Magma is less dense then the rock above so it is constatly  trying to force itself to the surface. When it does and the magma cools it makes new land mass.
Hawaiian Eruption
Pelean Eruption
Phreatic Eruption
Plinian Eruption
Vulcanian Eruption
Strombolian Eruption
Submarine Eruption
Surtseyan Eruption
Subglacial Eruption
Underwater Valcanoe's are also known as Submarine Volcanoe's. They are fissures in the earths surface from which magma can erupt. The vast majority are located near areas of tectonic plate movement. When the lava spuces out and upon contact with water it forms a solid cust around it.
High temps, and pressure under the earths crust causes rock to melt down to a liquid. The  liquid is known magma. 
When a volcano erupts it comes out with gas, rocks, glass and lots of ash.
 Permant which is more quiet and regular in eruptions, , moderet which are viloent and eragular. Paroxysmal are simular to the modeate but less violent(proxysmal means any sudden or violent out berst).
A Volcano erupts when pressure on the magma chambers causes the magma to rise.
Volcanoes
volcanoes are hot because of the molten lava, that comes from ,the earths core whitch cause's a eruption.