Volcanoes (Explained in Detail) – UPSC Notes/PDF

A volcano is a place where gases, ashes and molten rock material (lava) escape to the ground.

  • A volcano is called an active volcano i the materials mentioned above are being released or have been released out in the recent past.
  • The asthenosphere is the main source of this material. The asthenosphere contains the molten rock, which is called magma. When this magma comes out on the surface of the earth, it is called lava.
  • The material that reaches the ground includes ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon.

Types of Volcanoes

Volcanoes are classified based on nature of eruption and the form developed at the surface Major types of volcanoes are as follows:

Shield Volcanoes

Shied Volcano

The shield volcanoes are the largest of all volcanoes. These volcanoes are mostly made up of basalt (a type of lava that is very fluid when erupted). That is why these volcanoes are not steep. They become explosive only when water gets into the vent, otherwise, they are characterised by low-explosivity.

The examples of shield volcanoes are Mauna Loa, Kilauea (Hawaii), Mouna Keo etc.

Composite Volcanoes

Composite Volcano’s Structure

The materials that come out of these types of volcanoes are cooler than the basalt. These volcanoes often result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground.

The examples of these volcanoes are Mount Saint Helens, Mount Fuji, Mount Pelee etc.

Eruption of Mount Saint Helens


These are the most explosive of the earth’s volcanoes. They are usually so explosive that whey they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. The examples of these type of mountains are Kilauea (Hawaii), Mount Loa, Yellowstone Caldera etc.

Intrusive Volcanic Landforms

The lava that releases during volcanic eruptions, on cooling develops into igneous rocks. The cooling of magma can either take place outside or inside of the earth. When the magma cools inside the earth, it assumes different forms. These forms are called ‘intrusive form’.

Following are the types of intrusive forms:-


Source: Wikipedia

A large body of magmatic material that cools in the deeper depth of the crust develops in the form of large domes. They appear on the surface only after the denudation process removes the overlying material. They cover a large area. Batholiths are the cool portion of the magma chamber.

Some examples of Batholith are: Aswan Granite batholith in Egypt, the Cape Coast batholith in Ghana, Darling batholith in South Africa.


These are large dome-shaped intrusive bodies with a level base and connected by a pipe-like conduit from below. It can be regarded as the localised source of lava that finds its way to the surface. The Karnataka the plateau is spotted with domed hills of granite rocks.


A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body.


A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of the anticline in the folded igneous country. Such wavy materials have a definite conduit to source beneath in the form of magma chambers (subsequently developed as batholiths). These are called the Phacoliths.


The near horizontal bodies of the intrusive igneous rocks are called sill or sheet, depending on the thickness of the material. The thinner ones are called sheets while the thick horizontal deposits are called sills.


When lava makes its way through cracks and the fissures developed in the land, it solidifies almost perpendicular to the ground. It gets cooled in the same position to develop a wall-like structure. Such structures are called dykes. These are the most commonly found intrusive forms in the western Maharashtra area. These are considered the feeders for the eruptions that led to the development of the Deccan traps.

Related Post

Formation of Volcanoes (Introduction)