Volcanic eruptions are of two types: (i) centeral eruptions, (ii) Fissure eruptions.
Central Eruptions: In central eruptions, the emission of lava takes place through a vent. Such a volcano usually exhibits a cone-and-crater arrangement. This volcano may have parasitic cones on the flanks of the main structure. Etna, a European volcano, has more than 200 parasitic cones.
Fissure Eruptions: In fissure eruptions, basic lavas issue relatively quietly from linear fissures in the earth’s crust These lavas cover a very large area before solidifying, and considerable thickness of rocks is formed from repeated eruptions. This vast accumulation of solidified lava is called ”plateau basalts”. An example of fissure eruptins is the ”Deccan Traps”, which cover an area of about 1,000,000 square kilometers in central India. Ocean floor basalts are also the product of fissure eruption.
Although volcanic eruptions are sometimes violent, many are very quiet. The factors which determine the nature of eruptions are: (i) temperature of magma, (ii) composition of magma, and (iii) amount of dissolved gases. The first two factors, i.e. composition and temperature, mainly affect the magma’s viscosity. The viscosity of a magma is directly related to its silica content. The lower the percentage of silica in magma, the greater, the higher the fluidity. Thus basaltic lavas are very fluid.
The gases play a major role in causing volcanic eruptions. They provide the force, which drives magma out from the cone. Highly viscous magmas often plug the vent thereby trapping the gases below. This results in a buildup of pressure, which leads to explosive types of eruptions. Many gradations exist between these two extremes. Chief types of the volcanic eruptions are as follows. They are arranged in order of their increasing violence.
- Hawaiian Type: In the Hawaiian type eruptions, the mobile ava of basaltic composition is discharged relatively quietly. The lava flows out from lava pools occupying craters or from fissures.. Sometimes fountains of lava are formed due to the rapid emission of gases. This type of eruption is characterized by Hawaiian volcanoes particularly Mauna Loa and Kilauea.
- Strombolian Type: In the strombolian type eruptions, the viscous lavas erupt with moderate explosion Clots of incandescent lava are thrown out to form bombs or lumps of scoria. Stromboli volcano in Sicily exhibits such eruptions.
- Vulcanian Type: This type of eruption is more violent than the Strombolian types. The viscous lava solidifies quickly thereby causing severe explosions. This results in the ejection of solid rock fragments of all sizes including cinders, ash, and dust. Dark clouds of dust are usually seen.
- Vesuvian Type: In this case the lava highly charged with gases erupts with extreme violence. Large amount of gases and dust are thrown up into the air, which forms a cauliflower-like cloud at a great height.
- Plinian Type: It is the most violent type of the volcanic eruption in which a blast of uprushing gses rises to great heights.
- Pelean Types: In this type of eruption, the most viscus ava is thrown out. The blocked vent is cleared due to t violent explosions The hot gases and volcanic ash spread very widely through the air.