About VolcanoeS......

Volcanoes result when molten material from the earth’s interior, called magma, forces its way through weakened areas in the lithosphere, that is, the earth’s rigid outer shell. All volcanoes have beneath them a magma chamber. This chamber is joined to the earth’s surface through a conduit known as a volcanic vent. The magma is extruded on to the surface with explosive force. The extruded magma is called lava, which solidifies in various forms soon after exposure to the atmosphere. In particular it does so around the vent, , building up the characteristic volcano cone.
The lava that solidify can take three different forms. The first form is a thin, plastic surface which is dragged into rope like fluids. The second form is a thick hard skin which is broken up so as to present a fragment, jagged surface. The third form is very much like the former except the letter has a much smoother surface. The gasses which escape as the magma nears the surface are often released with explosive violence, which shatters the rising material and shoots it into the air. This then falls to the ground as solid fragments. These solid fragments are known as pyroclasts, and they are of many sizes, shapes and textures. The finest particles are called dust; then, increasing size come ash and bombs. Some volcanic bombs are huge. In some cases, crust and ash can be carried much farther in the atmosphere.
Although volcanoes are frequently explosive, the explosions do not occur as sudden,rapid events. In many cases a stream of gas will rush continuously from a volcanic vent for several hours at a time, carrying pyroclastic rocks with it. In most explosions the ejected material is carried to considirerrable heights before it falls back to the surface. Volcanic activity can range from the quiet extrusion of lava to the violent ejection of pyroclastic rocks. The deposits from each activity produce their own charactheristic surface atructures. The most common type of volcano emits both lava and pyroclastic rocks at different times, and so forms a composite cone with alternating layers of the two materials. Examples are mounts etna, Vesuvius and Stromboli in Italy, and Fujiyama in japan. Volcanoes are generally restricted to belts of seismic activity. Most of the world’s active volcanoes are concentrated in areas which coincide with plate margins. These plate margins are simply large scale fissure eruptions and are called constructive palte margins. There are also destructive plate margins. Not all volcanoes are, however, associated with plate margins. Some , such as those in east Africa, occur in continental interiors. Others are scattered about the ocean floor,well away from oceanic ridges.
The reasons why volcanoes occur are not yet fully understood. One view is that the magma rises through fractures in the plates, but this in itself does not explain why such fractures exist in the first place . one possibility is that the lithosphere acts as a thin membrane which distorts, and acquires zones of wezkness, when it moves.