3.4 Glaciers and Ice Ages

Glaciers, massive bodies of ice that persist year-round, shape Earth's surface through erosion and deposition. From alpine glaciers in mountains to continental ice sheets, these icy giants carve landscapes and leave behind distinctive landforms, influencing our planet's geography.

Ice ages, driven by orbital variations and changes in atmospheric composition, have profound effects on Earth's climate and ecosystems. By studying past glacial periods, scientists gain insights into Earth's climate system, helping us understand and predict future climate change.

Glaciers and their types

Types of glaciers

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• Alpine glaciers are found in mountainous regions and include:
  • Cirque glaciers, which form in bowl-shaped depressions on mountainsides
  • Valley glaciers, which flow down pre-existing river valleys
  • Piedmont glaciers, which form when valley glaciers spill out onto flatter plains at the base of mountains
• Continental glaciers, also known as ice sheets, are much larger than alpine glaciers and can cover vast areas of land
  • Examples include the present-day ice sheets in Antarctica and Greenland, which cover entire continents

Characteristics and formation of glaciers

• Glaciers are massive bodies of ice that persist year-round and flow under their own weight due to gravity
  • Glacial ice forms from the accumulation and compaction of snow over many years
  • As snow accumulates, it compresses the underlying layers, transforming them into dense, crystalline ice
• Glaciers move downslope or outward from their source region due to gravity and the pressure of the overlying ice
  • The movement of glaciers is slow, typically ranging from a few centimeters to a few meters per day
  • The speed of glacial movement depends on factors such as the thickness of the ice, the steepness of the slope, and the temperature of the ice

Glacial erosion and deposition

Processes of glacial erosion

• Plucking involves the glacier freezing onto bedrock and removing large chunks of rock as it moves downslope
  • Water seeps into cracks in the bedrock and freezes, expanding and loosening the rock
  • As the glacier moves, it plucks out the loosened rock fragments, leaving behind jagged, irregular surfaces
• Abrasion occurs when rocks and debris embedded in the base of the glacier scrape and polish the bedrock beneath
  • The embedded rocks act like sandpaper, creating smooth surfaces and parallel scratches called striations
  • Glacial abrasion can also produce grooves, chattermarks, and polished surfaces on the bedrock
• Freeze-thaw action happens when water seeps into cracks in the bedrock, freezes, and expands, causing the rock to break apart
  • This process is also known as frost wedging or ice wedging
  • Repeated cycles of freezing and thawing can significantly weaken and fragment the bedrock, making it more susceptible to erosion

Glacial deposition and landforms

• Glacial deposition occurs when the glacier melts and releases the sediment it has been carrying, creating various landforms
  • The size and shape of the deposited sediment depend on factors such as the glacier's velocity, the amount of meltwater, and the underlying topography
• Moraines are ridges of glacially deposited sediment, classified based on their location relative to the glacier
  • Lateral moraines form along the sides of the glacier, while medial moraines form where two glaciers merge
  • Terminal moraines are deposited at the end of the glacier, marking its maximum extent
• Drumlins are elongated, teardrop-shaped hills composed of glacial till (unsorted sediment)
  • They are formed beneath the glacier and are aligned with the direction of ice flow
  • Drumlins can range in size from a few meters to several kilometers in length
• Eskers are long, winding ridges of sand and gravel deposited by meltwater streams flowing beneath or within the glacier
  • They can extend for several kilometers and are often used as natural roadbeds in glaciated regions

Landforms of glacial activity

Erosional landforms

• Cirques are bowl-shaped depressions carved into mountainsides by the erosive action of glaciers
  • They often feature a steep headwall and a flat or overdeepened floor, where a small lake (tarn) may form after the glacier has melted
  • Cirques are the starting points for alpine glaciers and can enlarge over time through continued glacial erosion
• Arêtes are sharp, narrow ridges formed when two cirques erode back-to-back
  • They are characterized by steep, knife-like edges and can be challenging to traverse
  • Examples of arêtes include the Matterhorn in the Alps and the Garden Wall in Glacier National Park, Montana
• Horns are steep, pyramid-shaped peaks resulting from the erosion of multiple cirques
  • They form when three or more cirques erode the sides of a mountain, leaving a distinctive pointed peak
  • The Matterhorn in the Alps is a classic example of a glacial horn
• U-shaped valleys are created by the erosive power of valley glaciers, which widen and deepen pre-existing river valleys
  • They are characterized by steep, parallel walls and a flat, wide floor, in contrast to the V-shaped profiles of river-carved valleys
  • Yosemite Valley in California is a well-known example of a U-shaped glacial valley
• Hanging valleys are tributary valleys left "hanging" above the main glacial valley due to the more rapid erosion of the main glacier
  • They often feature waterfalls, as the tributary streams cascade down to meet the main valley floor
  • Bridal Veil Falls in Yosemite National Park is an example of a waterfall formed by a hanging valley

Depositional landforms

• Moraines are ridges of glacially deposited sediment, classified as lateral, medial, or terminal based on their location relative to the glacier
  • Lateral moraines form along the sides of the glacier, while medial moraines form where two glaciers merge
  • Terminal moraines are deposited at the end of the glacier, marking its maximum extent
  • The Outer Lands of Cape Cod, Massachusetts, are an example of a terminal moraine complex
• Drumlins are elongated, teardrop-shaped hills composed of glacial till, formed beneath the glacier and aligned with the direction of ice flow
  • They can range in size from a few meters to several kilometers in length
  • The Drumlin Field in Wisconsin is a well-known example, featuring thousands of drumlins formed during the last ice age
• Eskers are long, winding ridges of sand and gravel deposited by meltwater streams flowing beneath or within the glacier
  • They can extend for several kilometers and are often used as natural roadbeds in glaciated regions
  • The Denali Highway in Alaska follows the path of a large esker formed by glacial meltwater

Causes and effects of ice ages

Causes of ice ages

• Variations in Earth's orbit (Milankovitch cycles) are a primary driver of ice ages
  • Eccentricity refers to the shape of Earth's orbit, which varies from nearly circular to more elliptical over a 100,000-year cycle
  • Obliquity is the tilt of Earth's axis, which varies between 22.1° and 24.5° over a 41,000-year cycle
  • Precession is the wobble of Earth's axis, which affects the timing of seasons over a 26,000-year cycle
• Changes in atmospheric composition, particularly reduced greenhouse gas concentrations, can contribute to cooling and the onset of ice ages
  • Lower levels of carbon dioxide and methane in the atmosphere allow more heat to escape Earth's surface, leading to global cooling
• Variations in solar output, such as decreased solar activity during the Maunder Minimum (1645-1715), can also influence Earth's climate and potentially contribute to cooling

Effects of ice ages

• During glacial periods, sea levels drop due to water being locked up in ice sheets, exposing land bridges and altering global ocean circulation patterns
  • The Bering Land Bridge, which connected Asia and North America during the last ice age, allowed for the migration of humans and other species between the continents
• Ice ages have significant effects on Earth's climate, ecosystems, and biodiversity
  • Colder temperatures and reduced precipitation during glacial periods lead to the expansion of grasslands and tundra biomes, while forests contract
  • Many species adapt to the changing conditions through evolutionary processes, while others may face extinction due to habitat loss or competition
• Positive feedback mechanisms, such as the ice-albedo feedback, can amplify the cooling effect during an ice age
  • As ice sheets expand, they reflect more of the sun's energy back into space (higher albedo), further cooling the planet
  • This feedback loop can intensify and prolong the cooling trend, until other factors (e.g., increasing greenhouse gases) eventually offset the cooling and lead to warming

Studying past ice ages

• The Pleistocene Epoch, which began about 2.6 million years ago and ended 11,700 years ago, is characterized by multiple glacial-interglacial cycles
  • During this time, Earth experienced several major glacial periods, interspersed with warmer interglacial periods
  • The most recent glacial period, known as the Last Glacial Maximum, occurred about 26,500 to 19,000 years ago
• Scientists study past ice ages using a variety of methods, including:
  • Analysis of ice cores, which provide a record of past atmospheric composition and temperature
  • Examination of glacial landforms and sediments, which offer insights into the extent and dynamics of past glaciations
  • Study of fossil records, which reveal how ecosystems and species responded to changing climate conditions
• Understanding past ice ages helps scientists better comprehend Earth's climate system and predict future climate change
  • By studying the causes, effects, and cyclical nature of past glaciations, researchers can improve climate models and anticipate the potential impacts of current and future climate change on our planet