Magma once formed inside the earth tends to move upward. When intrudes into the crust, it forms intrusive igneous rocks.
When it extrudes ore or ejects to the surface of the earth it gives rise to volcanoes.
A valcano is a coni! hill with a nearly circular opening known as vent, through which hot gases, va molten liquid – the lava and rock fragments are ejected from beneath the earth, vent is connected to the magma chamber by a pipe or conduit, This type of val is called central type. In another case known as fissure eruption, the lava is eru through long fissures with absence of large amounts of gases and rock fragme Geysers, fumaroles and some hot springs are distinctly phenomena of volcani However, they are related to ground water only.
2.10.1. Description of a volcano: Typically a volcano is generally coni commonly with gentle slope in all directions. At the top of cone is a circular pit known as crator which varies in diameter from meters to even few kilometers. Crators are formed due to une accumulation of rock fragments and lava.
This cone and crator structure is typical of central type of valcanic eruption. In this case, there is a pipe or conduit through which eruptions take place. Lava moving upward, at times cuts through the country rocks forming dykes. When these dykes come to the surface they form secondary or parasitic cones and crators.
In some cases, sills are formed. In some valcanoes, large nearly circular pits are formed, either due to violent explosions blowing up at the summit, or due to subsidence along circular fractures. These pits are termed as calderas. The first one is called as explosion caldera and the second type is subsidence caldera.
Lava cones are conical structures, gently sloping, cover wide area, formed mainly by quiet type of volcanoes. Cinder cones are formed around the volcanic pipe by accumulation of loose fragmentary materials known as cinders. Composite or mixed cones are intermediate between cinder cones and lava cones; consist of layers of pyroclastic materials with intercalations of lava flows. They are also called as strato – volcanoes. Shield valcano resembles warrior’s shield, covers large areas with very gentle slopes.
In case of fissure eruptions, lavas are piled up flow after flow over extremely large areas resulting in enormous thickness giving rise to volcanic plateau. The Deccan plateau is an example of fissure eruption.
Fumaroles are fissures or vents through which gases come out. Hot springs are abundant in volcanic regions. Geysers are smimilar to hot springs through which hot gases and hot water are ejected intermittently in regular periodic intervals.
They are abundant in Iceland and Newzealand. Yellow stone National Park at California is one of the best examples. Meteor crators are features very similar to volcanic crators, but are formed due to impact of meteorites. They often form lakes, known as crator lakes. Mud volcanoes look like volcanoes, but are made up of mud and are found in oil-field regions. Some of the volcanic plateaus are so much flat that, they give rise to plains, known as lava plains. Similarly, ash showers form plains known as ash plains.
Hot gases, molten lava and rock fragments are ‘ through volcanoes.
Gaseous products always accompany volcanism. Waterv is the chief constituent, constitutes more than 90% of the volcanic When ejected, it forms clouds. Other gases are carbon dioxide, nit sulphur dioxide, hydrogen sulplide, hydrogen, sulphur, chlorine, am carbon monoxide, boron, fluorine, hydrochloric acid, phosphorous, a and many others.
The hot molten liquid ejected through a vole known as lava. Lavas vary greatly in chemical composition, which isn responsible for their viscosity. There are mainly two types of lavas- lava and basic lava. Acid lavas are rhyolitic or dacitic.
Basic lava is basalt. Trachytic and andesitic lavas are intermediate in composition, erupted through fissures are basaltic in nature. Lava on solidific produce different types of forms like blocks lava, ropy lava and pi lava. The surface of the lava flow in many instances contain num vesicles formed due to escape of volatiles forming vescicular struc Often, the vescicles are filled in by secondary minerals known amygdules also amygdales and the resulting structure is amygdaloidal.
Lava moving upward often explodes the solidified r of the crust. These are knwon as pyroclasts or pyroclastic materials, rock formed after solidification is known as pyroclasitc rock. Pyroclasts vary in shape and size. The larger angular fragments are kno as volcanic blocks while volcanic bombs are somewhat rounded.
Small fragments are called lapilli or cinders. Still smaller fragments are vole- sand, dust and ash respectively. Dust and ash on consolidation form are known as tuff. Some pyroclasts are spongy due to escape of gases and resultant mass becomes spongy. Extreme stage of vesciculation produ a rock termed as pumice that floats on water.
Classification of volcanoes:
Volcanoes have been classified into central type and fissure type according to the nature of pipe and volcanic erruptions.
Depending upon the activity, the volcanoes are classified as active, dormant and dead or extinct. Volcanoes which erupt very often like the Vesuvius are classed as active volcanoes. Dormant volcanoes erupt intermittently with lapse of considerable period of time between successive eruptions. Other volcanoes which have not shown any eruption within recorded period of history are said to be dead or extinct. An extinct volcano might still be in a dormant stage.
The volcanoes are also classified according to their nature of eruptions into: (i) explosive, (ii) intermediate (iii) quiet type. In the first case, gases escape with violent explosion with consequent blowing up of the upper part of the neck of the volcano with formation of huge quantities of pyroclasts. In the intermediate type, explosion takes place in the begining with the formation of pyroclasts and at later period lava is erupted quietly. In the last type, lava is erupted quietly without much explosion.
Volcanoes are also classified depending upon the nature of activity of particular type of eruption. They are:-
(i) Hawaiian type:
Typical of the volcanoes of the Hawaiian Islands, in which case, more fluid lava is ejected without much explosion and without ejection of gases or pyroclasts.
(ii) Strombolian type:
It is typical of the Stromboli volcano in the Mediterranean. Less fluid basic lava is ejected, at times with violent explosions. Pyroclasts are dust, bomb etc.
(iii) Vulcanian type:
Named after the Vulcano in the Mediterranean near Stromboli. Viscous lavas are ejected, form a crust over the volcano, so that, when next eruption takes place, the earlier formed crust is broken with violent explosion. Large amount of pyroclasts aret up from this type at volcano. Composition of lava in succ eruptions vary appreciably. Lava compostion varies from vole volcano of this type.