What are Volcanoes?
- A volcano is a vent or fissure in Earth’s crust through which lava, ash, rocks, and gases erupt.
- An active volcano is a volcano that has erupted in the recent past.
- The mantle contains a weaker zone known as the asthenosphere.
- Magma is the material present in the asthenosphere.
- Material that flows to or reaches the ground comprises lava flows, volcanic bombs, pyroclastic debris, dust, ash, and gases. The gases may be sulfur compounds, nitrogen compounds, and trace amounts of argon, hydrogen, and chlorine.
Magma vs Lava: The difference
- Magma is the term used to denote the molten rocks and related materials seen inside the earth. A weaker zone of the mantle called the asthenosphere usually is the source of magma.
- Once this magma came out to the earth’s surface through the vent of a volcano, it is called the Lava. Therefore, Lava is nothing but the magma on the earth’s surface.
- The process by which solid, liquid, and gaseous material escape from the earth’s interior to the surface of the earth is called Volcanism.
Types of Lava- Acidic and Basic
Basic Lava (Basaltic)
Acidic Lava (Granitic)
- Tephra- Ash dust+cinders thrown out by volcanic eruptions
- Fumaroles- Small vents from which steam and gas shoot out.
Types of Volcanoes
- Volcanoes are classified based on the nature of eruption and the form developed at the surface.
- The Shield volcanoes are the largest of all the volcanoes on the earth, which are not steep.
- These volcanoes are mostly made up of basalt.
- They become explosive if in some way water gets into the vent, otherwise, they are characterized by low-explosivity.
- The lava that is moving upwards does so in a fountain-form and emanates the cone at the vent’s top and then develops into a cinder cone.
- Eg: Hawaiian shield volcanoes
- Composite Volcanoes are characterized by outbreaks of cooler and more viscous lavas than basalt.
- They are constructed from numerous explosive eruptions.
- Large quantities of pyroclastic material and ashes find their way to the ground along with lava.
- This material gathers near the vent openings resulting in the creation of layers.
- Mayon Volcano in the Philippines, Mount Fuji in Japan, and Mount Rainier in Washington are the major composite volcanoes in the world.
- The major composite volcano chains are the Pacific Rim which is known as the “Rim of Fire”.
- Calderas are known as the most explosive volcanoes of Earth.
- They are generally explosive.
- When they erupt, they incline to collapse on themselves rather than constructing any structure.
- The collapsed depressions are known as calderas.
Flood Basalt Provinces
- Flood Basalt Province volcanoes discharge highly fluid lava that flows for long distances.
- Many parts of the world are covered by thick basalt lava flows.
Mid-Ocean Ridge Volcanoes
- These volcanoes are found in oceanic areas.
- There exists a system of mid-ocean ridges stretching for over 70000 km all through the ocean basins.
- The central region of this ridge gets frequent eruptions.
Structures made by Volcanoes
- The lava that is released during volcanic eruptions on cooling develops into igneous rocks.
- The cooling may take place either on reaching the surface or from the inside itself.
- Depending on the location of the cooling of lava, igneous rocks are classified as:
Volcanic Igneous rocks (Extrusive igneous rocks):
Cooling of the rock occurs at the surface of the earth. E.g. Basalt, Andesite etc.
Plutonic Igneous rocks (Intrusive igneous rocks):
Cooling takes place in the crust and not over the surface. E.g. Granite, Gabbro, Diorite etc. Intrusive igneous rocks are classed into the following types according to their forms.
Batholiths:A large body of magmatic material that cools in the deeper depth in the form of a large dome. These are granitic bodies. They sometimes appear on the earth’s surface when the denudation processes remove the overlying materials.
Laccoliths:Large dome shaped intrusive bodies with a level base and pipe-like conduit from below. Resembles a composite volcano structure, but beneath the earth. (Eg: Karnataka Plateau)
Lapoliths:They are saucer-shaped, concave to the sky.
Phacoliths:Wavy materials which have a definite conduit to source beneath.
Sheets/ sills:They are the near-horizontal bodies of intrusive igneous rocks. Thinner ones are called sheets and while thick horizontal deposits are called sills.
Dykes:When the lava comes out through cracks and fissures, they solidify almost perpendicular to the ground to form wall-like structures called dykes. (Eg: Deccan traps in Maharastra region).
Distribution of Volcanoes:
Most of the volcanoes in the world are found in three well-defined belts:
- The Circum-Pacific Belt (The Pacific Ring of Fire).
- The Mid-World Mountain Belt.
- The African Rift Valley Belt.
List of Important Volcanoes by descending Height
Ojos del Salado
Other Important Volcanoes
Mt. St. Helens
USA (Washington State)
Tanzania (Dormant or extinct)
Italy (Also called the ‘Lighthouse of Mediterranean’)
Volcanoes in India
Andaman and Nicobar (Only active volcano in S. Asia)
Andaman and Nicobar (Dormant)
- Old Faithful geyser- Yellowstone National Park (USA)