There are two types of volcanoes – sub-aerial and submarine. But the same kinds of materials may be ejected from the above two types. However, in case of sub-aerial volcanism, the materials ejected are subjected to mechanical and chemical weathering before being deposited on the sea floor.

If the volcanic materials are first deposited on the land, they are later transported to the sea by the rivers. But the lighter and more finely divided materials may be carried over the sea by the wind. Pumice, however, floats in the water for some time because it is very light.

Windborne volcanic materials are deposited in large amounts over a large area. Volcanic materials can be distinguished by their physical or chemical characteristics provided they are not highly altered. The unaltered volcanic materials consist of lava fragments, pumice, volcanic glass and mineral grains.

Most of the volcanic materials are found near the areas where volcanic activity has taken place. Volcanic materials constitute a special type of sediment. Submarine volcanism is very common in certain areas.

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As we are aware, the Pacific Ocean is dotted with volcanic islands which form as a result of the accumulation of volcanic ejects up to the sea surface. In other oceans also there are numerous volcanic islands.

However, there are many such accumulations of volcanic materials which could not reach the sea surface. Remember that it is rather impossible to recognise the deposits of volcanic origin when they are highly altered.

In certain parts of the sea floor, basaltic lava flows are the major constituents. In certain localities bare lava flows are encountered, while in certain other localities they are hidden under a thin layer of other sediments.