What is Volcano? or Definition of volcanoes
Volcanoes (بركان) are conical or dome-shaped structures built by the emission of lava and it contained gases from a restricted vent in the earth’s surface. The volcanoes are having truncated tops representing the crater, that acts as the avenues for the magma to rise
Volcanoes take many forms, and the activity that is associated with their eruption is highly varied. The activity of volcanoes differs in the amount and type of material ejected. The size, temperature and composition of the material ejected determine the shape of the volcano or the form of extrusion volcanoes also differ in the violence and the timing of successive eruptions
Types of Volcanoes
- A volcano is ‘Active’ when it is erupting intermittently or continuously. A volcano which has not erupted for a long time is known as ‘Dormant’, whereas an ‘Extinct’ volcano is one, which has stopped eruption over a long time.
- On the basis of mode of eruption as well as on the basis of nature of eruption, different types of volcanoes have been recognized.
Basing on the mode of eruption volcanoes are classified as:
- Central types: Where the products escape through a single pipe (or vent).
- Fissure types: Where the ejection of lava takes place from a long fissure or a group of parallel or closed fissures.
Based on the nature of eruption,
- Explosive Type:- In which case the lava is of acidic (felsic) in nature and because of their high degree of viscosity, they produce explosive eruptions.
- Quiet types: In this case the lava is of basaltic compostion (mafic lava), which is highly fluid and holds little gas, with the result that the eruptions are quiet and the lava can travel long distances to spread out in this layers.
Besides the above, a number of other types of volcanoes have been identified according to their degree of explosive activity and nature of eruption. They are as follows
- Hawaiian type:- Silent effusion of lava without any explosive activity.
- Strombolian type: periodic eruption, with a little explosive activity
- Vulcanian type: Eruption takes place at longer intervals and the viscous lava quickly solidifies and gives rise to explosions of volcanic ash.
- Vesuvian type: Highly explosive volcanic activity and eruptions occur after a long interval (measured in tens of years).
- Plinian type: – The most violent type of Vesuvian eruption is sometimes described as Plinian. Here huge quantities of fragmental products are given out with little or no discharge of lava
- Palean type: This is the most violent type of all the eruption. They are characterized by eruption of ‘nuees ardentes’
It includes both positive as well as negative relief features. The High or elevated relief features comprising of hills, mountains, cones, plateaus or upland plains are some of the examples of positive relief feature, while the low lying relief features like craters, calderas, tectonic depression etc. represent the negative relief features.
- Positive relief features: – These features are formed due to both quiet as well as explosive volcanic activity, and some of which are as follows:
- Hornitos: These are very small lava flows
- Driblet Cones: The most acid lava often gives rise to quite small conelets and are known as driblet cones.
- Cinder cones: – These are volcanoes of central type of eruption, steep-sided with uniform slopes of 30 to 40 degrees.
- Lava cone. These are built up of lava flows, due to heaping of lava during quiet type of eruption. It is also known as lava or ‘Plug-dome’.
- Composite cone:- These are made up alternatively of pyroclastic material and lava. Due to rude stratification, they are also known as ‘Strato-volcanoes’.
- Shield-volcanoes:- These are made up of lava alone and due to quiet type of eruptions, whereby piling up of flow after flow of fluid lava, a rounded dome like mass is produced
- Spatter-cone. Small cones formed on lava flows where breaks occur in the cooled surface of the flow allowing hot gases and lava to be blown out.
Volcanic plateau:- These are formed because of fissure type of eruption.
- Negative relief features:
- Crater:- This is a depression located at the summit of the volcanic cone.
- Calders:- Sometimes because of violent volcanic explosion the entire central portion of the volcano is destroyed and only a great central depression, named a ‘caldera’ remains. The calderas may also be formed due to erosion and enlargement of the crater.
- Lava-tunnels: the more mobile lavas of basic composition, when erupted onteh surface in the form of flows quickly consolidate and form a solid crust while the interior may still remain fluid. Under such conditions the enclosed fluid lava drain out through some weak spots lying at the periphery of the flow, forming what is known as ‘lava tunnel’.
- Cone in cone topography:- After an explosion destroys an existing crater, a new built smaller cone with its own crater is built up. This is known as cone in cone topography
Explosion-pits are also negative relief features of volcanoes
A volcanic eruption comprises solid, liquid and gaseous materials. Fragments of rocks ejected during an explosive eruption are called ‘pyroclastic materials’. The pyroclastic materials of various size grades are known differently as follows
- Volcanic blocks (or bombs):- Diameter of the fragments is always above 32 milimeters
- Cinder or lapilli:- Here the diameter is between 4mm to 32 mm
- Ash:- These particles range in size between 0.25mm to 4 mm
- Fine-ash: Minute particles of diameter less than 0.25 mm constitute the fine ash.
- Tuff:- Rocks made up of ash and fine ash are known as tuffs and the welded ruffs are known as ‘Ignimbrite’.
- Agglomerates: Pyroclastic rocks consisting mainly of fragments larger than 20 mm in diameter, are known as agglomerates.
Other products of Volcanism
- Porus or spongy: Masses of solidified frothy lava is known as Pumice or Scoria. But scoria is having a little dark colour and course texture than that of pumice.
- Spilitic lava: These are albitic (soda-rich) lava and it produce pillow structure.
- Gaseous product:- Steam forms the most important constituent of volcanic gases and contributes nearly 90% of the total content of volcanic gases. Gases like carbon-dioxide, nitrogen Sulphur-dioxide, hydrogen-sulphite, boric acid vapours, phosphorous arsenic-vapour etc. forms a part of the volcanic gases.
- Nuee ardente:- An incandescent cloud of gas and volcanic ash, violently emitted during the eruption of pelean types of volcanoes.
Features Associated with the Decaying phases of Volcanism
- Fumaroles: – These are fissures or vents through which volcanic gases are ejected
Fumaroles emitting sulfurous vapor are called ‘Salfataras’ and those which emit carbon-dioxide and boric-acid vapor are known as Mofettes and Safoni respectively
- Hot Springs: These are fissures through which host water escapes. The water usually gets heated with increased temperature below, may be magnetic or radioactive heat.
Calcareous deposits formed from hot springs are known as Travertine tufa and similarly, siliceous deposits are called Siliceous sinters
- Geyser: – Hot springs ejecting boiling water and steam at regular intervals are geysers. Siliceous deposits formed around geysers are known as ‘geyserite’
What are Pseudo Volcanic features?
Pseudo-volcanic features:- Mud-volcanoes, meteor-craters are of non-volcanic origin and are exampes of pseudo-volcanic features.
Cause of Volcanism
- The release of high pressures, which build up within magma chambers below the ground surface.
- Accumulation of radio-active heat produces magma; of course, other factors like frictional heat and the increase of heat with depth causes the formation of magma. Their eruption on the earth’s surface causes volcanism.
Distribution of Volano
- They are concentrated in a narrow-belt called Circumpacific Ring of Fire. Where the volcanoes are located on the high young folded mountains.
Other volcanic areas include the scattered areas in the Pacific, particularly the Hawaiian Islands, a belt that includes Arabia, Madagascar and the rift valley of Africa, the Mediterranean belt, the volcanoes of West-indies and those of Iceland.