UNIT-10 GLACIERS

OBJECTIVES

After studying this unit, you should be able to:

Ø  Define glaciers and their types

Ø  Explain glacial movement and erosion

Ø  Identify glacial landforms

Ø  Understand the role of glaciers in shaping land

INTRODUCTION

Snowfall and Snow Fields: In high altitude and cold regions, snow remains unmelted for many years. Continuous accumulation forms thick snow fields.

Snow Line: The lowest elevation where snow remains permanently throughout the year is called the snow line.

It varies with climate:

·       Near sea level in polar regions.

·       Up to 6000 m in tropical mountains.

Neve (Firn): Accumulated snow becomes compact and granular due to melting and refreezing. This intermediate stage of snow is called neve or firn.

Formation of Glacier: With continuous snowfall, the snow becomes compressed into ice. When ice thickness reaches about 60–90 m, its own weight causes it to flow slowly downhill. A glacier is therefore a large mass of ice formed from recrystallized snow that moves slowly over land like a river.

Movement of Glaciers: Glaciers move very slowly due to their viscous nature. They gradually melt at their ends. When glaciers reach the sea, large floating ice masses called icebergs break off.

Classification of Glaciers: Glaciers are classified based on their form and location.

1. Cirque Glacier: Small glacier found in a bowl-shaped depression called a cirque. Located at the head of mountain valleys.

2. Valley Glacier (Alpine Glacier): Glaciers that flow down mountain valleys like rivers. Common in Alps and Himalayas.

3. Piedmont Glacier: Formed when valley glaciers emerge from mountains and spread out over lowlands. Example: Malaspina Glacier (Alaska).

4. Ice Sheet (Continental Glacier): Very large glaciers covering extensive land areas.  Example: Antarctica Greenland.

GLACIAL MOVEMENT

Rate of Glacial Movement: Glaciers move much more slowly than rivers. The average rate of glacier movement is only a few feet per day. Most Alpine glaciers move about one meter per day. Some glaciers in Greenland have been recorded to move up to three meters per day.

Nature of Glacier Movement: The movement of glacial ice is known as glacial flow. The centre of a glacier moves faster than the sides because the sides experience friction with the valley walls. The surface of the glacier moves faster than the deeper layers. At curves in the valley, the convex side moves faster than the concave side. Since different parts of the glacier move at different speeds, this motion is called differential movement.

Mechanisms of Glacier Movement: Glacial movement occurs mainly by two processes: internal flow and basal sliding.

Internal Flow: When the thickness of ice reaches about 50–60 meters, the pressure causes the ice to behave like a plastic material. The ice slowly deforms and flows downhill under its own weight.

Basal Sliding: In this process the entire glacier mass slides over the ground surface.
This sliding occurs mainly in the lower part of glaciers where meltwater reduces friction.

Causes of Glacier Motion: Glaciers move mainly due to the force of gravity which pulls them down the slope. Melting and refreezing of ice under pressure also helps the glacier to move forward. Re-crystallization and expansion of ice also contribute to glacier movement.

Accumulation and Ablation: Glaciers continuously gain and lose ice. Snow accumulation and ice formation occur above the snow line and this area is called the accumulation zone. Below the snow line melting of ice takes place and this region is called the ablation zone.

Advance and Retreat of Glaciers: If accumulation of snow is greater than melting, the glacier advances forward. If accumulation and melting are equal, the glacier remains stationary. If melting is greater than accumulation, the glacier retreats.

FEATURES OF GLACIERS

Bergschrund: A Bergschrund is a large crack that occurs at the head of a glacier. It forms where the moving glacier ice separates from the stationary ice attached to the mountain slope.

Crevasses: Crevasses are deep cracks formed in glacier ice due to stress during movement. They develop when the glacier flows over uneven surfaces or when it stretches while moving.

Types of Crevasses: Transverse crevasses form across the glacier when it moves over steep slopes. Longitudinal crevasses form parallel to the direction of glacier movement when the glacier spreads sideways. Marginal crevasses develop near the sides of the glacier due to friction with valley walls.

Moraines: Moraines are accumulations of rock fragments carried and deposited by glaciers.

Lateral Moraine: Lateral moraine forms along the sides of a glacier where rock debris falls from valley walls onto the glacier.

Medial Moraine: Medial moraine forms when two glaciers meet and their lateral moraines join together in the centre.

Terminal Moraine: Terminal moraine is a ridge of debris deposited at the front or end of a glacier when the ice melts.

Melt water Streams: During summer melting of glacier ice produces streams of water on the surface of the glacier. This melt water may flow through channels within the glacier and finally emerge near the glacier terminus.

Glacier Tables: Glacier tables are formed when large rock blocks protect the ice beneath them from melting. As the surrounding ice melts away, the rock remains standing on a pillar of ice.

Moulins: Moulins are vertical shafts formed in glaciers by melt water flowing into crevasses. These shafts allow surface water to reach deeper parts of the glacier.

GEOLOGICAL WORK OF GLACIERS

The geological activity of glaciers is known as glaciation, which means modification of land surface by moving ice. Glaciation includes erosion, transportation and deposition. Evidence of past glaciation was strongly supported by Louis Agassiz, who proved that large parts of Europe were once covered by glaciers. Glacial erosion mainly occurs beneath the glacier, while deposition occurs near the glacier margin.

EROSION BY GLACIERS

Glacial erosion mainly occurs through plucking and abrasion.

Plucking (Quarrying): Plucking is the removal of rock fragments by glaciers. Melt water enters cracks and joints in rocks and freezes. The expansion of ice loosens rock fragments which are then carried away by the glacier.

Abrasion: Abrasion occurs when rock fragments embedded in glacier ice grind against the bedrock surface. This grinding produces fine powder called rock flour. Large rock fragments create scratches and grooves called glacial striations. These striations help determine the direction of glacier movement.

Special Erosional Features                                              

Roche Moutonnee: Roche moutonnee is an asymmetrical rock hill shaped by glacial erosion. The upstream side is smooth and gently sloping due to abrasion. The downstream side is steep due to plucking.

Crag and Tail: Crag and tail is formed when a resistant rock mass obstructs glacier movement. The steep side facing the glacier is called the crag. The gently sloping side protected from erosion is called the tail.

Cirques: A cirque is a bowl-shaped depression formed at the head of a glacier. It develops mainly by frost weathering and glacial erosion. After glacier melting, cirques often contain small lakes called cirque lakes.

Characteristics of Glaciated Valleys

U-Shaped Valley: Glacial valleys have U-shaped cross sections with broad bottoms and steep sides. This differs from V-shaped river valleys.

Truncated Spurs: Glaciers cut across projecting ridges of mountains forming truncated spurs, resulting in straighter valleys.

Hanging Valleys: Tributary valleys join the main valley at a higher level forming hanging valleys. These often produce waterfalls.

Arete: An arete is a sharp ridge formed between two cirques.

Col: A col is a gap formed when cirques erode back toward each other.

Horn: A horn is a pyramidal peak formed when several cirques erode a mountain from different sides. A famous example is the Matterhorn.

Fjords: Fjords are deep, narrow sea inlets with steep sides formed when glacial valleys are submerged by sea water. They occur along the coasts of Norway, Sweden, Alaska, and Chile.

TRANSPORTATION BY GLACIERS

Glaciers carry rock debris of all sizes including clay, sand, pebbles and boulders. Unlike rivers, glaciers do not sort their sediments. The transported debris may occur:

·       On the glacier surface

·       Along the sides

·       At the bottom of the glacier

Fine rock powder produced by grinding is called rock flour.

DEPOSITION BY GLACIERS

Deposits left by glaciers are collectively called glacial drift. These deposits are of two types:

1) Glacial till (direct deposits)

2) Glacio-fluvial deposits (meltwater deposits)

1. Glacial Till : Glacial till is an unsorted mixture of sediments deposited directly by glaciers. Large transported boulders found in till are called glacial erratics.

Moraines: Moraines are ridges of glacial debris.

Ground Moraine: Ground moraine is a sheet of debris deposited beneath a melting glacier.

Terminal Moraine: Terminal moraine forms at the end of a glacier. It marks the furthest advance of the glacier.

Lateral Moraine: Lateral moraine forms along the sides of a glacier.

Recessional Moraine: Recessional moraine forms when a glacier temporarily stops during its retreat.

Drumlins: Drumlins are smooth elongated hills formed by glacial till. They are aligned parallel to the direction of glacier movement.

Erratic Boulders: Erratics are large boulders transported far from their original source by glaciers. They differ from the local bedrock.

2. Glacio-Fluvial Deposits: Meltwater streams from glaciers deposit sorted sediments like sand and gravel.

Valley Train: Sediments deposited by meltwater streams within valleys are called valley trains.

Outwash Plains: Outwash plains are broad plains of sand and gravel formed by glacial meltwater streams.

Kettle Holes: Kettle holes are depressions formed when buried blocks of ice melt within glacial deposits.

Kames: Kames are conical hills made of stratified sand and gravel deposited by glacial meltwater streams.

Eskers: Eskers are long winding ridges of sand and gravel deposited by streams flowing beneath glaciers.

Glacial Lakes: Glacial lakes form when meltwater accumulates behind ice dams created by glaciers. A famous example is Lake Agassiz.

Varves: Varves are annual layers of sediment deposited in glacial lakes. Each pair of layers represents one year.

·       Summer layer – coarse and light colored

·       Winter layer – fine and dark colored

CAUSES OF GLACIATION

Glaciation refers to the formation and expansion of glaciers due to significant decrease in global temperatures. During glacial and interglacial periods, the earth’s temperature fluctuated considerably. A reduction of about 6°C in average temperature in middle latitudes is enough to produce glacial conditions. Several hypotheses have been proposed to explain these climatic changes.

1. Heat Distribution Hypothesis

This hypothesis explains glaciation through changes in the movements of the Earth.
Variations in the eccentricity of Earth’s orbit and the inclination of its axis affect the distribution of solar heat.  These periodic changes lead to variations in climate. Extreme variations in these factors could result in the formation of large glaciers. Evidence suggests that a major ice age occurred during the Carboniferous Period apart from the glaciation in the Pleistocene Epoch.

2. Solar Radiation Hypothesis

This hypothesis is based on variations in the amount of solar radiation reaching Earth. The solar constant represents the amount of heat received at the outer surface of Earth’s atmosphere. Measurements by the Smithsonian Institution showed that solar radiation can vary slightly over time. The theory was proposed by Sir George Simpson.

According to this theory, increased solar radiation raises temperatures and increases atmospheric circulation. This results in more precipitation and cloud formation. In high latitudes, increased snowfall and reduced melting promote the growth of glaciers.
Eventually, continued temperature increase destroys the glaciers and an interglacial period begins. When solar radiation decreases again, glaciers expand and another glacial period begins. This theory suggests that multiple glacial cycles occurred during the Pleistocene Ice Age.

3. Solar Topography Hypothesis

This theory was proposed by Richard Foster Flint. It combines the effects of solar radiation variations and continental topography. The theory states that large-scale glaciation occurs mainly when continents are elevated and extensive. High land areas encourage accumulation of snow and formation of glaciers. When minimum solar radiation coincides with high continental areas, glaciers advance when solar radiation increases, glaciers begin to melt and retreat. This hypothesis attempts to explain glaciation more satisfactorily than earlier theories.