice.
   
   
   

4. Antarctic Ice Sheet
   The Antarctic Ice Sheet covers nearly 14 million square kilometers and holds around 60% of the world's freshwater. If it melted, global sea levels would rise by about 60 meters.
   
   

5. Greenland Ice Sheet
   The Greenland Ice Sheet spans about 1.7 million square kilometers, covering 80% of Greenland. Its complete melting would raise sea levels by about 7.4 meters.
   
   
   

6. Icefields
   Icefields are similar to ice caps but are generally smaller and more influenced by the underlying terrain. They don’t form a dome shape like ice caps.
   
   

7. Global Ice Distribution
   Over 99% of Earth's freshwater ice is contained in the Antarctic and Greenland Ice Sheets. Together, they store over 68% of the world's fresh water.
   
   

8. Icebergs
   Icebergs are large chunks of ice that break off glaciers or ice sheets and float in the ocean or lakes. They can range from small to massive and are dangerous to ships.
   
   

9. Bergy Bits and Growlers
   Smaller icebergs, called bergy bits and growlers, are harder to spot and pose significant danger to ships, especially in cold waters.
   
   

10. How Glaciers Form
    Glaciers begin forming when snow accumulates year-round, compacts under its weight, and eventually turns into firn (intermediate ice) and then solid glacier ice.
    
    

11. Firn Formation
    Firn is an intermediate stage of glacial ice, where snow has been compressed into dense ice that is about two-thirds as dense as water.
    
    

12. Glacial Ice Crystallization
    Over time, firn transforms into solid ice as air pockets are squeezed out, and the ice crystals grow larger, sometimes reaching the size of a fist.
    
    

13. Glacier Mass Balance
    Glaciers grow in the accumulation zone (where snow adds more mass than melts) and shrink in the ablation zone (where melting or calving exceeds accumulation).
    
    

14. Glacier Flow and Gravity
    Gravity causes glaciers to flow slowly downhill, shaping the land beneath them and creating features like valleys, fjords, and moraines.
    
    

15. Glacial Features
    Glaciers leave behind features such as moraines, eskers, drumlins, and fjords, which are shaped by their movement and the debris they transport.
    
    

16. Glacier Retreat and Advance
    A glacier retreats when melting exceeds accumulation, and it advances when snowfall surpasses melting. This balance is affected by climate conditions.
    
    

17. Global Glacial Distribution
    Most of the world’s glaciers are found near the poles, in high-altitude regions like the Himalayas, Alps, and Andes, but glaciers can be found on every continent except Australia.