The one line answer is: volcanoes erupt due to combined pressure of accumulating magma in the magma chamber and the pressure caused by various dissolved gasses and water vapors finding their way out through a newly formed or existing weak spot on the earth crust. The more detailed answer will explain precisely how this entire process takes place deep under the earth surface leading to volcanic eruption. The causes of volcanic explosion can be explained under the following four headings which cover underlying factors that contribute to volcanic eruption:
Increase in Magma Quantity:
The core of the earth is made of molten rocks and this molten rock has very high temperatures and pressure. This pressure is balanced by the weight of the surrounding rocks. There are crevices and openings between rocks in which the magma flows. The magma melts surrounding rocks which increase the size of magma in those crevices forming chambers filled with magma. These magma filled spaces between rocks are called magma chambers. As magma in these chambers eats into surrounding rocks by melting it, the size of magma chambers is increased. Any addition of molten rocks in chambers adds to the volume of magma and further increases its pressure. Magma with molten rocks become more buoyant and move upwards.
Increase in Magna Pressure and Weak Spot on Earth Crust:
As magma in chambers push upwards, its finds way through crevices and spaces formed due to geological activity such as earthquakes. As molten rocks move upwards to the earth crust through crevices and spaces, it may get mixed up with water sources and form water vapor. The dissolved gasses in magma such as sulphur dioxide and carbon dioxide also are released from the magma. Combined pressure of magma, all gasses and water vapor exert tremendous pressure and escape through the existing weak spot or a new opening formed due to seismic activity.
A very simple experiment can explain volcanic eruption. A full blown balloon when slightly pricked bursts and the spot of the prick ruptures haphazardly. The prick caused a weak point and the gas in the balloon escaped with a gust from the tiny hole rupturing it haphazardly and created a loud sound. Had there been no prick, the balloon gas would not have erupted from the surface. In volcanoes eruption also the weak spot is very important. And, at the same time, those factors that cause gas to build up and lava size becoming expansive are also important causes responsible for volcanic eruption.
Volcanic Regions and Volcano Structure:
Theoretically, though a volcano may erupt anywhere and anytime, it is not so because for a volcano eruption to erupt there is a process and conditions responsible for volcanic eruption. When conditions lead to processes, then only a volcano can erupt. For the same reasons, some specific regions on earth are better known for volcanoes because of regular seismic activity causing earthquakes and ruptures in the earth’s crust. These conditions depend upon the rocks conditions and geological activity at the site. Once those conditions are formed, volcano eruption follows a set of processes. Almost every volcano eruption undergoes a process. It can be a slow or a quick process. Volcanoes are of four main types: Stratovolcanoes, are composite volcanoes; Shield volcanoes are in a large area; Supervolcanoes, are very large volcanic eruptions taking place once in thousands of years; and Cider Cone Volcanoes, are steep slopes and cover small areas and are located on larger craters.
Prediction of Volcano Eruption:
While the reason why volcanoes erupt is more or less a well understood phenomenon there are several aspects of the genesis of magma and its effect on volcanoes that are not fully understood. If it was not so it would have been possible to predict eruption of each and every volcano. Japan’s recent Mount Ontake volcano eruption (in 2014) which caused loss of many lives could not be predicted. But the advance of Iceland’s Bardarbunga volcanic eruption was monitored with great accuracy by using GPS technology, volcanic gas sniffers and by monitoring seismic activity. Bardarbunga volcanic eruption was predicted very accurately.