Igneous rocks result from crystallization of magma and occur in a wide variety of forms of different shapes and sizes. The major subdivision is into the extrusive or volcanic rocks and the intrusive or plutonic rocks.
Extrusive rocks occur as lava flows and pyroclastic ash or debris that is ejected into the air during eruptions. These kinds of rock often occur in characteristic volcanic cones. Submarine lava flows form characteristic pods called pillows.
Intrusive rocks also occur in a variety of forms. Vertical sheets of igneous rock are called dykes. Horizontal sheets, parallel or near parallel to layering are known as sills. Fatter pods of crystalline rock are called laccoliths. Very large crystalline bodies are called batholiths.