Hence, igneous rocks are often formed by processes operating at plate boundaries . Exceptions to this are volcanoes such as those in the Hawaiian Islands which erupt in plate interiors where hot spots have developed. Classification of igneous rocks is based upon composition and texture. Typical of rocks in continental crust Texture of igneous rocks - governed in part by rate of cooling slow cooling deep in crust results in larger crystals and coarsely crystalline texture rapid cooling near or at the surface results in smaller crystals and finely crystalline textures. Glassy textures which have no mineral crystals present are formed by extremely rapid cooling.
Eruption is quiet because there is little gas in the magma, and what there is escapes quietly.
Eruption includes alternating quiet eruption of lava and explosive eruption of solids , because there is more gas in the magma. The gas builds up and escapes quickly in the more explosive events. Resulting rocks are more sialic or felsic than those of shield volcanoes. These volcanoes are typical of subduction zones . Calderas - greatly enlarged craters, larger than 1 km in diameter, formed either by collapse of the summit of the volcano associated with previous eruption of large amounts of lava, or by violent explosion of the summit, associated with the extremely rapid escape of large amounts of gas.