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Volcanoes

(i) A sudden & abrupt explosion in earth crust through which Magma, gases, dust, smoke & solid material burst out.
(ii) Volcanic activity is connected with crustal disturbances, closely related with regions that have been intensely folded or faulted.
(iii) Magma while thrusting its way upto the surface may cool & solidify within crust as Plutonic rocks resulting in intrusive landforms.
(iv) Magmas that reach the surface & solidify, form extrusive landforms.

Sills & Dikes (Common Intrusive Landforms)

(i) When an intrusion of molten magma is made horizontally along the bedding planes of sedimentary rocks, the resultant intrusion is called a Sill.
(ii) Similar intrusion when injected vertically as narrow walls of igneous rocks within the sedimentary layers are termed as Dikes.

 Laccoliths An igneous mound with a dome shaped upper surface & a level base, fed by a pipe like conduit from below
 Lopolith An igneous intrusion with a saucer shape
 Phacolith A lens shaped mass o f igneous rock occupying the crest of an anticline or the bottom of a syncline & being fed by a conduit from beneath
 Batholith A large emplacement o f igneous intrusive rock, mainly granite, that forms from cooled magma deep in the Earth’s crust.
Extrusive Landforms
(i) Lava or molten magma ejects at a very high pressure through a pipe known as Volcano’s neck or vent.
(ii) Top portion of volcano is known as crater and a crater lake is formed when rain water gets accumulated in.
(iii) Some volcanoes may have greatly enlarged depressions like cauldron known as Calderas.
(iv) Volcanic dust or ash (finer particles) that emerges out of volcano travels round the world & falls as black snow, which can bury house & people.
(v) The coarser fragmental rocks are collectively called as Pyroclasts which include cinders, pumice & volcanic bombs.
Types of Volcanoes
1. Active Volcano
(i) Keeps on ejecting volcanic material at frequent intervals.
(ii) Ex - Etna (Italy), Stromboli (Sicily - largest island in the Mediterranean Sea, near Italy)
(iii) Mt Stromboli → Lighthouse of the Mediterranean
2. Dormant Volcano
(i) One in which eruption has not occurred for a long time but can occur any time in future
(ii) Barren Island (Andaman), Versuris (Italy)
3. Extinct Volcano
(i) No eruption has occurred in historic times & possibility of future eruption is also remote
(ii) Mt. Popa (Myanmar). But we can never be thoroughly sure about them.
(iii) Vesuvius (Bay of Naples near Italy) & Mt. Krakatau (Sunda straits b/w Java & Sumatra) were thought to be extinct & yet both erupted violently.
Some Volcanic Eruptions:
(i) Mt. Vesuvius
(ii) Mt. Krakatau-Krakatau is a small island in the sunda straits midway between Java and Sumatra
(iii) Mt.Pelee-Westindies
Distribution of Volcanoes in the World
(i) There are mainly three volcanic belts, besides many volcanoes which are outside these belts.
(a) Circum-Pacific belt - known as Ring of fire & houses around 2/3rd of world’s Volcanoes
(b) Mid-Continental belt - This belt has various volcanoes of the Alpine mountain chain, Mediterranean Sea (Stromboli, Vesuvius, Etna etc.).
(c) Mid-Atlantic belt - This belt includes the volcanoes of the Mid-Atlantic Ridge.
Geysers
(i) A spring characterized by intermittent discharge of water ejected turbulently and accompanied by steam.
(ii) The phenomenon is associated with a volcanic region in which the water below is being heated beyond its boiling point
(iii) Is often triggered off by the gases seeping out of the heated rocks.
(iv) Examples include Iceland; New Zealand & Yellowstone park of USA
(v) The world’s best known geyser is perhaps old faithful in Yellowstone National park, Wyoming
Hot Springs
(i) A spring that is produced by the emergence of geothermal heated groundwater from the Earth crust.
(ii) Is more common than geysers.
(iii) The water rises to surface without any explosion
(iv) Such springs contain dissolved minerals which may be of medical value
(v) Examples Include Iceland, Japan & Hawaii.



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