The nature of a volcanic eruption is determined largely by the type of materials Ejected from the volcanic vent. According to the phases of the activity that is whether it is Mostly explosive and effusive or mixed. A volcano emits different proportions of volcanic Ejects.
Volcanologists have characterized the phase of activity of the volcanoes, as
5) Plinian and
as exemplified by the prevailing most characteristic habits of certain well known modem volcanoes. The explosive ness of a magma depends upon its viscosity and gas content. If it contains no Gas it has almost no explosive power. Viscosity is influenced by the chemical composition and It is diminished by increasing temperature and to some extent by the gas content. As the Temperature and gas content of a magma may change, the explosive nature of the magma Also may change so that the same volcano, at different times, may go through different Phases of explosive activity. Andesitic and rhyolitic lavas are more viscous than those of Basalts and therefore are often associated with explosive types.
Hawaiian Type: This is the mildest phase of activity exemplified by many volcanoes in the Hawaiian Islands and hence the name Hawaiian type. They discharge very quiet flows of Thinly liquid lava not accompanied by the explosive escape of gases. The lava emitted is very Hot and highly fluidal and thin. Gas is given off but rather quietly jets and fountains of Incandescent spray may be thrown up by the rapid emission of spurting gases. When Caught by a strong wind the bulbs of molten lava are drawn out into long glassy threads Known as Pele’s hair named after the Hawaiian goddess of fine Pele. Mouna Los is the giant Among the Hawaiian volcanoes with vast outpourings of basaltic lavas.
Strombolian Type: When less mobile lava is exposed to air in a crater the gases escape More sporadically with moderate explosive which may be rhythmic or almost continuous Clots of lava often incandescent are blown out to form bombs or lumps of scoria, while in Phases of more intense activity the explosions may give rise to luminous clouds. The Volcano of Stromboli is in the Mediterranean Sea just north of Sicily in the Lipari Islands, It has been continuously active since the drawn of history, its eruptions being as a rule very Much rhythmic consisting of mild explosions. A Volcano is said to be in the Strombolian Phase when it ejects clots of incandescent lava and the magma in its crater and docs not crust Over between consecutive explosions.
Vulcanian Type: This is the phase of activity exemplified by volcano in Liparl Islands, Its Lava is more viscous and pasty and quickly crusts over solidly between successive eruptions. Gases accumulate and gather sufficient strength beneath the congealed cover and below off At longer intervals shattering the crust formed after an earlier eruption, hurling angular Fragments of rocks with great explosive violence. The resulting volcanic clouds are dark and Dense and characteristically assume convoluted or “Cauliflower” shape as the gases and Dust ascend high and expand. They are not incandescent even in nights. The major eruptions Of several volcanoes begin with a vulcanian phase. Vulcanian is the term often used for Eruptions in which only cold material is blown out. Activity of this type is mostly likely to Occur during the cleaning out of a volcanic vent after the volcano has been dormant for quite sometime.
Vesuvian Type: This is the paroxymal extension of strombolian and vulcanian types, the New and characteristic feature being the extremely violent explosition of AA which has Become highly changed with gases during a long interval of mild activity. Inconsequence of The preliminary removal the contents of volcanic pipe, down to a considerable depth, Often as a result of escape of the lava through lateral fissures and vents, the overhead Pressure on the underlying magma is relieved. The magma then bursts into explosive forth And expels itself as vast luminous and incandescent clouds of cauliflower form. These Clouds ascend to very great heights and from them showers of ashes are widely distributed.
Plinian Type: The most violent e of eruptions sometimes culminate in a Stupendous blast of up rushing gas which rises to a height of several miles and then spreads Out into an expanding cloud of globular masses of gas and vapor. This phase was first Observed in Pliny during the catastrophic eruption of vesuvius.
Pelean Type: The pelean phase is the most violently explosive of all lt’s eruption clouds are Denser than even those of Vulcanian. In this type the limit high viscosity and explosiveness Is reached. Upward escape is prevented by the growth of an abstructive dome above the Conduit. Intermittent spurts of tightly compressed magma force their passage through the Lateral cracks and each of these sweeps down the slopes as in intensively hot avalanche of Self-explosive fragments of lava lubricated by constantly expanding gases and vapors. These downward rolling explosive blasts, one of which nearly wiped out St. Pierre in 1902, Are generally known as Nue’es ardents. But as a matter of fact, they do not glow as the French term suggests but are ink black in color and hence it is best to describe them as Pelean clouds.
The extraordinary feature of a pelean clouds are that is ejected horizontally from Beneath the lava plug in the summit of the volcano instead of blasting the plug out vertically Upward. It is extremely hot and it travels at great speed carrying-with it enormous quantity Indies.
Submarine volcanoes: violent submarine volcanic eruptions have been observed in the Volcanic belt of Aleutian Islands and in the south central Pacific Ocean area. Cones may be built few hundred meters above sea level but when volcanic activity declined the small Volcanic Islands are soon destroyed by the waves. Panteller a small Island near Sicily, is such A submarine volcanic island.
Mud Volcanoes: Gases issuing out at the surface may carry with them particles of sand and Clay which are deposited at the vent and as the process continues a small cone is built up Water present, the sand and clay form mud which dries up and hardens the surface of the Mount. Gases accumulate below the hardened surface until the pressure is sufficient to blow Off the top of the cone in the miniature scale of a volcano. Therefore they are called mud Volcanoes and are found mostly in areas where petroleum and natural gas bearing strata Occur below, as is the case in the bank region of Russia. They are also found in the regions Of volcanic origin in Burma.
Features Associated with decadent Volcanism: Many volcanoes give off steam and other Gases during the dormant intervals between eruptions. Even long after the volcanoes have Ceased to be active, these emanations may continue to issue from the crators or their flanks. Also large bodies of magma that have ascended to the higher levels of the crust without Reaching the surface on solidifying give oft their dissolved gases which escape through Fissures to the surface. These emanations issue at the surface either as gases when they are Called fumaroles or as hot waters when they are called hot spring or geysers.
Causes of Volcanism:
- Release of high-pressures, which build-up within magma chambers below the ground
- Accumulation of radio-active heat produces magma. Of course other factors like
Frictions heat and the increase of heat with depth cause the formation of magma and
Their eruption on to the earth’s surface causes volcanism.