Geosystems Ch 17 Glacial Landscapes

GLACIAL LANDSCAPES

Chapter 17 Overview

  • Examination of glacial landscapes, including the types of glaciers, the processes behind their flow and mass balance, and significant glacial periods, specifically during the Quaternary and Pleistocene epochs. These studies are crucial for understanding past climate change and predicting future environmental shifts.

A. Glacier Types

  1. Ice Sheet

    • Definition: A continent-sized dome of ice, typically exceeding 50,000 km² in area, that flows outward radially in all directions. These massive ice bodies can be several kilometers thick at their center.

    • Examples: Antarctica (covering over 14 million km²) and Greenland (covering over 1.7 million km²), which hold the vast majority of the Earth's freshwater ice.

  2. Ice Cap

    • Definition: A smaller, dome-shaped mass of ice, typically less than 50,000 km² in area, that covers uplands or plateaus. Its flow is generally unconstrained by underlying topography.

    • Examples: Iceland, particularly the Vatnajökull ice cap, and areas of Baffin Island (Canada).

  3. Alpine Glacier

    • Definition: Also known as a valley glacier, this type of glacier flows down mountain slopes or within pre-existing valleys, constrained by the surrounding topography. They are typically much longer than they are wide.

    • Examples: Glaciers in the Alps, Cascades (North America), Himalayas (Asia), and coastal ranges of Alaska.

  4. Cirque Glacier

    • Definition: A small, armchair-shaped glacier confined to the upper, bowl-shaped reaches (cirques) of a mountain valley, often representing the remnant of a once larger alpine glacier.

    • Example: Many small glaciers found in Glacier National Park, Montana.

  5. Outlet Glacier

    • Definition: A fast-flowing arm or stream of a larger ice sheet or ice cap that drains ice from its interior and flows out onto an adjacent valley or coastal plain, often confined by bedrock topography.

  6. Tidewater Glacier

    • Definition: A glacier that flows directly into the ocean or a large lake, terminating at a floating ice shelf or at a calving ice margin where icebergs break off. These glaciers are highly sensitive to oceanic temperatures.

B. Glacier Flow and Mass Balance

1. Snow/Ice Metamorphism

  • Process: Deeply buried snow undergoes a series of transformations as air is expelled due to compaction and pressure, leading to crystal re-arrangement and increased density. This process forms granular snow, then firn, and finally solid glacial ice.

    • Density Variations:

      • Snow: Approximately 0.1 - 0.3 g/cm³ (freshly fallen to compacted)

      • Firn: Approximately 0.7 - 0.8 g/cm³ (intermediate stage of compacted, recrystallized snow)

      • Ice: Approximately 0.8 - 0.9 g/cm³ (solid glacial ice, approaching that of pure ice, 0.917 g/cm³)

  • Result: The immense weight of accumulating ice causes deeper layers to deform plastically and flow once the ice mass reaches a minimum thickness of around 50 meters (150 feet). This flow occurs through internal deformation and, where meltwater is present, basal slip.

2. Glacier Flow

  • Mechanism: Glacial ice flows primarily by two mechanisms:

    • Plastic deformation: Deeper layers of glacier ice (below 50 m) deform internally as ice crystals slide past one another and recrystallize under pressure, at rates typically ranging from 10 to 100 feet (or several tens of meters) per year.

    • Basal slip: The entire glacier mass slides over its bedrock base, lubricated by a thin layer of meltwater, especially in temperate glaciers.

  • Erosion: As a glacier flows, it actively erodes bedrock through:

    • Abrasion: Rock fragments embedded in the ice scrape and polish the bedrock.

    • Plucking (or Quarrying): Meltwater penetrates cracks in the bedrock, freezes, expands, and dislodges blocks of rock, which are then incorporated into the glacier.

  • Surface Features: The upper 50 meters of ice are generally brittle due to lower pressure and colder temperatures, and are carried passively above the flowing internal layers. This brittle layer often cracks to form crevasses, which are deep fissures resulting from differential stresses during flow over uneven terrain or changes in gradient.

3. Mass Balance

  • Formula: Mass Balance = Accumulation – Ablation

    • Accumulation: Refers to all processes adding mass to a glacier, including snowfall, rainfall, avalanches, and the refreezing of meltwater. This typically occurs in winter or at high elevations near the glacier's head.

    • Ablation: Involves all processes leading to the loss of ice from a glacier, including surface melting, sublimation (direct evaporation of ice), internal melting, and calving (breaking off of icebergs). This mostly happens in summer at lower elevations near the ice margin.

  • Equilibrium Line: The point or elevation on the glacier where winter accumulation precisely equals summer ablation over the course of a year, separating the glacier into two distinct zones. This line can shift annually based on climatic conditions.

    • Accumulation Zone: The upper part of the glacier where net accumulation of snow and ice exceeds ablation.

    • Ablation Zone: The lower part of the glacier where net ablation of ice exceeds accumulation.

  • Types of Glacier Mass Balance:

    • Stable Glacier: Occurs when accumulation equals net ablation and flow past the equilibrium line, resulting in a relatively stationary ice margin, indicating a balanced state.

    • Advancing Glacier: Occurs when accumulation exceeds ablation, leading to an increase in ice volume, greater flow rates, and the advancement (growth) of the ice margin downslope or outward.

    • Retreating Glacier: Occurs when ablation exceeds accumulation, resulting in a net loss of ice volume, deceleration of flow, and the retreat (shrinkage) of the ice margin. This is often observed during periods of warming climate.

    • Dead Ice: Refers to isolated blocks or stagnant bodies of melting ice that are no longer thick enough or connected to the main glacier to flow internally, eventually melting in place.

C. The Ice Age – Quaternary / Pleistocene Glaciations

1. Overview of the Quaternary Period

  • Time Frame: Spanning from 2.6 million years ago to the present, a period primarily characterized by fluctuating global temperatures and recurrent glaciations.

  • Notable Events: This era is marked by repeated advances and retreats of vast continental ice sheets, particularly across North America and Eurasia. Major glacial maxima, or peak ice extent, occurred approximately every 100,000 years, driven largely by Milankovitch cycles (variations in Earth's orbit and axial tilt).

    • Last Glacial Maximum (LGM): Occurred around 20,000 years ago, when ice sheets covered their greatest extent during the last glacial cycle.

  • Epochs:

    • Pleistocene Epoch: 2.6 million to 10,000 years ago, characterized by multiple alternating glacial (cold, ice-covered) and interglacial (warm, ice-free) stages.

    • Holocene Epoch: 10,000 years ago to present, representing the current interglacial phase, which has generally been warmer and more stable than previous interglacials.

2. Current Glacial Coverage

  • Land Area Covered by Glaciers Today:

    • Worldwide: Approximately 10% of the Earth's total land surface.

    • Alaska: Roughly 5% of its land area.

    • Canada: About 2% of its land area.

  • Land Area Covered at Glacial Maximum (LGM):

    • Worldwide: Approximately 25% of the Earth's total land surface was covered by ice.

    • Alaska: As much as 33% of its land area.

    • Canada: A vast 97% of its land area was beneath the Laurentide Ice Sheet.

D. Continental Glaciers – Processes and Landforms

1. Glacial Landforms

  • Definition: Distinctive geological features and sediment deposits produced by the erosional and depositional actions of large continental ice sheets, observable today in regions previously covered by Pleistocene ice sheets.

  • Types of Landforms:

    • Proglacial Landforms: Features formed in advance of the glacier edge or beyond the ice margin, often by meltwater action.

    • Subglacial Landforms: Features formed underneath the ice due directly to the movement and pressure of glacial ice.

2. Glacial Till

  • Definition: Unsorted, non-stratified sediment eroded, transported, and deposited directly by glacial ice. It is an amalgam of rocks, gravel, sand, and mud, characterized by its angularity and wide range of clast sizes, reflecting the powerful, indiscriminate nature of glacial transport.

  • Types of Till:

    • Basal Till (Lodgement Till): Densely compacted till deposited directly under the flowing ice as it grinds and smears sediment onto the bedrock.

    • Meltout Till (Ablation Till): Less compacted till deposited directly from melting ice without significant reworking by water, often forming irregular hummocks.

  • Erratic Boulders: Large boulders, often composed of a different rock type than the local bedrock, that have been transported great distances (hundreds of kilometers) by glacial ice and deposited during glacial retreats. Their origin helps reconstruct the flow paths of ancient ice sheets.

3. Glacial Moraines

  • Definition: Accumulations of glacial till that form distinct ridges or mounds at the edges or terminus of a glacier due to the constant transport and deposition of sediment by ice flow.

  • Types of Moraines:

    • End Moraines: Arcuate or crescent-shaped ridges of till located at the foot or terminus of a glacier, marking the farthest extent or a pause in retreat.

      • Terminal Moraines: Mark the absolute farthest advance of a glacier, representing its maximum extent.

      • Recessional Moraines: Formed during temporary halts or minor readvances of the glacier margin during an overall period of glacial retreat.

    • Lateral Moraines: Ridges of till found along the sides of valley glaciers, formed by accumulated rockfall from the valley walls onto the glacier margin and subsequent deposition.

    • Medial Moraines: Linear ridges of till formed in the middle of a larger glacier, resulting from the convergence of two tributary glaciers, where their adjacent lateral moraines merge.

4. Subglacial Landforms

  • Formation: Sediments deposited beneath the ice are significantly affected and modified by the intense pressure and movement of the overlying ice and subglacial meltwater flow.

  • Notable Features:

    • Drumlin: An elongated, asymmetrical hill composed of glacial till, often with a blunt, steeper stoss (up-ice) side and a gentler, tapering lee (down-ice) side. Drumlins are streamlined by overriding ice flow, and their orientation indicates the direction of past glacier movement, often forming fields of