Volcanism Types Based on Outflow of Lava

On the basis of outflow of lava, volcanism can be classified into four types- Exhalative, Effusive, Explosive and subaqueous Volcanism.

Volcanism Types Based on Outflow of Lava

Exhaltive volcanism - vapour or fumes

  • Exhalative volcanism is characterized by the discharge of materials in the gaseous form. It includes gases like steam, Sulphur Dioxide, carbon dioxide, carbon monoxide, hydrogen, nitrogen, hydrogen sulphide, hydrochloric acid etc.
  • These gases can escape to the earth's atmosphere from the vents which can be in the form of hot Spring, geyser, solfataras, and fumaroles etc. The landforms associated with exhaustive volcanism are sinter mounds, mud volcanoes and the cones of the of the precipitated minerals etc.




Effusive Volcanism - Lava outpouring


  • When the outflowing lava has low viscosity and high fluidity, various gases coming out of the volcano can escape easily. The erupting magma forms lava flows, and these eruptions are known as effusive volcanic eruptions.
  • Erupting Lava is poor in silica content such as basalt, and they flow through very large distances. The Deccan traps are formed by the effusive outpouring of Lava. Many parts of the Deccan trap has developed into the finely grained basalt plateau.


Lava outpouring


Explosive volcanism - violent ejection of the solid material


  • If the Magma has a large amount of gases which get trapped inside it, the pressure increases and builds until the magma erupts explosively out of the vent. The explosive eruptions can produce pyroclastic flows sweeping down the valleys, which can destroy everything which comes under their path.
  • Explosive volcanism leads to the fragmentation and ejection of the solid materials through the volcanoes. The various materials coming out of the vents of an explosive volcanism are
    1. Tephra, which includes all the fragmented rejects coming out of the volcanoes.
    2. Volcanic ashes, which are the finest and sand-sized tephra.
    3. The gravel-sized particles which are in the molten or in the solid state are known as Lapilli.
    4. The Boulder-sized solid ejects are known as blocks.
    5. When the lumps of Lava are thrown out of the volcano, they are known as volcanic bombs.
    6. The layers of the volcanic dust and ashes are known as tuff.
  • The lighter and smaller particles like the lapilli and ash can travel for large distances and can remain suspended in the atmosphere for longer periods of time. Volcanic bombs and blocks, which are heavier in weight fall near to the vents of the volcanoes.


Explosive eruption



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Subaqueous volcanism

  • Subaqueous volcanism takes place in the underwater oceans and seas, and other water bodies. In the underwater ocean floors, the outflowing lava comes in the contact of water and consolidates to produce structures, which are like the heap of pillows. These types of structures belonging to the Precambrian age have been found in the Karnataka region.
  • When the outflowing lava is highly viscous and erupts at lesser depths, they develop a structure of glassy margins on the pillows leading to the formation of hyaloclastite. Hyaloclastites are most commonly found in Iceland.


pillow lava


Negative effects of Volcanism

  • Volcanic eruptions release gases like Sulphur dioxide, carbon dioxide, carbon monoxide etc in the atmosphere. These gases are hazardous for the people, agriculture, and environment.
  • The sulphur dioxide released from volcanic eruptions can cause acid rain which is harmful to humans as well as biodiversity and environment. The release of Sulphur aerosols in the stratosphere can lower the surface temperature leading to the depletion of Ozone layer.




Classification of volcanism based on the mode of eruptions


  • Hawaiian eruption: in the Hawaiian eruption, the basalt lava comes out in the form of effusive outpouring from the craters or fissures. In a single flow, the lava spreads over wide areas, or it flows down the valley is like lava rivers. The great basalt plateaus of Colombia is its example.
  • Strombolian eruption: in the Strombolian eruption, more viscous lava has erupted like a fountain at the regular intervals of about 15 minutes. The Stromboli volcano is located on the Lipari island near Italy.
  • Vulcanian Eruption:this volcanic eruption is associated with a short, violent and a relatively smaller eruption of viscous lava. During volcanic eruptions, volcanic bombs, blocks and ashes etc are ejected in the surrounding areas. The volcano becomes dormant for several decades and even for centuries after each eruption cycle.
  • Pelean eruption: it occurs as a result of the flow of very viscous, gas-rich and acidic lava erupting violently over the crater rim. The lava and gases do not move upwards toward the sky but spread downslopes as a nuee ardente.
  • Icelandic type eruption: it is characterized by the outflow of molten basaltic lava flowing from the long and parallel fissures and often leading to the formation of Lava plateaus.


Counterclockwise: a Plinian eruption column, Hawaiian pahoehoe flows, and a lava arc from a Strombolian eruption.

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