🌋Physical Geology Unit 13 – Glaciers and Glacial Landscapes

What Are Glaciers?

• Glaciers are large, persistent bodies of ice that form on land and move under the influence of gravity
• Consist of compacted snow that has accumulated over many years and recrystallized into ice
• Require specific climatic conditions to form, including cold temperatures and sufficient snowfall
• Play a crucial role in Earth's water cycle by storing water in solid form and releasing it gradually
• Glacial ice appears blue due to the absorption of red light and the scattering of blue light within the ice
• Glaciers cover approximately 10% of Earth's land surface, primarily in polar regions and high mountain ranges (Greenland, Antarctica, the Himalayas)
• Act as sensitive indicators of climate change, as they respond to variations in temperature and precipitation

How Glaciers Form and Move

• Glaciers form through the accumulation and compaction of snow over many years
• Snow that survives the summer melt season is called firn, which is an intermediate stage between snow and glacial ice
• As new snow falls on top of the firn, the weight compresses the underlying layers, causing the snow crystals to recrystallize into denser, interlocking ice crystals
• Glacial movement occurs through a combination of internal deformation and basal sliding
  • Internal deformation involves the gradual flow of ice within the glacier due to the force of gravity
  • Basal sliding occurs when meltwater at the base of the glacier lubricates the ice, allowing it to slide over the underlying bedrock
• The rate of glacial movement varies depending on factors such as the glacier's size, slope, and the presence of meltwater at its base
• Glaciers in temperate regions tend to move more quickly than those in polar regions due to the presence of more meltwater
• Glacial movement can result in the transportation of rock debris, which is deposited as the glacier retreats or melts, forming various glacial landforms (moraines, erratics)

Types of Glaciers

• Glaciers can be classified into two main types: alpine glaciers and ice sheets
• Alpine glaciers, also known as valley glaciers, form in mountainous regions and are confined by the surrounding topography
  • Examples of alpine glaciers include cirque glaciers, which occupy bowl-shaped depressions on mountain slopes, and hanging glaciers, which cling to steep mountainsides
• Ice sheets are large, continuous masses of ice that cover vast areas of land, often spanning hundreds of thousands of square kilometers
  • The two main ice sheets on Earth today are the Antarctic Ice Sheet and the Greenland Ice Sheet
• Ice caps are smaller than ice sheets and typically cover less than 50,000 square kilometers
  • Ice caps are found in high-latitude regions and can cover entire mountain ranges or islands (Vatnajökull in Iceland)
• Outlet glaciers are channels of ice that flow from an ice sheet or ice cap through valleys or fjords to the sea
  • The Jakobshavn Glacier in Greenland is a well-known example of an outlet glacier
• Tidewater glaciers are glaciers that terminate in the ocean, often calving icebergs into the sea (Hubbard Glacier in Alaska)
• Rock glaciers are composed of a mixture of ice and rock debris and are found in high-altitude, arid regions (the Andes, the Rocky Mountains)

Glacial Erosion Processes

• Glaciers are powerful agents of erosion, capable of reshaping the landscape through various processes
• Abrasion occurs when rock fragments embedded in the base of the glacier scrape and polish the underlying bedrock
  • This process can create smooth, striated surfaces and glacial polish on the bedrock
• Plucking involves the glacier freezing onto and dislodging large chunks of bedrock, which are then transported by the ice
  • Plucking is responsible for the formation of jagged, angular features such as cirques and arêtes
• Frost wedging is the process by which water seeps into cracks in the bedrock, freezes, and expands, causing the rock to fracture and break apart
  • This process is enhanced by the presence of a glacier, which provides a constant supply of meltwater
• Glacial meltwater can also contribute to erosion through the formation of subglacial streams and rivers
  • These streams can transport sediment and carve channels into the bedrock, creating features such as glacial troughs and tunnel valleys
• The combined effects of abrasion, plucking, frost wedging, and meltwater erosion can dramatically alter the landscape, creating distinctive glacial landforms (U-shaped valleys, cirques, horns)

Glacial Landforms and Features

• Glaciers create a wide variety of landforms and features through erosion and deposition
• Cirques are bowl-shaped depressions carved into mountainsides by the erosive action of alpine glaciers
  • Cirques often have steep headwalls and a flat or gently sloping floor, and may contain small lakes called tarns
• Arêtes are sharp, knife-like ridges that form between adjacent cirques as the glaciers erode the mountainside from multiple directions
• Horns are steep, pyramid-shaped peaks that result from the erosion of a mountain by cirque glaciers on three or more sides (the Matterhorn in the Alps)
• U-shaped valleys are created by the erosive action of valley glaciers, which widen and deepen the valley through abrasion and plucking
  • U-shaped valleys have steep, parallel walls and a flat or rounded bottom, in contrast to the V-shaped valleys carved by rivers
• Hanging valleys are tributary valleys that are left "hanging" above the main glacial valley due to the more rapid erosion of the main glacier
  • Hanging valleys often feature waterfalls where the tributary stream enters the main valley (Yosemite Valley in California)
• Fjords are long, narrow, and deep inlets of the sea that form when a glacial valley is submerged by rising sea levels after the glacier has retreated (Sognefjord in Norway)
• Moraines are accumulations of rock debris transported and deposited by glaciers
  • Terminal moraines mark the farthest advance of a glacier, while lateral moraines form along the sides of the glacier, and medial moraines form where two glaciers merge
• Erratics are large boulders that have been transported by glaciers and deposited far from their origin, often resting on bedrock of a different composition

Climate Change and Glaciers

• Glaciers are highly sensitive to changes in climate, particularly variations in temperature and precipitation
• The mass balance of a glacier refers to the difference between the amount of snow and ice accumulated through precipitation (accumulation) and the amount lost through melting and sublimation (ablation)
  • A positive mass balance indicates that the glacier is gaining mass, while a negative mass balance indicates that it is losing mass
• In recent decades, many glaciers worldwide have experienced a negative mass balance due to increasing global temperatures
  • This has led to widespread glacial retreat, with glaciers shrinking in size and their termini (endpoints) moving to higher elevations
• The retreat of glaciers has numerous consequences, including rising sea levels, changes in local and regional water resources, and impacts on ecosystems and human communities
  • Glacial meltwater is a crucial source of freshwater for many regions, and the loss of glaciers can lead to water scarcity and increased competition for water resources
• The rate of glacial retreat varies depending on the region and the specific glacier, but the overall trend is one of accelerating ice loss
  • The Greenland and Antarctic ice sheets are of particular concern, as their melting has the potential to cause significant sea-level rise in the coming centuries
• Studying the response of glaciers to climate change is crucial for understanding the impacts of global warming and for developing strategies to mitigate and adapt to these changes
  • Glaciers serve as valuable indicators of climate change, providing a visible and measurable record of the effects of rising temperatures on the Earth's surface

Studying Glaciers: Methods and Tools

• Glaciologists employ a variety of methods and tools to study glaciers and their response to climate change
• Field observations involve direct measurements of glacial characteristics, such as ice thickness, surface elevation, and flow velocity
  • These measurements are often made using GPS, radar, and laser altimetry techniques
• Remote sensing techniques, such as satellite imagery and aerial photography, allow researchers to monitor changes in glacial extent and surface features over large areas and long time scales
  • Multispectral and radar satellites can provide detailed information on glacial surface properties, such as snow cover, albedo, and ice velocity
• Ice cores drilled from glaciers and ice sheets provide a valuable record of past climatic conditions
  • The chemical composition, dust content, and air bubbles trapped in the ice can reveal information about temperature, precipitation, and atmospheric composition over thousands of years
• Glacial geology involves the study of landforms and sediments created by glacial processes
  • By examining the distribution and characteristics of glacial features, researchers can reconstruct the extent and behavior of past glaciations
• Numerical modeling is used to simulate the behavior of glaciers under different climatic scenarios
  • Models can help predict future changes in glacial mass balance, sea-level rise, and the response of glaciers to various forcing factors (temperature, precipitation)
• Glacier monitoring programs, such as the World Glacier Monitoring Service, coordinate the collection and dissemination of data on glacial changes worldwide
  • These programs rely on a network of researchers and institutions to provide standardized measurements and assessments of glacial mass balance and behavior
• Interdisciplinary collaborations between glaciologists, climatologists, hydrologists, and other Earth scientists are essential for understanding the complex interactions between glaciers and the global climate system

Glaciers in Earth's History

• Glaciers have played a significant role in shaping Earth's landscape and climate throughout its history
• During the Pleistocene epoch (2.6 million to 11,700 years ago), Earth experienced a series of glacial-interglacial cycles, with ice sheets advancing and retreating in response to changes in Earth's orbital parameters (Milankovitch cycles)
  • At the peak of the last glacial period (Last Glacial Maximum, ~20,000 years ago), ice sheets covered much of North America, northern Europe, and parts of Asia
• The growth and decay of ice sheets during the Pleistocene had far-reaching effects on global sea levels, atmospheric circulation patterns, and the distribution of flora and fauna
  • Sea levels were up to 120 meters lower than present during the Last Glacial Maximum, exposing vast areas of the continental shelves
• The repeated advance and retreat of glaciers during the Pleistocene played a crucial role in the evolution and dispersal of many species, including humans
  • The expansion of ice sheets and the resulting changes in sea level and climate created barriers and corridors for species migration, influencing patterns of genetic diversity and adaptation
• The Pleistocene glaciations also left a lasting imprint on Earth's landscape, creating many of the distinctive glacial landforms and features observed today
  • The Great Lakes in North America, the fjords of Norway, and the rolling hills of the American Midwest are all products of glacial erosion and deposition during the Pleistocene
• The study of past glaciations, through the analysis of glacial landforms, sediments, and ice cores, provides valuable insights into the mechanisms and consequences of global climate change
  • Understanding the behavior of glaciers during the Pleistocene can help inform predictions of future glacial responses to anthropogenic climate change and the potential impacts on sea level, water resources, and ecosystems
• The ongoing retreat of glaciers worldwide, in response to current global warming, represents a significant departure from the glacial-interglacial cycles of the Pleistocene
  • The rate and magnitude of contemporary glacial retreat are unprecedented in the context of the last several thousand years, underscoring the profound influence of human activities on the Earth's climate system