In a word, an eruption is a phenomenon that underground substances are spouted rapidly from a crater by rising magma.
　Mechanism of eruptions is so complex and can change by a condition at that time, so we can’t say “this mountain erupt this type” sweepingly. But we can sort them generally

　First high-degree magma rise and strata are heated. Vapor occurs and pressure gets higher there. The vapor is spouted from a crater, together with broken pieces of rock and volcanic ash. At that time, magma is not spouted. Explosion makes a thundering sound. And a hollow the same size as volume of spouted rocks is left at a crater.

　Magma touch water such as lake, sea or underground water and vapor is made massively (imagine that you drop little water to heated empty kettle). Magma is also spouted together with vapor because it come shallow spot. Rocks that enveloped water can fly about 5km ahead.

　Because underground high-temperature and high-pressure place become higher and higher, inside pressure get much higher. And volatile substances having magma (such as vapor, hydrogen sulfide, sulfur dioxide and so on) foam and explode. Moreover, they spout gas, vapor, rocks etc. to the surface of the earth together with thundering sound.

　The scale of volcanic eruptions is usually following order.
Phreatic Explosion ＜ Phreatomagmatic Explosion ＜ Magmatic Explosion

　Form of eruption is a thing that expresses characteristics of volcanic activity. However, there are many cases that forms are different from each eruption in every volcano.

　Rocks, volcanic bombs, volcanic ash are spouted rapidly by relatively small-scale eruption. In Japan, this type often occurs. This is dangerous type because pressure of gas dissolving magma becomes higher and rocks fly from a crater suddenly. It occurs not sequentially but there’re many cases that repeat it few minutes ~ few hours’ intervals. Windows face volcano can be broken because air vibrate accompanying explosion.

　In this type, volcanic spout magma, volcanic bombs from crater by relatively short cycle. High temperature or hardening magma fly high and described an arc, drop near the crater. When the scale is big, volcanic bomb fly about 1km ahead. After that low-viscosity lava can very rapidly. But this type isn’t massive eruption, so it’s relatively safe.

　Rocks lay thick covering wide area together with large-scale eruption. Thick smoke rise to a height of ten hundreds of meters making a mushroom cloud and it make many volcanic products lie on the lee. Rising volcanic ash reach the stratosphere. After that, it can be flowed toward east by westerlies and can fall about hundreds of kilometers ahead (this is only in Japan). So fire volcanic ash can turn round the earth. After so much magma erupted, a caldera can be made.

　This is a type that low-viscosity magma flow from a crater or a crevice. Small-scale eruption is repeated at short interval. Magma gathering in a crater makes a lake and magma boil slowly there. Then, volcanic bombs (etc.) are spouted by little eruption. When the activities become more active, volcanoes spout magma like a fountain.

　In this type, extremely high-viscosity magma is pushed out. Small scale and high temperature pyroclastic flow occurs at the same time when volcanoes erupt. This can cause a disaster, so this type is the most dangerous.