Igneous rocks are produced by the crystallization and solidification of molten magma. Plutonic rocks or intrusive rocks are rocks formed when magma cools and solidifies below the earth's surface. Depending on their silica content, they are called (in ascending order of silica content) gabbro, diorite, granite and pegmatite Volcanic rocks or extrusive rocks are rocks formed when lava cools and solidifies on the earth's surface. Depending on their silica content, they are called (in ascending order of silica content): basalt, andesites, dacites, rhyolite, pumice and obsidian.