Table of Contents
- Volcanic landforms are divided into extrusive and intrusive landforms based on weather magma cools within the crust or above the crust.
- Rocks formed by cooling of magma within the crust are called ‘Plutonic rocks’.
- Rocks formed by cooling of lava above the surface are called ‘Igneous rocks’.
- In general, the term ‘Igneous rocks’ is used to refer all rocks of volcanic origin.
Extrusive Volcanic Landforms
- Extrusive landforms are formed from material thrown out during volcanic activity.
- The materials thrown out during volcanic activity includes lava flows, pyroclastic debris, volcanic bombs, ash and dust and gases such as nitrogen compounds, sulphur compounds and minor amounts of chlorine, hydrogen and argon.
Conical Vent and Fissure Vent
- A conical vent is a narrow cylindrical vent through which magma flows out violently. Conical vents are common in andesitic (composite or stratovolcano) volcanism.
- A fissure vent, also known as a volcanic fissure or eruption fissure, is a narrow, linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few meters wide and may be many kilometers long. Fissure vents are common in basaltic volcanism.
- These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins. The central portion of this ridge experiences frequent eruptions.
- The lava is basaltic in nature (Less silica and hence less viscous).
- Cools slowly and flows through longer distances.
- The lava here is responsible for see floor spreading.
Composite Type Volcanic Landforms
- They are conical or central type volcanic landforms.
- Along with andesitic lava, large quantities of pyroclastic material and ashes find their way to the ground.
- Andesitic lava along with pyroclastic material accumulates in the vicinity of the vent openings leading to formation of layers, and this makes the mounts appear as composite volcanoes.
- The highest and most common volcanoes have composite cones.
- They are often called strato – volcanoes.
- Stromboli ‘Lighthouse of the Mediterranean’, Mt. Vesuvius, Mt. Fuji etc. are examples.
Shield Type Volcanic Landforms
- The Hawaiian volcanoes are the most famous examples.
- These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted.
- These volcanoes are not steep.
- They become explosive if somehow water gets into the vent; otherwise, they are less explosive.
- Example: Mauna Loa (Hawaii).
Fissure Type Flood Basalt Landforms [Lava Plateaus]
- Sometimes, a very thin magma escapes through cracks and fissures in the earth’s surface and flows after intervals for a long time, spreading over a vast area, finally producing a layered, undulating (wave like), flat surface.
- Example: Deccan traps (peninsular India), Snake Basin, U.S.A, Icelandic Shield, Canadian Shield etc..
- After the eruption of magma has ceased, the crater frequently turns into a lake at a later time. This lake is called a ‘caldera’.
- E.g. Lake Toba, Indonesia, the largest volcanic crater lake in the world. Crater Lake in Oregon, USA.
- Lonar in Maharashtra is an example of meteor crater lake and not volcanic caldera.
- A cinder cone is a steep conical hill of loose pyroclastic fragments, such as either volcanic clinkers, cinders, volcanic ash, or scoria that has been built around a volcanic vent.
Intrusive Volcanic Landforms
- Intrusive landforms are formed when magma cools within the crust [Plutonic rocks (intrusive igneous rock)].
- The intrusive activity of volcanoes gives rise to various forms.
- These are large rock masses formed due to cooling down and solidification of hot magma inside the earth.
- They appear on the surface only after the denudation processes remove the overlying materials.
- Batholiths form the core of huge mountains and may be exposed on surface after erosion.
- These are granitic
- These are large dome-shaped intrusive bodies connected by a pipe-like conduit from below.
- These are basically intrusive counterparts of an exposed domelike batholith.
- The Karnataka plateau is spotted with dome hills of granite rocks. Most of these, now exfoliated, are examples of laccoliths or batholiths.
- As and when the lava moves upwards, a portion of the same may tend to move in a horizontal direction wherever it finds a weak plane. It may get rested in different forms. In case it develops into a saucer shape, concave to the sky body, it is called Lapolith.
- A wavy mass of intrusive rocks, at times, is found at the base of synclines or at the top of anticline in 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.
- These are solidified horizontal lava layers inside the earth.
- 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 the 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.