In the following paragraphs, only those elevated forms which are more common and more significant have been described. Here, it may be mentioned that the forms of various types of volcanoes are dependant on the nature of eruption and the variety of the erupted material.

However, the later denudation may considerably modify the original form of a volcano. The following are the major forms of volcanoes built chiefly by central eruption.

1. Composite Cones 2. Cinder Cones or Ash Cones

3. Shield Volcano 4. Acid Lava Cones

ADVERTISEMENTS:

5. Lava Domes 6. Lava Plugs

7. Parasite Cones 8. Spatter Cones

1. Composite Cones:

The most common and typical volcanoes are the composite cones. All the great volcanoes are generally composite cones partly of ash and partly of lava in alternating beds.

ADVERTISEMENTS:

This type of volcano is built up over a long period of time as the result of a large number of both major and minor eruptions. In the early stages of its growth, the form is conical, but the shape is usually not so perfect, because the streams of lava flow out unevenly.

As the volcano attains greater height, the column of lava in the central vent also rises higher and higher to reach the crater. The pressure on the walls of channel goes on increasing with the result that the lava finds it easier to force its way through the sides of the hill than to rise up to the crater.

In great volcanoes, therefore, the streams of lava come up from the fissures in the walls, and so the secondary cones are often formed.

One of the typical examples of a composite volcano in Europe is Stromboli, in the Lipari Islands in Italy. Its eruptions are frequent and mild, often occurring at intervals of an hour or so.

ADVERTISEMENTS:

The glows of the hot lava on the clouds of smoke and condensed moisture above the crater have caused it to be named the lighthouse of the Mediterranean.

Because of the alternating sheets of lava and pyroclastic material composite cones are also known as strato cones. The structure of this type of cone indicates the alternating periods of ex­-plosive and quiet eruptions.

The typical examples of composite cones are to be seen in most of the world’s larger volcanoes, such as Fujiyama (Japan), Vesuvius (Italy), and Cotopaxy (Ecuador) etc. The volcanic mountains of this type are also represented by Shasta, Hood, Rainier in Western United States, and Mayon in the Philippine Islands.

2. Cinder or Ash Cones:

ADVERTISEMENTS:

Cinder or Ash Cone is built as a result of the explosive type eruption in which there is abundance of pyroclastic material. These cones attain only moderate height of a few hundred meters.

These cones grow around a crater. Many such cones develop large craters which are cleared and enlarged by successive eruptions.

In humid regions the finer material is washed away making fans at the foot of the slopes. However, due to the resistant nature of the coarser material, the cone retains its original form.

3. Shield Volcanoes:

ADVERTISEMENTS:

Basic lava cones are built by the mobile basic lava. It may be pointed out that if the lava is mobile and fluid, it flows for great distances. Such cones are built by volcanoes that emit chiefly lavas from their craters or through cracks in their sides.

Fragmented material may be present in relatively small amounts, but the main structure is a series of lava flows. The fluidity of the lava as it comes from the volcano is usually great; therefore, the flows may extend to the base of the mountain and over the surrounding country.

On this account the slopes of these domes usually are much gentler than those of composite cones or endogenous domes rarely exceeding 6° or 8°. The volcanoes of the Hawaiian type best illustrate shield domes.

These great basic-lava cones are often known as shield volcanoes’-Worcester. The great volcanoes of Hawaii have vast cones with a small angle of slope. Mauna Loa and Mauna Kea of Hawaii Islands are the typical examples.

ADVERTISEMENTS:

Shield volcanoes also occur in the Samoa group and in Iceland. These volcanoes are also known as exogenous domes or shield domes.

4. Acid Lava Cones:

Such cones are built by highly viscous lava rich in silica. Naturally these viscous lavas have low mobility. As soon as they are emitted from a volcano, they take no time in cooling and solidifying.

This type of viscous lava builds high cones of steep slopes. This type of cone is also known as Strombolian type of cone.

5. Lava domes:

Lava domes resemble shield domes except their greater size. They are built due to the accumulation of solidified lavas around the volcanic vents.

These domes may be formed either by the more viscous lavas rich in their silica content, or they may also be formed by more fluid basic lava deficient in silica and rich in iron and magnesium content. Lava domes are also known as Cumulo-domes.

6. Lava plugs:

When composite cones become extinct, it often happens that their vents are filled with solidified lava. As a result of denudation the cones are worn away and the more resistant plug is ultimately exposed. These plugs are sometimes called volcanic necks.

7. Parasite Cones:

These cones are associated with the composite cones. Sometimes a later explosive activity blows off the top of the main composite cone, so there forms a much larger crater. Within this enlarged crater, a secondary cone may develop.

Remember that parasitic cones usually grow on the flanks of the main cone. Etna, in Sicily, has a larger number of secondary cones on its flanks. Some volcanoes have several major cones which are known as ‘multi- pie Volcanoes’.

It may be pointed out in these cones a number of subsidiary pipes branch out from the main conduit through which lavas and other volcanic materials are emitted.

These materials are deposited around the newly created vents on the flanks of the original cone. In this way, many small sized cones come into existence on the flanks. There are 200 parasite cones on the slopes of Etna.

8. Spatter Cones:

These are the small volcanic cones which form on lava flows both from central and fissure eruptions. They are only a few meters high. When the gas bubbles burst through the lava and accumulate there, small irregular cones come into existence.