The kind of magma that is found in the most explosive volcanoes, the composite volcanoes, is andesitic magma. It is magma that is quite high in silica content which makes it thick, sticky and gooey. Great lumps of this sticky magma cool at the top of the volcano to form a sealed cap. Magma beneath the cap builds up and pressure mounts and eventually becomes too much to be contained and the magma violently erupts out from the top, blowing out the cap and shooting miles up into the air and in all directions.
The kind of magma found in gentle volcanoes such as shield volcanoes is runny in consistency. It has very minimal amounts of silica and is not thick but loose. It therefore tends to trickle out of the top of the volcano rather than erupt or explode.
