In its oxide or silicate form, silicon is used to make concrete, bricks, glass, ceramics, and soap. Silicon metal is also the base material for making silicones used in such products as synthetic oils, caulks and sealers, and anti-foaming agents.
Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite. Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals.
Let's start with igneous rocks. Igneous rocks form when melted rock, or magma cools and hardens. Sometimes the igneous rocks form in the ground (intrusive igneous rocks), and sometimes they form outside of the ground (extrusive igneous rocks). Rocks deep in the earth melt and form molten, or melted, rock called magma.