The cinder cone above and the close up at right are made of basalt . Intrusive igneous rock In trusive , or plutonic igneous rock forms when magma is trapped deep in side the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth's surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies. Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size.
Intrusive rocks have a coarse grained texture.
The image at right shows granite , an intrusive igneous rock. Simple rock classification chart with more rock and mineral links. Or, use the back button on your browser to return to the previous page. This site is a cooperative endeavor of the US Geological Survey Western Earth Surface Processes Team and the National Park Service .