Volcanoes and Volcanic Landforms

What a Volcano Is

A volcano is a place where gases, ashes and molten rock material — lava — escape to the ground. You have likely seen photographs of volcanoes; the dramatic cone shape is familiar, but the definition is broader than that. Any opening through which these materials reach the surface counts as a volcano.

A volcano is called active if the materials are being released or have been released in the recent past. The word "recent" is deliberately loose — geologically speaking, it can span human history.

The source of the molten material lies below the solid crust. The mantle, which sits beneath the crust, has a higher density than the crust. Within the mantle is a weaker zone called the asthenosphere, and it is from this zone that molten rock materials find their way to the surface. While the material is still in the upper mantle, it is called magma. Once it starts moving towards the crust or reaches the surface, it is referred to as lava.

The material that reaches the ground includes:

• lava flows
• pyroclastic debris
• volcanic bombs
• ash and dust
• gases such as nitrogen compounds, sulphur compounds, and minor amounts of chlorine, hydrogen and argon

Classification of Volcanoes

Volcanoes are classified on the basis of the nature of eruption and the form developed at the surface. The major types are as follows.

Shield Volcanoes

Barring the basalt flows, shield volcanoes are the largest of all volcanoes on the earth. 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. Because the lava is so fluid, these volcanoes are not steep — the lava spreads out widely before cooling, giving the volcano a broad, gentle, shield-like profile.

They become explosive only if water somehow gets into the vent; otherwise, they are characterised by low explosivity. The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent, which develops into a cinder cone.

Composite Volcanoes

These volcanoes are characterised by eruptions of cooler and more viscous lavas than basalt. Because the lava is thicker and does not flow easily, these volcanoes often result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground. This material accumulates in the vicinity of the vent openings, leading to the formation of layers. This layering is what makes the mountains appear as composite volcanoes — they are built up of alternating layers of lava and pyroclastic material.

Calderas

These are the most explosive of the earth's volcanoes. They are usually so explosive that when they erupt, they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas. Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity to the surface.

Flood Basalt Provinces

These volcanoes outpour highly fluid lava that flows for long distances. Some parts of the world are covered by thousands of square kilometres of thick basalt lava flows. There can be a series of flows, with some flows attaining a thickness of more than 50 metres. Individual flows may extend for hundreds of kilometres.

The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province. It is believed that initially the trap formations covered a much larger area than they do today.

Mid-Ocean Ridge Volcanoes

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.

Volcanic Landforms: Intrusive Forms

The lava released during volcanic eruptions, on cooling, develops into igneous rocks. The cooling may take place either on reaching the surface or while the lava is still in the crustal portion. Depending on the location of cooling, igneous rocks are classified as:

• volcanic rocks — cooling at the surface
• plutonic rocks — cooling in the crust

The lava that cools within the crustal portions assumes different forms. These forms are called intrusive forms. The main types are described below.

Batholiths

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 denudational processes remove the overlying materials. They cover large areas and, at times, assume a depth that may be several kilometres. These are granitic bodies. Batholiths are the cooled portions of magma chambers.

Lacoliths

These are large dome-shaped intrusive bodies with a level base, connected by a pipe-like conduit from below. They resemble the surface volcanic domes of composite volcanoes, except that they are located at deeper depths. A lacolith can be regarded as the localised source of lava that finds its way to the surface. The Karnataka plateau is spotted with domal hills of granite rocks; most of these, now exfoliated, are examples of lacoliths or batholiths.

Lapolith, Phacolith and Sills

As the lava moves upwards, a portion of it may tend to move in a horizontal direction wherever it finds a weak plane. It may then rest in different forms:

• Lapolith — develops into a saucer shape, concave to the sky. …