Volcanoes are constructive and Destructive.
Constructive effects:
(i) Volcanoes provide nutrients to the surrounding soil as volcanic ash contains minerals beneficial to plants.
(ii) Some plateaus and plains are formed due to volcanic eruption.
(iii) They cool off the earth removing heat from the interior.
Destructive effects:
(i) They cause earth quakes, fast floods, mud slides and rock falls.
(ii) Dust and Ash that come out of volcanoes are very harmful to living things

(i) The inner most layer of the earth, called the core has two parts, the outer core and the inner core.
(ii) The outer core is rich in iron and is in liquid state.
(iii) The inner core is made of Nickel and Ferrous which is in solid state.
(iv) The central core has very high temperature and pressure.

There are three major zones of volcanic activities in the world. They are:
(i) The Circum - Pacific belt
(ii) The Mid continental belt
(iii) The Mid Atlantic belt
Circum Belt :
This is the volcanic zone of the convergent oceanic plate boundary. It includes the volcanoes of the eastern and western coastal areas of Pacific Ocean. This zone is popularly termed as the Pacific Ring of Fire which has been estimated to include two-thirds of the world's volcanoes.     

(i) Volcanoes are also grouped according to-their periodicity of eruptions such as active, dormant and extinct.
(ii) These names refer to the state of activity rather than the types of volcanoes
(iii) Active Valcanoes that erupt frequently are called active volcanoes. Most of the active volcanoes lie in the Pacific Ring of Fire belt which lies along the Pacific coast.
(iv) Mt. Stromboli in Mediterranean Sea, St.Helens in USA, Pinatubo in Philippines.
(v) Dormant volcanoes have shown no sign of activity for many years but they may become active at any time. It is also called Sleeping Volcano
(vi) Vesuvius mountain of Italy, Mt Fujiyama in Japan, Mt. Krakatoa of Indonesia are famous examples of this types.
(vii) Extinct The top of extinct volcanic mountains have been eroded. Mt Popa of Myanmar and Mt. Kilimanjaro and Mt. Kenya of Africa are examples of extinct volcanoes.

(i) Earthquakes may cause changes in the earth's surface. Vibrations often set landslides in mountainous regions.
(ii) A greater danger in an earthquake is the falling of buildings.
(iii) Fire is another great danger. Underground water system is naturally disturbed by such movements.
(iv) An earthquake which originates below or near the sea causes great disturbance in the water.
(v) Tsunami, a Japanese term, is the name given to the huge wave caused in the sea by an earthquake