Intrusive Igneous Rocks

Intrusive igneous rocks form in magma chambers that are deep underground. Click on image for full size (31K GIF) Courtesy of Boise National Forest

Many kilometers below the Earth’s surface, molten rock called magma flows into cracks or underground chambers. There, the magma sits, cooling very slowly over thousands to millions of years. As it cools, elements combine to form common silicate minerals, the building blocks of igneous rocks. These mineral crystals can grow quite large if space allows.

Rocks that form in this way are called intrusive igneous or plutonic rocks. The mineral crystals within them are large enough to see without a microscope. There are many different types of intrusive igneous rocks but granite is the most common type.