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TOPIC 3
MAJOR
FEATURES OF THE EARTH’S SURFACE
Earth is the fifth largest planet in the solar system.
Its surface is approximately 510 millions square kilometers. This means that
earth is very large. The surface of the earth is made up of two main features
LAND and WATER BODIES The land's surface area is estimated at 29.2% of the
total area of the earth's surface, while water covers the remaining 70.8%. Most
of the land exists in large blocks called Continents. Likewise, most of the
water is contained in large water bodies called seas and oceans.
CONTINENT
Continent
is a major landmass rising from the ocean floor. Formerly the continents were big landmass known
as Pangaea. Due to the forces operating continuously, the landmass was separated into two
parts namely Laurasia to
the northern hemisphere and Gondwanaland to the Southern hemisphere.
Further separation or drifting led to
the formation of the present
continents which include Africa.
Antarctica, Asia. Australia, Europe,
North America and Southern
America.
The boundaries of the continents with the exception of Asia and Europe were filled with water. Ural
Mountains separated Europe and Asia.
While other continents are
separated by water bodies called seas
and oceans. For instance African
continent and Asia to the north are separated by Mediterranean sea and Red sea. Africa and South America are separated by Atlantic ocean, etc.
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| Formation of Continents |
The land surface occupies
29% of the surface of the globe, and the remaining 71% is covered by water. The
land surface forms seven continents. A continent is a major landmass rising
from the ocean floor. It includes islands adjacent to the continent. There are
seven continents namely, Africa, Asia, South America, North America, Europe,
Australia and Antarctica. These continents are surrounded by the following
oceans: The Indian Ocean, the Atlantic Ocean, the Pacific Ocean, the Arctic
Ocean and the Southern ocean.
There is more land surface
in the northern hemisphere than in the southern hemisphere but there is more
water surface in the southern hemisphere than in the northern hemisphere. The
continents are broader in the northern hemisphere. The seven continents that
make up the globe are explained below:
1.
Asia: Asia
is the largest of all continents. It covers more than one third of the land surface
of the earth. It is approximately one and a half times the size of Africa. Its
total area is about 45.6 million square kilometres. Asia stretches from 0° to
67°N and from 30°E to about 18°E. The Ural Mountains form the boundary between
Asia and Europe. This continent is attached to Africa by the narrow Isthmus of
Suez which has been dug to form the Suez Canal. The continent is bordered to
the North by the Arctic Ocean, to the East by the Pacific Ocean, and to the
South by the Indian Ocean.
2.
Africa:
Africa is the second largest continent with an area of about 3.6 square
kilometres. Africa extends from 37°N to 35°S and from 50°W to 50°E and it is
crossed by Tropics of Cancer and Capricorn. Thus the greater part, about three
quarters of the whole area lies in the tropics. Africa is bordered to the north
by the Mediterranean Sea, to the west by the Atlantic Ocean, and to the East by
the Indian Ocean.
3.
South America:
South America is the third largest continent and it is about two-thirds the
size of Africa. Its size is about 24.3 million square kilometres. It lies
between 10°N and 50°S and between 35°W and 80°W. This continent is bordered to
the east by the Atlantic Ocean, to the West by the Pacific Ocean, and it is
joined to North America by the Isthmus of Panama.
4.
North America:
North America is the fourth continent in size and it is slightly more than half
the size of Africa. Its size is about 17.9 million square kilometres. If
extends from 10°N to 65°N and from 60°W to 160°W. It is bordered to the west by
the Pacific Ocean, to the East by the Atlantic Ocean, and the North by the
Arctic Ocean.
5.
Antarctica:
Antarctica is the fifth continent in size and it is about one-third the size of
Africa. Its area is about 11.4 million square kilometres. This is the
southernmost continent, forming a circle at the South Pole and extends south of
661⁄2°S. It is surrounded by the southern ocean. The continent is mostly
uninhabited.
6.
Europe:
Europe is the sixth continent in size and it is about two-fifth the size of
Africa. The size of Europe is 9.8 million square kilometres. Most of Europe
lies between 40°N and the 1 Arctic circle, and between 10°W and 60°E. It lies
to the west of Asia, separated by the Ural Mountains. Europe is bordered to the
north by the Arctic Ocean, to the west by the Atlantic Ocean, and to the south
by the Mediterranean Sea.
7.
Australia:
Australia is the smallest continent and it is about a quarter of the size of
Africa. Its size is about 8.5 million square kilometres. Australia is
approximately 10°S and 40°S and between 115°E and 150°E. The islands of New
Zealand to the south east of Australia are part of this continent. The
continent is bordered to the west and north by the Indian Ocean, to the east by
the Pacific Ocean, and to the south by the Southern Ocean.
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MAJOR RELIEF FEATURES OF CONTINENTS
The surface of the continents is not smooth. It has mountains, hills, rivers and valleys, plateaus, and plains. Mountains are landforms which have high relief generally over 300 metres above the surrounding area. Hills are landforms that have moderate relief generally between 150 and 300 metres above the surrounding area. Plateaus are extensive highland areas with more or less uniform summit level, bounded by one or more slopes falling steeply away, sometimes rising on one or more sides by steep slopes to mountain ridges. Plains are continuous sketches of comparatively flat land not much above sea level, sometimes gently rolling or undulating.
Mountains
There are four types of
mountains. These are the Fold Mountains, Block Mountains, Residual Mountains,
and Volcanic Mountains. These mountains are all named according to the way they
were formed.
Fold Mountains
Fold Mountains are formed by wrinkling or
(folding) of the Earth’s crust. Fold Mountains usually form parallel
ranges which extend for hundreds of miles across a continent. Thus, Fold
Mountains are the most extensive ranges in the world. For example, the
Rocky Mountains in North America vary in width from 640 to 1,600
kilometres and are about 5,000 kilometres in length. These types of
mountains have some of the highest peaks in the world. Mount Everest in
the Himalayas is 8,848 metres above sea level and the Aconcagua in the
Andes is 7,003 metres above sea level.
Examples of Fold Mountains include the Himalayas in Asia; the Rockies in North America; the Andes in South America; the Alps in Europe; the Atlas in North Africa; the Cape ranges in South Africa; the Appalachians in the USA; and the Great Dividing Ranges in Australia.
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| Fold Mountain |
Block Mountains are formed when a movement in
the earth’s crust forces the rocks to break instead of folding. As a
result enormous cracks or faults are formed. When two sets of faults run
parallel to each other and the ground between is forced to rise up, a
block (fault) mountain is formed. Usually Block Mountains do not extend
over wide areas as Fold Mountains do. Examples of Block Mountains are the
Usambara, Uluguru and Ruwenzori Mountains in Africa; the Vosges and Black
Forest Mountains in Europe; and Mount Sinai in Asia.
Volcanic Mountains
Volcanic mountains are formed from the piling
up and cooling of hot molten lava and ashes that are thrown out from the
earth’s interior after a volcanic eruption. Some of the volcanic mountains
existing today were built up by a single eruption, but others were built
by several eruptions. Volcanic eruptions are still taking place in some
parts of the earth. Among the existing volcanic mountains, some still
experience periodic eruptions, for example, the Vesuvius in Italy; the
Krakatoa in Indonesia; the Mufimbiro in Uganda; and the Oldoinyo-Lengai in
Tanzania.
The Volcanic Mountains that still experience periodic eruptions are called active volcanic mountains. The Volcanic Mountains which erupted once in historical times and are no longer active are said to be dormant. In this group are included the Kilimanjaro and Meru mountains in Tanzania. Those volcanic mountains which have never experienced eruption and have shown no signs of erupting again are said to be extinct (dead). Included in this group are mountains Kenya, Elgon, Ngorongoro and Rungwe in East Africa; and Demavend in Iran.Volcanic mountains are usually conical in shape and mostly contain craters or depressions at their summits, for example, mountains Fujiyama and Kilimanjaro. Sometimes the craters are filled with water to form crater lakes.
Residual Mountains
Residual mountains are formed when an area of
highland remains standing above the general level of land after the rivers
and other natural agents have lowered the surface of the surrounding area.
Sometimes such highlands are called mountains of denudation. These
mountains may in some cases appear as isolated hills but in other cases
they appear as long ridges, generally steep on one side (the scarp slope)
and gentle on the other side (dip slope). Examples of residual mountains
are the Ahaggar Mountains of central Sahara; the Sekenke hills of Singida
in Tanzania; the Admawa mountains of eastern Nigeria; the Highlands of
Scotland; the Sierras of central Spain; and the Mesas and Buttes of the
western plateau of the United States.
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| Residual Mountains |
Plateaus
In geology and earth
science, a plateau (plural: plateaus or plateaux), also called a high plain or
tableland, is an area of highland, usually consisting of relatively flat
terrain that is raised significantly above the surrounding area, often with one
or more sides with steep slopes.
The largest and highest
plateau in the world is the Tibetan Plateau, called the "roof of the
world”. The Tibetan plateau covers approximately 2,500,000 km2 at
about 5,000 m above sea level.
The second-highest plateau
is Deosai National Park (also known as Deoasai Plains) at an average elevation
of 4,114 m and is located in the Skardu District of Gilgit-Baltistan, in
northern Pakistan.
The third-largest plateau
is the Antarctic Plateau, which covers most of central Antarctica, where there
are no known mountains, but rather 3,000 m or more of ice.
Other plateaus in the world
include the Colorado Plateau (North America); the Great Central Plateau,
Ahagger Plateau and Fouta Djallon Plateau (Africa); Brazilian Plateau (South
America), Mexican Plateau and Laurentian Plateau (North America); Arabian
Plateau, Deccan Plateau and Tibet Plateau (Asia).
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| Plateau |
Plains
A plain is a broad area of
relatively flat land. Plains are one of the major landforms, or types of land,
on Earth. They cover more than one-third of the world’s land area. Plains exist
on every continent except Antarctica. Plains occur as lowlands and at the
bottoms of valleys but also on plateaus or uplands at high elevations.
Plains in many areas are
important for agriculture because where the soils were deposited as sediments
they may be deep and fertile, and the flatness facilitates mechanization of
crop production; or because they support grasslands which provide good pasture
for livestock.
Plains vary widely in size.
The smallest occupy only a few hectares, whereas the largest cover hundreds of
thousands of square kilometres. For example, the Great Plains of North America
extends from Pyrenees Range on the French–Spanish border across northern Europe
and Asia, almost halfway around the world.
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| Plain |
BASINS
AND DRAINAGE OF THE CONTINENT
BASINS
A basin is a depression, or dip, in the
Earth’s surface. Basins are shaped like bowls, with sides higher than the
bottom. They can be oval or circular in shape, similar to a sink or tub you
might have in your own bathroom. Some are filled with water. Others are empty.
Basins are formed by forces above the ground (like erosion) or below the ground (like earthquakes). They can be created over thousands of years or almost overnight
Basins are formed by forces above the ground (like erosion) or below the ground (like earthquakes). They can be created over thousands of years or almost overnight
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| Basin |
The
major types of basins are river drainage basins, structural basins, and ocean
basins.
River Drainage Basins
A river drainage basin is an area drained by a river and all of its tributaries. A river basin is made up of many different watersheds.
River Drainage Basins
A river drainage basin is an area drained by a river and all of its tributaries. A river basin is made up of many different watersheds.
A watershed is a small version of a river basin. Every stream and tributary has
its own watershed, which drains to a larger stream or wetland. These streams,
ponds, wetlands, and lakes are part of a river basin. The Mississippi River
basin in the U.S., for instance, is made up of six major watersheds: the
Missouri, Upper Mississippi, Ohio, Tennessee, Lower Mississippi, and
Arkansas-Red-White Rivers.
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| River Drainage Basin |
Structural
Basins
Structural basins are formed by tectonic activity. Tectonic activity is the movement of large pieces of the Earth’s crust, called tectonic plates. Tectonic activity is responsible for such phenomena as earthquakes and volcanoes. The natural processes of weathering and erosion also contribute to forming structural basins.
Structural basins form as tectonic plates shift. Rocks and other material on the floor of the basin are forced downward, while material on the sides of the basin are pushed up. This process happens over thousands of years. If a basin is shaped like a bowl, a structural basin is shaped like a series of smaller bowls, stacked inside each other. Structural basins are usually found in dry regions.
Structural basins are formed by tectonic activity. Tectonic activity is the movement of large pieces of the Earth’s crust, called tectonic plates. Tectonic activity is responsible for such phenomena as earthquakes and volcanoes. The natural processes of weathering and erosion also contribute to forming structural basins.
Structural basins form as tectonic plates shift. Rocks and other material on the floor of the basin are forced downward, while material on the sides of the basin are pushed up. This process happens over thousands of years. If a basin is shaped like a bowl, a structural basin is shaped like a series of smaller bowls, stacked inside each other. Structural basins are usually found in dry regions.
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| Structural Basin |
Ocean
Basins
Ocean basins are the largest depressions on Earth. Edges of the continents, called continental shelves, form the sides of ocean basins. There are five major ocean basins, coordinating with the major oceans of the world: the Pacific basin, the Atlantic basin, the Indian basin, the Arctic basin, and the Southern basin. Many smaller basins are often considered oceanic basins, such as the North Aleutian Basin, between the Pacific and Arctic Oceans
Ocean basins are the largest depressions on Earth. Edges of the continents, called continental shelves, form the sides of ocean basins. There are five major ocean basins, coordinating with the major oceans of the world: the Pacific basin, the Atlantic basin, the Indian basin, the Arctic basin, and the Southern basin. Many smaller basins are often considered oceanic basins, such as the North Aleutian Basin, between the Pacific and Arctic Oceans
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| Ocean Basin |
WATER BODIES
Water bodies includes the
folowing;
1. RIVERS
1. RIVERS
A river is a stream of water that flows
through a channel in the surface of the ground. The passage where the river
flows is called the river bed and the earth on each side is called a river
bank. A river begins on high ground or in hills or mountains and flows down
from the high ground to the lower ground, because of gravity. A river begins as
a small stream, and gets bigger the farther it flows.
The water in a river is called fresh water. It comes from rain or snow and it can usually be drunk safely, unless it has been polluted. The water in a sea cannot be drunk safely because it is salt water. Many plants, animals and people live near rivers. They need water to survive and, for humans, rivers were once the best means of transport
Tributaries
A tributary or affluent is a stream
or river that flows into a larger stream or main stem (or parent) river or a
lake. A tributary does not flow directly into a sea or ocean. Tributaries
and the main stem river drain the surrounding drainage basin of its surface
water and groundwater, leading the water out into an ocean.
Distributaries
A distributary, or a distributary channel,
is a stream that branches off and flows away from a main stream channel.
Distributaries are a common feature of river deltas. The phenomenon is known as
river bifurcation. The opposite of a distributary is a tributary.
Distributaries are found where a stream nears a lake or an ocean. They can also
occur inland, on alluvial fans, or where a tributary stream bifurcates as it
nears its confluence with a larger stream. In some cases, a minor distributary
can divert so much water from the main channel that it can become the main
route.
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| River |
Rivers are part of the hydrological cycle. Water
generally collects in a river from precipitation through a drainage basin from
surface runoff and other sources such as groundwater, springs, and the release
of stored water in natural ice and snowpacks (e.g. from glaciers).
Examples of rivers in Africa include the Nile,
Congo, Niger, Zambezi and Orange. In Tanzania we have rivers like Rufiji,
Ruvuma, Ruaha, Pangani, Wami and Malagalasi.
2. LAKES
A lake is a large body of water (larger and deeper than a pond) within a
body of land. As a lake is separated from the ocean, it is not
a sea. Some lakes
are very big, and people in the past sometimes called them seas. Lakes do not flow, like rivers, but many
have rivers flowing into and out of them.
Some lakes are artificial (man-made lakes) and are
constructed for industrial or agricultural use, for hydro-electric power
generation or domestic water supply, or for aesthetic or recreational purposes.
Examples of man-made lakes include Lake Nasser (in Egypt), Lake Kariba
(Zambia), and Lake Volta (Ghana).
Many lakes are man-made reservoirs
built to produce electricity, for recreation,
or to use the water for irrigation or industry,
or in houses.
The majority of lakes on Earth are fresh water, and most lie in the Northern Hemisphere at higher latitudes. Most lakes have at least one natural outflow in the form of a river or stream, which maintains a lake's average level by allowing the drainage of excess water. However, some lakes do not have a natural outflow and lose water solely by evaporation or underground seepage or both.
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| Lake |
The Great Lakes of the United States and Canada is the world's largest system of freshwater lakes. Lake Superior alone is the world's largest freshwater lake with an area of 82,414 sq km. The Caspian Sea is the largest lake in the world, with an area of 372,960 sq km. Lake Titicaca in the Andes Mountains of South America is the world’s highest lake at 3,800 m above sea level; while the Dead Sea is the lowest at 425 m below sea level.
3. OCEAN
An
ocean is defined as a body of saline water covering much of the earth. The
largest ocean is the Pacific. Its area is about 165.3 million square
kilometres. The second largest ocean is the Atlantic, which covers about 82.2
million square kilometres. The Indian Ocean, covering about 73.4 square
kilometres is the third largest, followed by the Arctic Ocean, covering about
14.0 million square kilometres.
An
ocean is defined as a body of saline water covering much of the earth. The
largest ocean is the Pacific. Its area is about 165.3 million square
kilometres. The second largest ocean is the Atlantic, which covers about 82.2
million square kilometres. The Indian Ocean, covering about 73.4 square
kilometres is the third largest, followed by the Arctic Ocean, covering about
14.0 million square kilometres.
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| Ocean |
Composition of Ocean Water
Ocean water contains a number of dissolved mineral
salts. These mineral salts include sodium chloride (common salt) which makes up
78% of all salt in the ocean water; and compounds of magnesium, potassium and
calcium. Most of the minerals in the ocean are a result of constant
accumulation since the formation of the oceans. However, a small amount of the
minerals come from the land, having been dissolved by water and brought into
the ocean by rivers. But the mineral salts in rivers are only in very small
quantities.
The saltiness of the ocean water is not the same
everywhere. Saltiness of the ocean water depends mainly on temperature which
affects the amount of salt that can dissolve in the water, the amount of fresh
water brought into the ocean by rivers and rainfall, and the amount of
evaporation taking place from the surface.
Water Temperature
Water is heated by the sun’s rays much more slowly
than land is. Water also loses heat to the air around it more slowly than the
land does. This causes the temperature of the sea water to vary only slightly
from season to season. In general, the temperature of the ocean water decreases
from the equator, where the surface temperature is 25°C to the polar regions
where the water is very cold (-2.2°C). But the decrease in temperature poleward
is not uniform because of the occurrence of warm and cold ocean currents. On
the other hand, water temperature decreases with depth in the tropics up to the
depth where the temperature is 1.1°C.
The table below shows the oceans and their size.
Water Movements
Ocean water is constantly in motion. There are two
types of movement. One is horizontal movement, which is in the form of ocean
currents and tides, and the other is vertical, which is the rising of
subsurface water and the sinking of the surface water. The movements of ocean
water are a result of density variations in the water which is particularly
important in vertical movements and winds which are particularly important in
horizontal movements.
An ocean current is the permanent or seasonal
movement of surface water in the ocean. There are warm and cold currents, the
ocean currents are set in motion by a combination of prevailing winds,
differences in density and temperature of the ocean waters, the rotation of the
earth, and the shape of landmass.
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| Ocean Currents |
Tides are the rising and the falling in the level
of water in the oceans, seas and lakes. They occur twice a day (in 24 hours).
The level to which tides rise and fall varies from day to day. On the days when
it rises to its highest level, it also falls to its lowest level. The rising
and falling is caused by the pull of gravity of the moon and the sun.
Waves are to and from movements of the surface
water. When water is thrown into waves, its surface gets a shape of ups and
downs. The highest part of the wave is called the crest and the lowest the
trough. The distance from one crest to the next, or from trough to trough is
called the wavelength. Waves travel in some definite direction, and give the
impression that they move forward, but in reality only the shape moves forward
while the water moves up and down. For example, a cork thrown into the water
does not travel with the waves, it moves up and down and to and fro, but not
forwards. A wave is driven on the shore by wind, and its height and force are
determined by the strength of the wind and the distance of open water over
which it has blown.
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| Water waves |
The Ocean Floor
The floor of the ocean is irregular. The major relief features of the ocean floor are explained below:
1. Continental shelf: This is a gentle-slope margin of a continent that forms the shallow areas of oceans. These shallow areas extend from the coast to a depth of about 200 metres towards the ocean, and usually end suddenly.
2. Continental slope: The continental slope is found at the point where the continental shelf forms a steep slope with the lower slope of the ocean floor towards the sea.
4. Ridge: A ridge is the raised part of the ocean floor. Some of these rides appear above the surface of the oceans as oceanic islands.
5. Ocean deep or trench: An ocean deep is a long, narrow depression (or trough) found on the ocean floor.
6. Deep sea plain (ocean plain): An ocean plain is the most extensive, flat area of the ocean floor. It is a monotonous and undulating area. A large part of the plain is covered by mud.
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| Ocean Floor |
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