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.