THE MAJOR FEATURES OF THE EARTH’S SURFACE

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THE MAJOR FEATURES OF THE EARTH’S SURFACE

The surface of the earth is made up of two main features: 1. Land and 2. Water bodies. The land surface is estimated at 29.2% of the total area of the earth’s surface while water covers the remaining 70.2%.  The most of the land exists in large block called continents; likewise, the most of the water is contained in large water bodies called seas and oceans. 

Definitions of Some Terms:

Canal is the small mass of land joining two continents. (e.g. sues canal). Gulf is the water inlet to the land (e.g. gulf of guinea, Mexico and Banguela). Strait is the narrow water path that separates one land mass from another (e.g. Gibraltar in morocco, Makasan in Indonesia). Peninsula is a part of land entering in a body of water (e.g. Msasani in Tanzania and Indian peninsula). Cape is piece of land getting in the sea (e.g. Cape of Good Hope in South Africa, Cape Verde, and Palmas).

A.    CONTINENTS

By definition, continent refers to a major landmass rising from the ocean floor.  Continent is surrounded by water bodies. Continents rise from ocean floors. There are seven continents on the earth’s surface. These are:

1)   Africa

2)   Asia

3)   Australia

4)   Europe

5)   Antarctica

6)   South America

7)   North America

Most of the continents are joined together by small mass of land, like Suez Canal, that connects Africa and Asia, Panama Isthmus join North America and South America.  The Northern hemisphere is covered by more land surface whereas southern hemisphere occupies small part of it.

All continents originated from the drifting apart of one Sialic known as Pangaea over million years ago. See the figure below:

   

Asia

Asia is the largest continent in the world covering more than one third of the earth’s surface. The continent has total areas of 43.6 million kilometer square and lies from 0⁰ to 67⁰N and 30⁰E about 180⁰E.

Africa

Africa is the second largest continent in the world, covers 30.3 million km². It Lies from 35⁰N and 37⁰N to 160⁰W.

North America

It is the third largest continent, which covers 25.3 Million Km². It extends from 10⁰N to 65⁰N and from 60⁰W to 160⁰W.

South America:

It is the fourth largest continent that covers about 17.6 million kilometer square extending from 10⁰N and 50⁰S and between 35⁰W and 80⁰W.

Antarctica

It lies within latitude 66⁰S covering the area of about 13.3 million km². Antarctica is the fifth largest continent in size and therefore it is the only the uninhibited continent.

Europe

Europe is the sixth largest continent in size that separated from Asia by Ural Mountain to East side. Its total area is about 10.5 million kilometer square.  

Australia

It is the smallest continent in size. Its total area is about 7.7 million kilometer square. The following table shows the size of continents:

                                                                                                                                                                 

Continent	Area (Km2)
Asia	43, 608, 000
Africa	30, 335, 000
North America	25, 349, 000
South America	17, 611, 000
Antarctica	13, 340, 000
Europe	10, 498, 000
Australia	7, 682, 000

The Major Feature of the Continents:

The surface of any continent is not smooth due to the agents of denudation and deposition. Features (relief features) are hills, mountains, rift valley, plains, plateaus and basins. Mountains, plains and plateaus forms the major relief features of the continents.

1.     Plains

Plain is an undulating land usually near to the sea level. Plains are large and nearly flat stretch of land that usually has no great change in its height. Plains are characterized by very gentle slopes and wide valleys. Such plain are described as rolling or undulating plains and Serengeti Plain is an example of such plain that found in Tanzania. Many plains are found in coastal areas of the continents, which are called coastal plains. Many of plains are caused by down warping of the earth’s crust. North Europe plain, Siberia plain, and the great central pain of America are example of the plain.

2.     Plateaus

Plateaus are extensive high altitude areas with more or less uniform summit levels. Plateaus are described as table-lands. Most of plateaus are resulted due to the warping of the earth‘s crust that caused by uplifting of some areas. Plateaus that are covered by thick layers of volcanic lava are called lava-plateaus. Brasilia highland, central plateau of Africa, Colombia plateau, Decan plateau of India and Ethiopian plateau are examples of the plateaus.

3.     Mountains

Mountain is the highland areas rising above mean sea level. There are various types of mountains formed depending to their mode of formation. There are major four types of mountains that named according to their mode of formation:

a)    Fold Mountains

b)    Block Mountains

c)     Volcanic Mountains

d)   Residual Mountains

(a)  Volcanic Mountains

Volcanic mountains are mountains formed from the pilling up and cooling of molten lava and ashes that are thrown out from the earth’s interior after volcanic eruption. Most of the volcanic mountains contains craters at their peaks and are conical in shape. Crater can be filled with water to form lake like Ngorongoro in Arusha. There are major three classifications of volcanic mountains:

                                                                                                                                                                              1. Active volcanoes: are volcanic mountains which still experiencing periodic eruptions and erupted in the recent periods. Mount Vesuvius and Stromboli in Italy, mount Mauna Loa in Hawaii, Krakatau in Indonesia, Mufumbiro in Uganda, Oldonyolengai in Tanzania are examples of active volcanoes.

2. Dormant volcanoes: are those mountains which have erupted once in the past but have remained inactive for fairly long period. These are called sleeping volcanoes which become active once again. Examples of dormant volcanoes are Kilimanjaro Mountain and Meru Mountain in Tanzania.

3. Extinct (dead) volcanoes: are those mountains which were active but not erupted for a very long time and have not shown any sign of erupting again. Elgon, Ngorongoro, Kenya and Rungwe in East Africa are examples of dead volcanoes.

(b)  Fold Mountains

Fold Mountains are formed by wrinkling of the earth’s crust caused by compressional forces. In them, they occur when sedimentary rocks are laid in horizontal layers which were later folded by compressional forces.  Most of the ranges in the world are formed by fold Mountains, like cape ranges in South Africa, Mount Everest of Himalaya, Atlas, Alps, Appalachians in U.S.A and the great divided ranges of Australia. The anticline is the up fold and syncline is the down fold of the fold mountain.

(c)   Block Mountains (Horst)

Block Mountain is an upland bordered by faults on one or more sides. Block Mountain formed due to being raised or tilted up by the earth movement along faults bounded block. Block Mountain does not cover large area like fold mountain. Examples of Block Mountain are mountain Sinai in Asia, Ruwenzori in Tanzania. Block Mountains are associated with rift valleys.

(d)  Residual Mountains

Residual mountains are formed by prolonged denudations. Sekenke hills in Singida Tanzania, Mesas and Buttes in US and Adamawa in Nigeria.

4       Rift Valleys (Grabens)

Rift valley is an elongated trough or depression formed when two sets of faults sink down. The Great East African rift valley is an example. Sometimes trenches are filled with water to form lakes. The bottom of a valley is called is called floor and valley’s side are called valley-walls or valley-slopes. 

5       Basins

Basin is the natural or artificial depression varying in size on the Earth’s surface. If basins are filled with or occupied by water, tend to form rivers or lakes and ocean basin. Examples of basins are Lake Victoria, Congo basin, Kalahari basin, Chad and Sudan basin.

6       Drainages

Drainage refers to the situation by which, water are removed from an area. Streams and rivers are responsible for draining the land surfaces. The processes of water to flow on the ground are known as overland flow, and if water percolates into the ground are called underground water. Surface flow of water from rainfall or snow-melt over the ground is known as water runoff.

The surface water from rainfall or snow-melt flows in small channels, which finally join to form large trenches of rivers are called tributaries. Small streams that branches off before they enter the sea or lake are known as distributaries. The area from which a river system collects its rain water is called river/basin/catchment area/drainage basin. The boundary between one catchment area and the next is formed by crest line of the surrounding highland called watershed/water divide/basin perimeter and the main river and its tributaries together forms river system. 

B.    WATER BODIES

Water body is any significant accumulation of water such as ocean, lake, river and other alike. Some water bodies are manmade (artificial). Navigable water bodies are called water ways. There are more water bodies in the southern hemisphere than the northern hemisphere.

1.     Oceans

Ocean is a large body of water that surrounds the landmasses of the earth. Ocean covers almost 360 million kilometer square which is equal to 71%. The Pacific Ocean is the ocean while Arctic Ocean is the smallest ocean.

Salinity of ocean water depends on the temperature, amount of fresh water entering in the ocean, nature of the rock of the ocean floor and additional of organic materials. The following is the table that shows the five large oceans in the world:

Ocean	Area (Km2)	Average Depth (M)
Pacific	155, 557, 000	4, 028
Atlantic	76, 762, 000	3, 926
Indian	68, 556, 000	3, 936
Southern	20, 327, 000	4, 000 to 5, 000
Arctic	14, 056, 000	1, 205

Types of Water Movement In oceans

Water moves in the oceans is into two main ways:

a)    Horizontal movement: e.g. tides and currents.

b)    Vertical movement: e.g.  Sinking and rising of water.

Ocean currents: Ocean current is the movement of surface water. There are two types of ocean currents: Warm ocean currents: these are ocean currents formed by masses of warm water e.g. Mozambique, Brazilia ocean currents. And the second is cold or cool ocean currents: these are ocean currents that formed by masses of cold water e.g. California, Benguela, and West Australia ocean currents. 

 Tides: Tides are periodic rise and fall in the level of water in the oceans and sea. Tides occur twice a day (in 24 hours). Tides are caused by the sun and to a greater exert the moon exerts a gravitational attraction on the earth’s surface.

   

Waves: Waves are the up and down movements of the surface water. There are two parts of lakes, sea and ocean waves: Trough: is the lowest part of the wave and Crest: is the highest part of the wave.

                                                                                                                                                                                    The distance between one crest to another is called wavelength. Waves are caused by winds which drive them to the shore. Wave travel in a defined direction, whereas water moves up and down.

2.     Lakes

Lake is a hollow in the earth earth’s surface in which water are collected in them. Lakes which are in great size are called seas like Caspian Sea, Dead Sea and Aral Sea. Most of lakes are permanent, but some are for temporary.

3.     Seas

Sea is an expense of salt water that covers most of the earth’s surface. Examples of seas are North Sea, Mediterranean Sea, China Sea, red sea etc.

4.     River

River is anybody of fresh water flowing from an upland sources to a large lake or sea fed by springs and tributary streams. River starts on hillsides as a small channels or rills. Some examples of rivers are river Nile, river Amazon, yellow river, and Rufiji River.  Rivers have the following parts:  channel is in which water flows and flat region or food plain of valley on either sides of the channel.

Nature of the Ocean Floor

Ocean floor is not smooth in nature but it contains varied reliefs. The relief of the ocean floor is as varied as that of the land surface; it is irregular in shape. There are features on the ocean floor that we do not see because they covered by water.  The following are the main features of the ocean floor:

1. Continental shelf: Continental shelf is the gentle sloping margin of a continent. The continental shelf is occupied by shallow water that extends from the coast to a depth about 180 to 200m towards the ocean basin. It is excellent for growth of plankton because shallow water allows sun rays to reach its floor.

2. Continental slope: Continental slope is the steep slope that extends from the edge of the shelf to the deep sea plain. The continental slope marks the edge of the continent at the beginning of the ocean basin.

3. Ocean ridge: Ocean ridge is the raised part of the ocean floor. It takes a form of ridge or plateau on the ocean floor as a sea bed. Mid Atlantic ridge and the Abaltros plateau are examples of ocean ridge which rise above the surface to form oceanic island.

 4. Ocean trenches: Ocean trenches are long, narrow depressions found on the ocean floors. The deepest known ocean trench is the Marianas Trench near the island of Guam in the Pacific Ocean. It is deepest part is 11911m below sea level.

5. Deep sea plains: Ocean deep sea plain is wide gently sloping surface with a depth of about 300m and 5500m. Deep sea plain is also called abyssal plain.

6. Continental islands: Island is a piece of land that is completely surrounded by water. Very small islands are called islets or keys. The following are three types of islands: Oceanic islands: formed due to volcanic activities, Continental islands: lies on continental shelf and Coral islands: formed from coral rocks.

TRIAL QUESTIONS

1. List down three sources of rivers.

2. Outline two movement of water in the large water bodies.

3. Giving example from each, mention two types of ocean currents.

4. Describe major two relief features of the earth.

5. With the aid of sketch map of East Africa, indicate the largest lakes.

6. Give the meaning of the following terminologies:

a)    Water divide

b)    Catchment area

c)     Underground flow

d)   Drainage

e)    Ocean deep

7. Differentiate between the following:

a)    Fold and block mountain

b)    Tributaries and distributaries

c)     Continental shelf and deep sea plain

d)   Tides and ocean currents

8. Write short notes about:

a)    Tides

b)    Waves

c)     Volcanic Mountain

d)   Valleys

e)    Islands.