Geography [4 Mark Question Answer]

About six important Convection Current Cells with over-riding six large plates have been identified below the Lithosphere. The Mid-Oceanic ridges from edges of the plates. For example, the Mid-Atlantic ridge is such a ridge. The molten matter from below adds new crust along such’ ridges. Taking the earth as a whole it has resulted in the spreading of the ocean floor at the rate of 1 cm to about 10 cm every year. It is called the constructive plate movement. At the same time, the crust at the other edges of the plates in oceanic trenches gets destroyed. It is called the destructive plate movement Consequently a balance exists and the Earth as a whole does not expand.

The surface of the earth is undergoing constant change. Some of these changes take place all of a sudden as in the case of volcanic eruptions and earthquakes but most of them are gradual and slow. Due to these changes, the sediments which were originally deposited in horizontal layers are found tilted bent broken and twisted. In certain regions, the structures which were once at sea-level in the Baltic Sea are now well above the water. Recently it has been found that the larger part of the coast of Scandinavia is rising relative to sea-level but that of the Southern extremity is sinking. Along some coasts, submerged forests and various human structures show that the land has not only risen but at places, it has also submerged. All this has happened or is happening due to the Earth’s movements. Thus we find that the Earth’s movements have modified the Earth’s surface.

Magma and Lava :
Magma :

1. Magma is hot sticky molten material.
2. It contains solutions to water and gases.
3. It comes out during volcanic eruptions.

Lava :

1. Lava is solidified magma.
2. Gases and water disappear after evaporataion.
3. It cools down as it comes into contact with the atmosphere.

 Adverse effects of volcanoes

1. Most of the recently formed volcanic areas are barren and forbidding to man.
2. The sudden flow of basic lava from an eruptive volcano may cause the total destruction of human life property and crops in the neighbouring areas.
3. The Karakatoa volcanic eruption (in 1883) caused such high sea waves that Karakatoa and several other neighbouring islands were completely destroyed.
4. Volcanoes eruption of Vesuvius in 79 A.D. buried and destroyed completely the two Roman towns of Pompeii and Herculaneum.

Beneficial effects of Volcanoes :

1. The soil made up of lava is very fertile.
2. Lakes are formed when the craters are filled up with waters.
3. Several minerals which are found deep down come up near the surface of the earth.
4. Lava flows have preserved many fossils which throw much light on the past life.

Obviously volcanoes will be found in those regions where the crust of the earth is weak because lava can easily be discharged from such places. These are found in areas of Fold mountains. There are three main belts of volcanoes :

1. Circum-Pacific Belt: This belt runs round the Pacific Ocean in Asia and America. It starts from Cape Horn goes along the Andes and the Rockies to Alaska. From there it turns westwards and passing through Aleutian Islands Japan and Formosa goes to Philippine Island. Here one branch goes to East Indies that is Java Sumatra and Bameo and other branch goes to New-Zealand. Some of the well-known Volcanoes are Karakatoa (on a hilly island between Sumatra and Java) Mayon (N. Philippines) Fujiyama (Japan) Chimborezo and Cotopaxi (Equador S. America).
2. Mid World Mountain Belts: This belt starts from the West Indies and passing through the Canavy Islands. The Mediterranean sea Caucasus mountains and Turkey reaches the Himalayas. Mt. Vesuvius (Italy) Mt Stromboli (Sicily) Etna (the Mediterranean sea) and Mt. Pelee (West Indies) are the important Volcanoes of this belt.
3. African Belts: This belt follows the Great African Rift Valley. This belt running through the Red Sea extends up to Africa. Important Volcanoes are Kilimanjaro Kenya Canary Islands St. Helena (Atlantic ocean).

About six important Convection Current Cells with over-riding six large plates have been identified below the Lithosphere. The Mid-Oceanic ridges from edges of the plates. For example, the Mid-Atlantic ridge is such a ridge. The molten matter from below adds new crust along such’ ridges. Taking the earth as a whole it has resulted in the spreading of the ocean floor at the rate of 1 cm to about 10 cm every year. It is called the constructive plate movement. At the same time, the crust at the other edges of the plates in oceanic trenches gets destroyed. It is called the destructive plate movement Consequently a balance exists and the Earth as a whole does not expand.

In the beginning all the continents were combined together known as ‘Pangasa’ which later on splitted away and separated. But still all continental and oceanic plates are sliding upon each other and result in various earthquakes and volcanoes. The continental drifting theory was discovered by German scientist Alfred Wagner in 1912 which supported this plate tectonics to prove the movement of the continental and oceanic plates working for changing the landforms of the crust.

The effects of volcanoes on human life are both positive and negative :
Destructive influences: The deposition of lava makes the area very porous which creates water problems. The major flow of hot lava of Etna destroyed the whole area and property in Sicily. Mt. Vesuvius destroyed the city of Pompeii seven times since 79 AD. and the city of Herculaneum.
Constructive Influences: Volcanoes have provided some fertile lands e.g. Java and Deccan plateau and areas of South Brazil. Various precious minerals come out on the upper part of the earth’s crust through eruption e.g. Diamonds of Kimberley and gold of Johannesburg in South Africa nickel deposits of Sudbury in Canada.

Two main types of volcanic eruptions are :

• Explosive and
• Quiet or Hawaiian.

1. The Explosive eruption is the most violent and destructive Explosion of pent up gases mainly steam cause enormous quantities of magma to be thrown into the air to form great clouds. Example Krakatoa volcano in Sunda Strait in Indonesia erupted in 1883.
2. The Quiet eruption cause and other gases to escape. There is no violent explosion. Lava flows the creators and flows down the sides of the cone.
   Example: Hawaiian volcano and Mauna Loa.

Prominent landforms associated with volcanoes are composite cones built of multiple materials erupted out from a volcano. Conical hills are common in volcanic areas Crater lake is formed along with the mouth of a conical volcano which is filled with water after cooling down of volcano. Due to repeated eruptions mouth of a volcano is converted into a large depression called a caldera. Lavashields are made by large amount of basic lava flows making volcanic shields with a gentle slope.
Lava plateau: is formed by large scale fissure eruptions covering extensive area e.g Columbia plateau in South America and the Deccan plateau of India.

Destructive Effects :

1. When the volcanoes erupt they destroy life and property. The hot lava together with ash and dust destroys not only human beings but also animals as well as plant life.
2. Agricultural fields are covered with layers of volcanic ash and dust making them unsuitable for cultivation.
3. Huge clouds are formed after the eruption of volcanoes causing heavy rains which result in floods and landslides.
4. Volcanoes also emit poisonous gases which pollute the environment and cause health problems.
5. Explosive volcanoes in ocean islands are followed ‘by high Tsunami waves. They flood the area and destroy property people animals and crops.

The landforms created on the earth’s surface are called Extrusive Landforms. These include volcanic plateaus volcanic mountains and volcanic plains.
(a) Volcanic Plateaus: These are derived from lava which flows from volcanic eruptions. The Plateau of Peninsular India especially the northwestern Deccan is an example of lava plateau. Other examples are South African plateau Columbian Plateau and Ethiopian plateau.
(b) Volcanic Mountains: These mountains are built from material ejected from the fissures in the earth’s crust. Volcanic mountains are the most diverse because there are great differences in volcanic eruptions as well as the materials they throw up.
(c) Volcanic Plains: These plains are vast and smooth formed by extensive volcanic flooding from volcanic centres. The Western Victorian Plains in Victoria Australia are the finest examples of Volcanic Plains.