Volcanoes are openings in the Earth’s crust through which molten material, gases and pyroclasts erupt. The nature of volcanic activity is largely controlled by the type of lava, tectonic environment, and internal heat dynamics of the planet.

There are two types of LAVA:

1. Basic lavas: These are the hottest lavas, about 1,000ºC.  and are highly fluid. They are dark coloured like basalt, rich in iron and magnesium but poor in silica. As they are poured out of the volcano, they flow quietly and are not very explosive. Due to their light fluidity, the flow readily with a speed of 10 to 30 miles per hour.

They affect extensive areas, spreading out as thin sheets over great distances before they solidify. The resultant volcano is gently sloping with a wide diameter and forms a flattened shield or dome.

1. Acid lavas: These lavas are highly viscous with a high melting point. They are lightcoloured, of low density, and have a high percentage of silica. They flow slowly and seldom travel far before solidifying. The resultant cone is therefore steepsided. The rapid congealing of lava in the vent obstructs the flow of the out-pouring lava, resulting in loud explosions, throwing out many volcanic bombs or pyroclasts.

Sometimes the lavas are so viscous that they form a spine or plug at the crater like that of Mt. Pelee in Martinique.

Causes of Vulcanism

1. Increases in temperature inside the (1ºc/32 metres) earth. Consequently, there is molten lava in the Asthenosphere.
2. Origin of magma because of lowering of melting point inside the earth (due to higher pressure)
3. Origin of gases and water vapour due to heating of water which reaches underground through percolation of rainwater and snowmelt water.

Ascent of Magma forced by enormous volume of gases and water vapour. Movement and splitting of the major and minor plates of the earth-most of the active volcanoes are found along the plate margins (mid-oceanic ridges and the ocean trenches (Benioff Zone or Ring of Fire). 

World Distribution of Volcanoes

1. The Circum-Pacific Belt: (Ring of Fire) A chain of 3200 km (2000 miles) in length. There are 80 active volcanoes, Acconcagua, Guayatiri (Andes Mts.), Fujiyama (3776 m), Cotopaxi (the highest volcano in the world 6035 m.)
2. The East African Rift Valley Belt: Mt. Kilimanjaro, Mt. Kenya.
3. The Mediterranean-Alpine-Himalayan Belt: The Himalayan zone is an exception : Mt. Ararat (Turkey), Elburz, Hindukush.
4. The Mid-Atlantic Ridge: Antilles, Azores, Mt. Pelle, St. Helens, St, Martinique (W. Indies.) Other significant volcanoes are Shasta, Rainier and Hooda (N. America).The Valley of Ten Thousand Smokes (Alaska). St. Helens (Washington, U.S.A.), Kilavea (Hawaii), Mt. Taal, Pinatubo and Mayon (Philippines).

Conclusion

Types of lava determine volcanic form and explosivity, while vulcanism itself is fundamentally driven by plate tectonics and mantle heat flow. Global distribution of volcanoes reflects these tectonic processes—concentrated in subduction zones, rift regions, mid-ocean ridges, and selective hotspots—shaping Earth’s topography and hazards.