Some volcanic eruptions are quiet. The lava oozes down the side of the volcano, like Mt. Kilauea in Hawaii or the Parcutin volcano pictured here. Others are explosive like Mt. St Helens. Volcano world has great video of actual volcano eruptions . When you learn about rock formation, you will learn that magma can be basaltic or granitic in composition.
Granitic lava contains high amounts of silica and is extremely thick.
Because it is thick, it gets trapped in the vents and builds up pressure. This pressure is released in violent explosions. Basaltic lava produces quiet, oozy eruptions that slide down the side of the volcano. The major gases which are associated with magma are carbon dioxide and water vapor. These gases increase the violence of the eruption.