A volcano is a vent, or opening, usually circular in form, through which gases, melted lava and fragments of rocks are ejected from the highly heated interior to the surface of the earth.
Crater: The narrow pipe, through which the magma comes out on the surface, is called Volcanic Pipe. The funnel or cup-shaped opening, at the top of the volcanic cone, is called as the Crater.
Crater Lake: When crater gets filled up with the rain-water, it forms as crater lakes for example Lonar lake in Maharashtra.
Caldera is formed due to subduction of the crater or expansion of the volcanic mouth due to successive eruptions. The largest ealdera in world is 'Aaso', which is in Japan. Lake Towa is in Indonesia which is known as super caldera.
Geyser: It is a special type of hot spring which sprouts hot water and vapour from time to time. The best examples are the Old Faithful and Exelsiar at the Yellow Stone National Park, in USA.
Fumaroles: Fumarole means such a vent throught which there is emission of gases and water vapour. Fumaroles impregnated with sulphur is called Solftara. Numerous fumaroles are found in the 'Katmai' mountains of the USA known as 'A Valley of Ten thousand Smokes' and the White Island fumarole in the Bay of Plenty, New Zealand, are also very famous.
Deccan Trapp: The basaltic lava, which comes out of the fissure voleanoes, flows slowly and gets solidified in the form of a thick shield. These layers or shields are called lava plateaux or Trapp. The best example of Trapp is the Deccan Trapp of India.
Classification on the Basis of Periodicity of Eruption :
- Active Volcanoes : These volcanoes constantly eject volcanic lavas, gases, ashes and fragmental materials. Etna and Stromboli of the Mediterranean Sea are the most significant examples of this category. Stromboli is know as the Light house of the Mediterrancean. Other volcanous of this category are - Cotopaxi of Equador (World's highers active volcano), Mt. Erebus of Antarctica (the only active volcano of the continent) and the Barren Island of the Andaman and Nicobar Islands, India.
- Dormant Volcanoes : These volcanoes become quiet after eruptions for some time and there are no indications for future eruptions but suddenly they erupt very violently. Important examples of this category are Vesuvius in Italy; Fujiyama in Japan; Krakatao in Indonesia, and the Narcondam Island of the Andaman & Nicobar.
- Dead or Extinct Volcanoes : These are those volcanoes which have not erupted since thousands of years and there is no indication of future eruption as well. Leading examples of this category are - Mt Kenya and Klimanjor, in Eastern part of Africa; Chimborajo in Equador; Popa in Myanmar; and Aconcagua in Andes mountains.
Two-third of the world's volcanoes are found in the coastal regions of the Pacific ocean, archipelagoes and oceanic island. Hence, the name given to this belt is the Fire Girdle of the Pacific or the Fire Ring of the Pacific.