1. UNIT-10 GLACIERS

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.

2. UNIT-11 LAKES AND SEAS

UNIT-11 LAKES AND SEAS

Lakes and seas act as important agents of external geological processes.  Lakes supply freshwater to rivers and act as settling basins in the drainage system. Oceans and seas supply moisture to the atmosphere, influencing the hydrological cycle. They also act as depositories for sediments transported from land by rivers.

Objectives

Ø  After studying this unit, you should be able to:

Ø  Describe lakes and seas as geological agents

Ø  Explain their role in erosion and deposition

Ø  Identify landforms created by lakes and seas

Ø  Understand their environmental importance

Introduction

Lakes : A lake is a natural inland depression or basin containing a large amount of water that is generally quiet or stationary. In some lakes there may be slow water currents, such as in Lake Erie. Lakes vary in size from small ponds to very large water bodies covering thousands of square kilometres. Examples of very large lakes include the Great Lakes of North America.

Most lakes occur above sea level at different altitudes. One of the highest lakes in the world is Lake Titicaca, located between Peru and Bolivia. The chemical composition of lake water varies widely. Some lakes contain fresh and pure water, while others contain highly saline water.
Examples of saline lakes include the Great Salt Lake and the Dead Sea.

Lake water chemistry mainly depends on the rocks through which the water flows and the concentration of dissolved minerals. Lakes also vary in depth from shallow depressions to very deep basins. The deepest lake in the world is Lake Baikal in Siberia, which is more than 5000 feet deep.

Lakes occur in mountain regions, plateaus, plains, valleys, and coastal areas. They are generally formed when surface drainage is obstructed. Geologically, lakes are temporary features, because they eventually fill with sediments.

Geological Work of Lakes

Most lakes have outlet streams that feed rivers, so they usually contain freshwater. Lakes regulate river flow by acting as natural reservoirs. During floods, water spreads over the lake basin and reduces flood damage downstream. During droughts, lakes release water slowly and help maintain perennial streams. Lakes also act as settling basins for sediments. Sediments carried by rivers settle on the lake floor when the water velocity decreases. For example, the Niagara River becomes clear after leaving Lake Erie because sediments settle within the lake.

Sediment deposition occurs gradually as sand, silt, and clay settle at different distances from the inlet. Although large quantities of sediments accumulate in lakes every year, the total is less than ocean deposition. Lakes influence the local climate by increasing rainfall and reducing temperature variations. Aquatic plants and organisms grow abundantly in many lakes. Their shells and plant remains accumulate and form fossil deposits that help scientists study ancient life.

In arid regions, lakes without outlets may evaporate, leaving behind mineral deposits. Common minerals deposited in such lakes include halite, gypsum, glauber salt, epsom salt, magnesium chloride, and calcite.

Lake erosion is similar to marine erosion, but it is usually less powerful. Large lakes may develop terraces, cliffs, caves, stacks, and arches along their shores. Lake deposition forms features such as beaches, bars, barriers, deltas, and terraces. A common freshwater lake deposit is marl, which is a mixture of clay and calcium carbonate.

Swamps

A swamp or marsh is a waterlogged area with saturated soil and abundant vegetation. Swamps are closely related to the water table level. Surface water infiltration is slow, and drainage is often poor in such areas.

Many swamps represent an intermediate stage between lakes and dry land. Some lakes gradually transform into swamps due to sediment accumulation and plant growth.

Swamps commonly occur in flat and poorly drained regions. Examples are found in north-central United States, parts of Canada, and eastern Russia.

Swamps accumulate large amounts of organic matter such as peat. Peat forms from partially decomposed plant material under waterlogged conditions. When dried, peat can be used as fuel and represents the first stage in coal formation.

Swamps also accumulate inorganic sediments such as clay and silt. Over geological time these deposits may form impure coal beds.

Swamps provide many useful natural products. Marl deposits are used in agriculture and cement manufacture. Certain lakes also contain diatomaceous earth, which is useful as an abrasive material. Reclaimed marshlands often produce fertile soils rich in humus for agriculture.

Origin of Lakes

Lake basins originate in several geological ways. Most lakes are formed through gradational processes, but some originate due to volcanic activity.

1. Lakes Formed by Glaciation

Glacial lakes are common in regions previously covered by glaciers. They may form by glacial erosion or deposition of glacial debris. Many lakes in Minnesota, Wisconsin, and New York were formed by glacial deposits.

Large lakes such as the Great Lakes were formed mainly by glacial erosion along pre-existing valleys. Cirque lakes formed by glacial erosion occur in mountain ranges such as the Himalayas, Alps, and Rocky Mountains.

2. Lakes Formed by Streams

Streams may create lakes through erosion and deposition processes. When a meandering river cuts off a loop, it forms a crescent-shaped ox-bow lake. Such lakes commonly occur on flood plains.

Lakes may also form on river deltas due to uneven sediment deposition. Examples occur in the deltas of the Nile River, Danube River, and Mississippi River.

3. Lakes Formed by Groundwater

In limestone regions, the collapse of cave roofs forms sinkholes. These depressions may fill with groundwater and create small lakes. Examples occur in Florida, Kentucky, and Tennessee in areas of karst topography.

Destruction of Lakes

Lakes are temporary geological features and may disappear gradually due to several natural processes.

a) Filling with Sediments: Streams flowing into lakes carry sand, silt, and clay. These sediments settle on the lake floor and gradually fill the basin. Over time the lake may become completely filled and disappear.

b) Filling with Organic Remains: In humid regions, aquatic plants grow abundantly near lake margins. When plants die, their remains accumulate and form bogs and marshes. Organic matter and animal shells gradually fill the lake basin.

c) Cutting Down of Outlets: Many lakes are dammed by glacial debris or loose sediments. Outlet streams may erode these dams, lowering the water level. Eventually the lake may be completely drained.

d) Removal of Ice Dams: Some lakes form when glaciers block valleys. When the glacier melts or moves away, the water escapes. This leads to the destruction of the lake.

e) Evaporation: In arid and desert regions, evaporation may exceed water supply. This causes lakes to shrink and eventually dry up.

f) Diastrophism: Earth movements such as earthquakes or faulting may drain lakes. Small lakes may lose water through cracks or fissures. Large lakes may disappear if their outlet regions are down-faulted.

Types of Lakes

Important lake types include saline lakes, alkaline lakes, and playa lakes.

1. Salt or Saline Lakes

Saline lakes contain high concentrations of dissolved salts.

They form in two ways:

1.     Accumulation of salts in lakes with no outlets.

2.     Separation of a part of the sea by sediment deposition or earth movements.

A major example is the Caspian Sea, the largest inland saltwater body. Another example is the Salton Sea in California, formed when sediments of the Colorado River blocked part of a desert depression. Saline lakes mostly occur in arid regions with interior drainage. High evaporation concentrates the dissolved minerals in water. A famous example is the Great Salt Lake in Utah. Another example is the Dead Sea, located in the Jordan River valley between Israel and Jordan.
Its surface lies about 1300 feet below sea level, making it one of the lowest lakes on Earth.

2. Alkaline Lakes

Alkaline lakes contain large amounts of alkaline carbonates such as sodium carbonate. They occur in regions such as Egypt, Hungary, and Venezuela. An important example is Mono Lake in California, which is rich in salt and soda. Similar alkaline lakes known as “dands” occur in the Pakistan sand province.

3. Playa Lakes

Playa lakes occur in desert regions. They form in shallow, flat-bottomed depressions created by weathering and wind action. During the rainy season, intermittent streams fill these basins with water. During dry periods the water evaporates, leaving salt deposits. Playa lakes are common in the Great Basin of the western United States. They also occur in deserts of Arizona and New Mexico.

Indian Lakes

Lakes are relatively few in India, especially in the peninsular region. Some lakes have formed along the coasts due to sand bars and delta deposits. Important coastal lakes include:

·       Pulicat Lake in Andhra Pradesh near Chennai

·       Chilika Lake in Odisha

1. Kolleru Lake

Kolleru Lake is a large freshwater lake in Andhra Pradesh, located between the deltas of the Krishna River and Godavari River.  It is shallow and elliptical in shape. The lake is gradually filling with sediments from streams such as Budameru River.

2. Sambhar Lake

Sambhar Lake is located near Jaipur in Rajasthan. It lies about 1200 feet above sea level and covers about 120 km². The lake often dries during summer and contains saline mud deposits.
It has been used as a source of common salt for centuries.

3. Lonar Lake

Lonar Lake is located in Maharashtra within the Deccan Traps. It is a circular depression about 1 mile in diameter and 300 feet deep. The lake contains sodium carbonate and sodium chloride deposits. It is considered a crater lake of volcanic origin.

4. The Nal Lake

Nal Sarovar is a saline lake located near Ahmedabad in Gujarat. It covers about 80 square kilometres. The water becomes highly saline during summer. It may have formed from a former arm of the sea cut off by sediment deposition. Here are short, clear, exam-ready notes with headings for the topic SEAS and Relief Features of Ocean Floor.

SEAS

In common usage, the terms sea and ocean are often used interchangeably to describe large bodies of salt water on the Earth’s surface. However, in scientific terminology, oceans are the largest bodies of salt water occupying huge basins between continents. Examples of oceans include the Pacific Ocean, Atlantic Ocean, and Indian Ocean.

Seas are smaller bodies of salt water that are partly enclosed by land. Examples include the North Sea, Yellow Sea, and Arabian Sea.

Many seas occur on the continental shelves and are therefore called epicontinental seas or shelf seas. Some shallow water bodies located within continental regions with limited connection to oceans are known as Epeiric seas. Examples include the Baltic Sea and Hudson Bay.

A Mediterranean Sea is a special type of epeiric sea that is deep and almost surrounded by land.
Examples include the Mediterranean Sea and Caribbean Sea.

Relief Features of the Ocean Floor

The mean sea level is used as the reference plane for topographic and geological surveys. Contrary to earlier belief, the ocean floor is not smooth. It has mountain ranges, plains, ridges, trenches, valleys, and volcanic peaks, similar to features found on land. About two-thirds of the Earth’s surface is covered by ocean basins. The important relief features of the ocean floor are described below.

1. Continental Shelf

The continental shelf is the gently sloping submerged edge of a continent extending from the shoreline.

·       It is usually less than 100 fathoms deep and may extend up to 320 km in width, though the average width is about 64 km.

·       The slope is very gentle, about 2 metres per kilometre.

·       The shelf may contain rock, sand, mud, or gravel deposits.

·       Changes in sea level due to diastrophism or erosion and deposition can modify the shape of the shoreline.

2. Continental Slope

·       Beyond the continental shelf, the sea floor descends steeply, forming the continental slope.

·       It marks the true boundary between the continental crust and deep ocean basin.

·       The slope extends downward until it gradually merges with the deep sea floor.

3. Deep Sea Floor

·       The deep sea floor lies beyond the continental slope.

·       It is not a smooth plain but contains submarine ridges, mountains, and plateaus.

·       One of the most important features is the Mid-Atlantic Ridge, which extends from Iceland to Antarctica.

·       The Hawaiian Islands are located on a long submarine ridge in the Pacific Ocean.

4. Submarine Canyons

·       Submarine canyons are deep, steep-sided valleys found on continental shelves and slopes.

·       They resemble river valleys on land and may have branching tributaries.

·       Many canyons occur near the mouths of rivers such as the Indus River, Hudson River, and Columbia River.

·       Some of these canyons extend 1,800–2,700 metres below sea level.

 5. Sea Mounts and Guyots

·       Sea mounts are submarine mountains rising more than 900 metres above the ocean floor.

·       Most seamounts have conical peaks, indicating volcanic origin.

·       Some seamounts have flat tops, which are called guyots.

·       These flat-topped mountains were probably eroded by waves when they were above sea level and later submerged due to subsidence of the ocean floor.

6. Trenches and Deeps

Ocean trenches are long, narrow, and very deep depressions on the ocean floor. They are especially common in the Pacific Ocean. Important examples include: Aleutian Trench,  Japan Trench, Mariana Trench, Philippine Trench, Java Trench, Puerto Rico Trench

The deepest parts of the ocean are found in these trenches. For example, the Mariana Trench reaches depths of about 10,692 meters. These trenches are located in tectonically active regions where earthquakes frequently occur.

7. Mid-Ocean Ridges

·       Mid-ocean ridges form a continuous underwater mountain chain across the oceans.

·       They extend for about 64,000 kilometres and cover nearly 20% of the Earth’s surface.

·       Their width ranges from 500 to 5,000 kilometres.

·       The ridges contain central rift valleys where magma rises from the asthenosphere.

·       This process creates new oceanic crust, making mid-ocean ridges important sites of sea-floor spreading.

 General Features of Seas

1. Nature of Sea Water

Sea water contains about half of the known chemical elements, but chlorides dominate and give sea water its salty nature. Mineral matter forms about 3.44% of the weight of sea water, showing that oceans contain enormous quantities of dissolved minerals. These dissolved substances have been supplied mainly by rivers and streams over millions of years.

Some elements such as iron, silica and calcium are removed from sea water by marine organisms during their life processes. The most abundant dissolved salt is common salt (sodium chloride), which forms nearly 78% of the total dissolved substances. Other important dissolved constituents include magnesium chloride, magnesium sulphate, calcium sulphate and potassium sulphate. It is estimated that if all dissolved salts were precipitated, they would form a layer about 50 meters thick on the ocean floor. Sea water also contains dissolved gases such as nitrogen, oxygen and carbon dioxide, which are supplied by marine organisms and submarine volcanic activity.

2. Temperature, Density and Pressure

The temperature of ocean water mainly depends on solar radiation and varies from the equator to the polar regions. Surface temperature is about 80°F near the equator but decreases to about 28°F near the poles. Temperature generally decreases with depth until about 600 fathoms, where it becomes nearly constant around 39–40°F. Cold water from polar regions moves towards the equator and plays an important role in ocean circulation and climate. The normal specific gravity of sea water is about 1.025, which is slightly higher than fresh water.
Density increases in cold polar seas and decreases in warm tropical seas. Changes in density caused by variations in temperature, salinity and pressure help produce ocean currents.
Pressure increases rapidly with depth according to hydrostatic laws and becomes extremely high in deep oceans.

3. Forms of Life in the Sea

The sea contains an enormous variety of plant and animal life ranging from microscopic organisms to giant whales. Lower organisms are far more numerous than highly evolved animals
higher animals include whales, seals, walruses, sea turtles and many species of fishes. Marine plants mainly consist of seaweeds that range from tiny diatoms to large algae many meters long.
The surface waters contain countless microscopic algae which use sunlight and chlorophyll to convert inorganic substances into organic food. These organisms form the basic food source for many marine animals living at different depths of the ocean. Marine organisms often possess hard parts made of calcium carbonate or silica, which later contribute to the formation of sedimentary rocks. Thus marine life plays an important role in rock formation and geological processes.

Life Zones of the Sea

1. Littoral Zone

The littoral zone lies between high tide and low tide levels and is regularly exposed and submerged by tides. This zone is strongly affected by waves, making living conditions difficult for organisms. Many organisms survive by attaching firmly to rocks or by burrowing into mud and sand. Some animals live in tidal pools, while others like sea urchins occupy holes in rocks.

2. Neritic Zone

The neritic zone extends from the low tide mark to the edge of the continental shelf.
Water depth is generally less than 130 meters, allowing sunlight to reach the bottom.
Food supply is abundant and therefore a very large variety of organisms live in this zone.
This region supports more marine life per unit area than any other part of the earth.

3. Bathyal Zone

The bathyal zone extends from about 70 fathoms to 1,000 fathoms depth. Only a small amount of sunlight reaches the upper part of this zone and plant life is limited. However, the sea floor supports many animal species which feed on organic material falling from upper waters.
Sediments in this zone often consist of planktonic shells, diatoms and sponge spicules.

4. Pelagic Zone

The pelagic zone includes the open waters of the ocean beyond the littoral zone. It contains both floating organisms known as plankton and free-swimming animals such as fishes. Algae and diatoms are the most common plants in this zone. The skeletal remains of these organisms contribute to the formation of sedimentary deposits on the ocean floor.

5. Abyssal Zone

The abyssal zone includes all parts of the sea below 1,000 fathoms depth. This region receives no sunlight and the temperature remains close to freezing. The pressure is extremely high and plant life cannot exist in this environment. Animals living here depend on organic material sinking from surface waters. Only highly specialized organisms can survive under such extreme conditions.

3. UNIT-12 WIND

UNIT-12 WIND

The Earth’s atmosphere is mainly composed of air, and wind is air in motion. Winds move due to differences in air pressure caused by unequal heating of the Earth by the Sun. Warm air rises while cooler air flows in to replace it, creating wind circulation.

Objectives

Ø  After studying this unit, you should be able to:

Ø  Explain wind as a geological agent

Ø  Understand erosion, transportation, and deposition by wind

Ø  Identify desert landforms

Ø  Describe the role of wind in landscape formation

 

Geological Work of Wind (Aeolian Processes)

Wind transports moisture, dust, and fine rock particles from one place to another. Since these materials are derived from land, the geological action of wind is more significant over land, especially in arid and semi-arid regions where vegetation is sparse.

Wind acts as a geological agent through three main processes:

1.     Erosion

2.     Transportation

3.     Deposition

However, wind is less effective than running water as an erosion agent.

2. Wind Erosion

Wind erosion mainly occurs in dry regions with little vegetation. It involves the removal of loose rock particles from the Earth's surface.

There are two main processes of wind erosion:

1. Deflation : Deflation is the removal of loose particles such as dust and fine sand by wind.

Features:

·       Occurs in arid regions with sparse vegetation

·       Wind picks up loose material and carries it away

·       Leaves behind coarser particles and desert pavement

 

Causes of deflation:

·       Strong winds

·       Whirlwinds and eddies

·       Lack of vegetation cover

 

2. Abrasion (Corrosion) : Abrasion or corrosion occurs when wind-blown sand acts like a natural sandblast and wears down rock surfaces.

Characteristics:

·       Sand particles strike rock surfaces

·       Rocks become polished, grooved, or fluted

·       Most effective close to the ground where sand concentration is highest

Common landforms produced by abrasion: Mushroom rocks, Pedestal rocks, Table rocks, Undercut hills

Example: Windows exposed to desert winds may lose their glass polish due to sand abrasion.

Ventifacts: Stones shaped and polished by wind abrasion are called ventifacts.

Types:

1.     Einkanters – stones with one polished face.

2.     Dreikanters – stones with three faceted faces formed by rotation of pebbles.

3. Wind Transportation

Wind transports materials depending on particle size, shape, and wind velocity.

Modes of Transportation

1. Suspension

Very fine particles like dust and clay are carried high in the air and transported over long distances.

Example: Dust storms, Volcanic ash transport

2. Saltation: Sand grains move in short jumping or bouncing movements.

Characteristics:

·       Most common method of sand transport

·       Particles move in a series of hops

·       Important in the formation of sand dunes

 

3. Surface Creep (Traction): Large sand grains and pebbles roll or slide along the ground due to wind pressure and collision with saltating particles.

Special Wind Movements

Whirlwind (Dust Devil) : A rotating column of air that lifts dust particles into the atmosphere.

Tornado: A violent rotating column of air capable of lifting heavy objects.

4. Wind Deposition

Wind deposition occurs when wind velocity decreases and it can no longer carry its load.

Deposited materials are mainly:  Sand, Dust, Clay

Deposits show sorting, with heavier particles settling first and finer particles travelling farther.

5. Sand Deposits and Sand Dunes

Wind deposits sand in heaps or ridges called sand dunes.

Formation of Dunes: Dunes begin forming when wind encounters:

Rocks, Vegetation, Surface irregularities

These obstacles reduce wind velocity and cause sand accumulation.

Structure of Sand Dunes: A typical dune has two slopes:

Windward slope

Gentle slope

Angle: 10°–15°

Leeward slope (Slip face)

Steeper slope

Angle: 20°–30°

Sand moves up the windward side and falls down the slip face, causing the dune to migrate slowly in the direction of wind.

Migration rate: Usually a few meters per year.

6. Types of Sand Dunes

1. Barchan Dunes: Barchan dunes are crescent-shaped dunes formed by winds blowing in one direction.

Characteristics:

·       Gentle windward slope

·       Steep leeward slope

·       Horns point downwind

Conditions:

·       Limited sand supply

·       Hard ground

·       Sparse vegetation

2. Seif (Longitudinal) Dunes : Seif dunes are long narrow ridges of sand.

Characteristics:

·       Formed by winds from slightly different directions

·       Parallel to wind direction

·       Common in deserts of Arabia and Sahara

3. Transverse Dunes : These dunes form perpendicular to the prevailing wind direction.

Characteristics:

·       Formed where sand supply is abundant

·       Occur in deserts and coastal regions

·       Appear as long ridges separated by troughs

7. Dune Structure: Sand dunes show cross-bedding due to continuous migration.

Typical features:

·       Well sorted sand

·       Mostly quartz grains

·       Layers inclined in different directions

Cross-bedding is a key indicator of ancient desert environments in sedimentary rocks.

8. Ripple Marks: Ripple marks form due to wind friction on sandy surfaces.

Characteristics:

·       Small ridges and troughs

·       Coarse grains accumulate at crests

·       Fine grains collect in troughs

They are common on:

·       Sand dunes

·       Beaches

9. Dust Deposits : Fine particles carried by wind settle when wind speed decreases.

Deposition may occur through: Rain, Snow, Calm atmospheric conditions

Examples: Mud rain, Dust-colored snow

These deposits are called Aeolian deposits.

10. Loess Deposits

Loess is a fine wind-blown deposit of silt.

Characteristics:

·       Yellow or buff coloured

·       Non-stratified

·       Composed mainly of quartz, feldspar, hornblende, and calcite

Thickness: Few meters to several hundred meters

Major regions:  China (largest deposits), Central United States, Europe (Rhine basin)

Importance of Loess

1.     Forms very fertile soils

2.     Can stand in vertical cliffs

3.     Associated with glacial deposits

4.     Indicates wind deposition during glacial periods

Deserts – Geological Processes

1. Desert Environment

Desserts are arid regions where wind acts as the main geological agent of erosion and deposition. These regions usually have clear skies, very dry atmosphere, and large daily temperature variations. Rainfall is extremely low and vegetation is very sparse or absent.

2. Rainfall and Surface Runoff

Although rainfall is rare in deserts, it often occurs as sudden cloudbursts. The heavy rain produces strong torrents that flow rapidly down slopes, causing intense erosion of the land surface. Many deserts are closed drainage basins, where water does not flow to the sea.

3. Alluvial Fans and Playa Lakes

Water flowing from gullies carries a large amount of sediment and spreads across the basin floor. The sediments are deposited in fan-shaped accumulations called alluvial fans. Temporary lakes called playa lakes form at the base of slopes where water collects and later dries up due to evaporation.

4. Wind Action in Deserts

Dry channels known as wadies act as natural pathways for wind movement. Wind carrying sand particles polishes and erodes the walls of these channels. Wind also transports fine dust and sand across long distances.

5. Formation of Sand Dunes and Desert Pavement

Wind may deposit sand in sand dunes, while finer particles are blown away from the surface. The remaining coarse fragments such as sand and pebbles are called lag stones. Over long periods, these fragments accumulate to form a desert pavement.

6. Desert Varnish

Lag stones often develop a dark shiny coating of iron and manganese oxides known as desert varnish, which forms due to chemical reactions on the rock surface.

7. Weathering in Desert Regions

Weathering in deserts is mainly mechanical (physical) weathering due to temperature changes and lack of moisture. As a result, the regolith is thin, discontinuous, and coarse-grained.