Igneous Rocks

The Granite Peak of Half Dome in Yosemite National Park

Igneous rocks compose a large portion of the Earth's surface, and they all form when molten rock cools and crystallizes. Igneous rocks are currently being formed wherever active volcanoes exist on Earth. They are also forming at various depths within the Earth. Due to the unique method of formation, igneous rocks have certain characteristics that help with identification. To distinguish between an intrusive and extrusive igneous rock, you must observe the rock's composition and texture. Texture is the size of the minerals present. Intrusive rocks exhibit a coarse-grained texture, while extrusive rocks can be fine-grained, glassy, or frothy. To differentiate between multiple igneous rocks with the same texture, simply observe the color of the minerals present. The color will indicate the rock's chemical composition which can help you identify the rock.