Why do volcanoes explode?

Answer 1

When rock inside the Earth becomes hot enough it melts. The molten magma is less thick than the solid rock surrounding it. Just as an object that is less thick than water will float on water, the relatively low density of the magma causes it to rise to the surface of the Earth. If the magma contains water and dissolved gasses, when the magma reaches the surface the water and dissolved gasses will suddenly expand into steam and gas, causing a violent eruption.
a volcano explodes because of the pressure and heat at the bottom of the volcano rocks melt and start to over flow in the volcano

Answer 2

If, however, you mean erupt rather than explode then it's something like:

Magma beneath the volcano comes under very high pressure, which forces it out the volcano, normally through the huge gaping hole at the top. However, very high pressure sometimes means that this hole is not enough, and magma (lava when outside the volcano) will burst out the sides of the volcano, such as the eruption at St. Helens.

I don't know much about science however, I don't know why or how the magma comes under pressure in the first place.

Answer 3

Volcanoes explode when built-up pressure from magma beneath the Earth's surface is released suddenly. This can be triggered by gases trapped in the magma, the accumulation of new magma, or a shift in the volcano's structure. The explosion can result in the release of ash, lava, gases, and rock fragments.

Answer 4

they have somthing called evaporation thatmakes it explode