Here is the explanation of the complex geologic processes that result in the formation of volcanos:
"Volcanos are defined as a "geological landform, consisting of fissure in the earth's crust, above which a cone of volcanic material has accumulated". Near Sicily is an island named "Vulcano" from which the term "volcano" is derived. Volcanos are a good example of nature and her unpredictable powers, and man's inferiority to her.
The earth is made up of plates which are in a constant state of motion. Usually volcanos develop on the boundaries of these plates. The plates have two types of movement, convergent and divergent. Convergent movement is when the plates come together and divergent movement is when they separate. A hot spot refers to an area on a plate that has been heated internally, from the mantle. A volcano can form in any one of these situations. It is not even uncommon for the majority of our volcanos to begin on the ocean floor.
The column of a volcano is brought about by the build-up of materials (fragments, dust, lava) that is expelled from the center of the earth. Basically the earth is made up of various layers. The molten iron core and surface crust have sandwiched between them a layer referred to as the mantle. The mantle is a solid formation of rock. Extreme amounts of heat and pressure cause this mantle rock to melt. This melted rock is known as magma. As the amount of magma increases, it a ascends to the surface. Magma that reaches ground level is then known as lava.
Repetitive eruptions of lava and solid matter results in the formation of the walls (cone) of the volcano. Hence the cone may be a result of millions of years of volcanic activity. The type of formation that develops depends on the type of eruption. The different types are caldera, cinder cones, shield volcanos, composite volcanos and strato-volcanos.
A caldera is the most volatile type. It spews out massive amounts of lava when it erupts and results in a noticeable depression in the ground. In the cinder cone type, large amounts of particles are ejected into the air and result in a steep and loosely constructed structure.
A shield volcano happens at divergent margins. When the plates separate, the melted rock will push its way to the surface. If the eruption occurs on the ocean floor, then a rift zone is formed (these are long stretches that fill with lava). Although on land, the gentle flow of lava forms a dome type mountain. This type of formation is usually spread out over an extensive area. The Hawaiian Islands are a good example of this type of formation. What actually happens is that plates slide over hot spots which are heated from the mantle. This causes the magma to rise and fall repeatedly, eventually creating a chain of volcanic islands.
Strata-volcanos occur when two plates come together and one slides under the other. The bottom plate ends up having parts melted from the heat of the mantle. This results in an increased amount a magma which forces its way to the surface.
It has taken millions of years to form the structures we see today. Numerous bouts of volcanic activity are responsible for the sloping and peaked formations of our earth's surface. It has been estimated that over three quarters of our earth's surface was formed in this way."
For more information see: