Landforms, Oceans and Continents Notes

Every landform has a beginning. Landforms once formed may change their shape, size and nature slowly due to continued action of geomorphic processes and agents. Due to change in climatic conditions and vertical & horizontal movement of land masses, intensity of process changes leading to new modifications in the landform.

Erosional Landforms formed due to Running Water

Valleys

A valley is an elongated low area running between hills of mountains. Formation of a valley starts from narrow rills which gradually develops into deep and wide valley. Types of valleys –

V-Shaped Valley – It is a narrow valley with steep sloped sides that appear similar to letter “V” from a cross-section.
Gorge – Narrow valley with steep rocky walls
Canyon – Valley with deep and steep sides with a river flowing along the bottom

Potholes and Plunge Pools

Potholes – These are the circular depressions formed over the rocky beds of hill streams.

Plunge Pools – Large and wide potholes formed at the foot of waterfalls are called Plunge Pools

Incised or Entrenched Meanders

These are deep wide meanders found cut in hard rocks.
Difference between incised meander and meander is that the incised meander is found on rock.

River Terraces

These are the surfaces marking old valley floor or flood plains.
They may be bedrock surfaces without any alluvial cover or alluvial terraces consisting of stream deposits
The river terraces may occur at the same elevation on either side of river. Such terraces are called paired terraces.

Depositional Landforms due to Running Water

Alluvial Fans

These are formed when stream flowing from higher levels break into foot slope plains of low gradient.
These are conical in shape
In humid area, alluvial fans have gentle slope while in arid areas, they have steep slopes.

Deltas

These are the wetlands that is formed when rivers empty their water and sediments into another water body.
As the delta grows, the river distributaries continue to grow in length and delta continues to build up into the sea

Floodplains

Flat area of land adjacent to a river. It stretches from banks of the river to outer edges of the valley
These plains are composed of sedimentary deposits brought by rivers or streams
Flood plains in a delta are called Delta Plain

Natural Leeves

These are low, linear and parallel ridges of coarse deposits along the banks of rivers.

Point Bars

These are also called Meander Bars.
They are found on the concave side of meanders of large rivers.

Meander

It is not a type of landform but only a type of channel pattern.
The series of regular curves formed in the channel of a river is called meander

Ox-Bow Lakes

It is a U-shaped Lake formed when a wide meander of river cuts off.

Erosional landforms due to Groundwater

Sinkholes

It is a circular opening at the top and funnel shaped towards the bottom.
These are formed due to chemical dissolution of carbonate rocks or collapse process.
These are covered up with soil mantle and appear as shallow mantle pools.
Sometimes, these are known as dolines to refer the collapse sinks.

Lapies and Limestone Pavements

Lapies are formed when water flows over a limestone surface.
Lapies gradually turns into smooth surface called limestone pavements

Caves

Caves are formed in areas where there are alternating beds of rocks with limestones or dolomites in between or in areas where limestones are dense and massive.
They normally have an opening through which cave stream are discharged.
Caves having openings at both the ends are called tunnels

Depositional landforms due to Groundwater

Stalactites

It is an icicle shaped formation that hangs from the ceiling of a cave.
It is produced by precipitation of minerals from water dripping through the cave ceilings

Stalagmites

It is an upward growing mound of mineral deposits that have precipitated from water dripping onto the floor of a cave.
Most stalagmites have rounded or flattened tips.
Difference between stalagmites and stalactites is that stalactites hang from the ceiling of cave while stalagmites grow from the floor of a cave

Erosional Landforms due to Glaciers

Cirque

These are the valleys formed by the glacial erosion
They are often found at the heads of glacial valleys
A lake of water can be seen within the cirque after the glacier disappears. Such lakes are called Tarn Lakes.

Horns and Serrated Ridges

These are formed through headward erosion of cirque walls

Glacial Valleys/Troughs

These are trough like and U-shaped with broad floors and relatively smooth and steep sides.
These may contain littered debris or debris shaped as moraines with swampy appearance.
Very deep glacial valleys filled with sea water are called Fiords

Depositional Landforms due to Glaciers

Moraines

These are accumulations of dirt and rocks that have fallen on to the glacier surface or have been pushed along by the glacier as it moves.
Lateral moraines are formed along the sides parallel to the glacial valleys
The moraine in the centre of the glacial valley is called medial moraine.

Eskers

These are formed due to melting of glaciers and flow of water on the surface of ice.
These are ridges made of sand and gravels deposited by glacial melt water flowing through tunnels

Outwash Plains

These are the plains formed at the foot of the glacial mountains or beyond the limits of continental ice sheets

Drumlins

These are smooth oval shaped ridge-like features composed mainly of glacial till with some masses of gravel and sand.
One end of drumlin, facing the glacier is called stoss and the other end is called tail.
Drumlins give an indication of direction of glacier movement.

Erosional Landforms formed due to Waves & Currents

Sea Caves

The lashing of waves against the base of cliff and the rock debris that gets smashed along with lashing waves create hollows
These hollows get widened and deepened to form sea caves

Sea Stacks

It is a geological landform consisting of steep or vertical columns of rock in the sea near the coast. It is formed by wave erosion.
These are temporary in nature

Depositional Landforms formed due to Waves & Currents

Beaches and Dunes

Beaches are formed from unconsolidated sand along the shorelines.
Most of the sediments making up the beaches comes from land carried by the streams and rivers or from wave erosion
Dunes are formed after beaches where sand is deposited and winnowed.

Bars, Barriers and Spits

A ridge of sand formed in the sea in the off-shore zone lying approximately parallel to the coast is called an off-shore bar.
An off-shore bar which is exposed due to further addition of bar is known as barrier bar.
Sometimes off-shore bars and Barrier bars get keyed up to one end of the bay and they are called spits
The barriers, bars and spits at the mouth of the bay gradually extend leaving only a small opening of the bay into the sea and the bay will eventually develop into a lagoon.

Erosional Landforms due to Winds

Pediments and Pediplains

Pediments are gently inclined rocky floors close to the mountains at their foot with or without thin cover of debris.
Such rocky floors form through the erosion of mountain front through a combination of lateral erosion by streams and sheets flooding.
When several pediments join, a relatively flat rocky surface is formed, called Pediplain.
Pediplain is the last stage of landform evolution and final stage of erosional process.

Playas

These are the shallow lakes where water is retained only for a short duration due to high evaporation.
These are prominent in desert areas.
The playas covered by salts are called Alkali flats

Deflation Hollows and Caves

Due to persistent movement of wind currents, bare soil getsblown out. This process creates shallow depressions called deflation hollows.
The deeper and wider deflation hollows are called caves

Depositional Landforms due to Winds

Sand Dunes

It is a landform composed of wind driven sand. It takes the form of a mound, ridge or a hill.
These are usually found in beaches or deserts
Crescent shaped dunes are called Barchans
When sandy surface is partially covered with vegetation, parabolic dunes are formed
When Barchan has only one wing or point, it is called Seif
When supply of wind is poor and wind direction is constant, longitudinal dunes are formed
When wind direction is constant and source of sand is elongated at right angles to the wind direction, transverse dunes are formed

Movement of Ocean Water

The Ocean water is dynamic. There are various factors which influence the movement of ocean water – temperature, salinity, density, external forces like that of the sun, moon and the winds. Ocean water moves horizontally as well as vertically. Horizontal movement is known as current or waves while vertical movement is known as Tides.

Waves

Waves are actually the disturbance (energy) and not the water as such, which moves across the ocean. Water particle only travel in a small circle as a wave passes.
The energy required for formation of waves is provided by wind energy.
Wind causes waves to travel in the ocean and the energy is released on the shorelines where it slows down due to friction between ocean water and sea floor.
The largest waves are found in the open oceans where there is no friction between ocean water and sea floor.
The height of a wave is determined by the strength of wind.
Origin of a wave can be determined by its shape and size – steep waves are young ones, generated by local winds; steady waves are generated from far off places.

Tides

The periodical rise and fall of the sea level, once or twice a day, due to the attraction of sun and moon, is called a tide. Two major forces which are responsible for formation of tides are gravitational pull between sun and moon and centrifugal force between sun and moon. Types of Tides:

Tides based on Frequency of Occurrence

Semi-diurnal tide – It contains two high tides and two low tides each day of approximately same height.
Diurnal tide – One high tide and one low tide during each day of approximately same height.
Mixed tide – The tides having variations in height are known as mixed tides. Such tides are found in North America and on many islands of Pacific Ocean

Tides based on position of Sun, Moon and Earth

Spring tide – When sun, moon and earth are in straight line, spring tide occurs. It happens twice a month, one on full moon and other on new moon.
Neap tide – When sun and moon are perpendicular to each other and the forces of sun and moon counteract one another. There is a 7 days’ interval between spring tides and neap tides.

Ocean Currents

An ocean current is a continuous and directed movement of sea water generated by a number of forces acting upon the water.

Forces Influencing the Ocean Current

Primary forces that initiate the movement of water
Secondary forces that cause the currents to flow

Primary forces that initiate the movement of water are of following types –

Heating by solar energy – It causes the water to expand. That is why, near the equator the ocean water is about 8cm higher in level than in middle latitude. This causes a gradient and water tends to flow down the slope.
Wind – Wind blowing on the surface of ocean water pushes the water to move.
Gravity – It tends to pull the water down and creates the gradient variation.
Coriolis force – It causes the water to move towards right in northern hemisphere and towards left in southern hemisphere causing large circular current.

Time between high tide and low tide is called ebb

Time between low tide and high tide is called flow/flood

Types of Ocean Currents

Based on Depth

Surface currents – These constitute about 10% of all water in the ocean. This type of current lies at upper 400m of the ocean.
Deep water currents – These constitute about 90% of all water in the ocean. Lies at a depth of below 400m of the ocean

Based on Temperature

Cold currents – These currents bring cold water into warm water areas. These are usually found on the west coast of the continents in low and middle latitudes and on the east coast in higher latitudes in Northern hemisphere
Warm currents – These currents bring warm water into cold water. These are found on the east coast of continents on northern and southern hemisphere and on west coast of continents in northern hemisphere.

Major Ocean Currents

The circular movement of water caused due to Coriolis force is called Gyre

Major Cold Currents and Oceans

Humboldt or Peruvian – current Pacific
Oyashio or Kuril or Okhotsk – Pacific
California – Pacific
Antarctica circumpolar – Pacific
Labrador – Atlantic
Canary – Atlantic
Benguela – Atlantic
Falkland – Atlantic
North-East monsoon current – Indian
Western Australian – Indian
South Indian ocean – Indian

Major Warm Currents and Oceans

North equatorial – Pacific
South equatorial – Pacific
Kuroshio – Pacific
East Australian – Pacific
North Pacific – Pacific
Alaska – Pacific
El Nino – Pacific
Tsushima – Pacific
Antilles – Atlantic
Brazillian – Atlantic
Florida Atlantic – Atlantic
Gulf stream – Atlantic
Norwegian – Atlantic
Irminger – Atlantic
Mozambique – Indian
Somali – Indian
Agulhas – Indian
South West Monsoon – Indian

Oceans and Continents

The continents across the Earth cover 29% of its surface and the rest part is covered by oceanic waters. The positions of the continents and oceans have been changing since the evolution of earth and it will continue to change in future. There are various theories of Formation of continents but the continental drift theory of Alfred Wegener holds water.

Continental Drift Theory

This theory was put by Alfred Wegener in 1912.
According to this theory, all the continents are formed out of a single continental mass surrounded by a mega ocean.
The super continent was named PANGAEA and the mega ocean was named as PANTHALASSA
About 200 million years ago, PANGAEA broke into two landmasses – Laurasia and Gondwanaland forming the Northern and Southern components respectively.
Laurasia and Gondwanaland continued to broke into further small fragments which developed into various continents that exist today.

The movement responsible for drifting of continents was caused by pole-fleeing force related to rotation of earth and Tidal force which is generated due to attraction between moon and Sun.

Evidence in support of continental drift

Matching of continents – The shorelines of Africa and America facing each other have remarkable matches.
Rocks of same age across the oceans – The marine deposits along the coastlines of South America and Africa are of Jurassic age.
Tillite – These are the sedimentary rocks formed out of deposits of glaciers. Similar sediments are found in India and in various parts of Southern hemisphere.

Plate Tectonics

In 1967, Mckenzie, Parker and Morgan came out with the concept of Plate Tectonics.

Tectonic Plate – It is an irregulary shaped slab of solid rock, composed of both continental and oceanic lithosphere.
A plate may be referred to as the continental plate or oceanic plate depending on which of two occupy a larger portion of the plate. For ex – Pacific plate is an oceanic plate and Eurasian plate is a continental plate.

Major Plates

North American
South American
Pacific
India-Australia-New Zealand
Africa with Eastern Atlantic floor plate
Eurasia and adjacent oceanic plate

Minor Plates

Cocos Plate – Between Central America and Pacific plate
Nazca Plate – Between South America and Pacific plate
Arabian Plate – Mostly found in Saudi Arabian landmass
Philippines Plate – Between Asiatic and Pacific Plate
Caroline Plate – Between Philippine and Indian Plate
Fuji Plate – North-East of Australia

Plate Boundaries

Divergent Boundaries – It exists between two plates when they move away from each other. The sites where plates move away are called spreading sites. For ex – Mid-Atlantic ridge, where American plate is separated from Eurasian plate.
Convergent Boundaries – It exists where one plate diffuses into another. The site where this diffusion takes place is called subduction zone.
Transform Boundaries – These boundaries are formed where the plates slide horizontally past each other

Rates of Plate Movement – The magnetic field of the Earth determines the rate of plate movement. The Arctic ridge has the slowest rate while East pacific rise near Chile has the fastest rate.

Oceans on the Earth

Pacific Ocean

It is the largest and deepest ocean of the world, explored by Ferdinand Magellan
Mariana Trench found in Pacific Ocean is the deepest trench with a depth of 11034 metres
Most of the islands of this ocean are of volcanic or coral origin

Atlantic Ocean

It is the second largest ocean but it has the longest coastline among all the oceans
It is the busiest ocean in the terms of trade and commerce

Indian Ocean

It is the only ocean named after a country and it is deeper than the Atlantic Ocean.
Continental islands – Madagascar and Sri Lanka
Volcanic islands – Mauritius, Seychelles, Andaman and Nicobar etc.

Arctic Ocean

Smallest and shallowest of all the oceans; lies within the arctic circle
The north pole lies in the middle of this ocean.
It has the least salinity of all the oceans

Ocean Floor Configuration

The ocean floor is segmented into three major divisions based on the depth and forms of relief –

1. Continental Margin – It is the transitional area between continental shores and deep-sea basins. It consists of continental shelf, continental slope, continental rise and oceanic trench.

Continental shelf – It is a portion of continent that is submerged under an area of relatively shallow water known as shelf sea.
Continental slope – It is a slope between continental shelf and continental rise
Continental rise – Low-relief area with accumulated sediments between continental slope and abyssal plains
Oceanic trench – These are the deepest part of an ocean. For ex – Mariana Trench of Pacific Ocean

2. Abyssal Plains – These are the plains between continental margins and mid-oceanic ridges.

3. Mid-Oceanic Ridges – This is an interconnected chain of mountain system within the ocean.

Minor relief features of Oceans

Seamount – It is a mountain with pointed summits, rising from the seafloor that doesn’t reach the ocean surface. These are volcanic in nature with a height of 3000- 4500m. for ex – Emperor seamount in Pacific Ocean
Submarine Canyon – It is a steep-sided valley cut into the seabed on continental slope, sometimes extending up to continental shelf. For ex – Hudson Canyon
Guyots – These are the flat-topped seamounts. These are under water volcanic mountains.
Atoll – Low islands found in the tropical oceans consisting of coral reefs. For ex – Addu Atoll in Maldives.

Temperature of Oceanic Waters

Ocean water gets heated up by the solar energy just as land. The process of heating and

cooling of ocean water is slower than that of land. Maximum temperature of the oceans is always at their surfaces because of direct incidence of sunlight. Heat is transmitted to

lower levels of oceans through convection. With the increase in depth, temperature

decreases but the rate of decrease is not uniform.

Factors affecting temperature of ocean water

Latitude – Temperature decreases from equator towards the poles because of decreasing insolation.
Unequal distribution of land and water – Oceans in the northern hemisphere receive more heat due to their larger contact with land than oceans in southern hemisphere.
Wind – Wind blowing from land towards the ocean resulting in decrease in temperature or vice-versa
Ocean currents – Warm oceanic currents increase the temperature in cold areas while cold currents decrease the temperature in warm areas

Salinity of Ocean Waters

Salinity is defined as the total amount of dissolved salts in sea water
It is calculated as amount of salt (in gm) dissolved in 1000 gm of sea water. It is expressed as Parts per Thousand (PPT)
Salinity of 24.7 PPT is considered as the upper limit to demarcate ‘Brackish Water’\
Highest saline sea – Dead Sea (238 PPT)
Salinity generally decreases with depth but there is distinct zone called Halocline, where salinity increases sharply.

Thermocline – The boundary region, from where the temperature decreases rapidly is

called thermocline