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

Intrusive igneous rocks, also called plutonic rocks, form deep below the Earth’s surface when magma, or molten rock, rises into a crack or an underground chamber within the Earth. The chamber is a little cooler than the molten rock. The slightly lower temperature allows the magma to cool very slowly over thousands to millions of years. As the molten material cools, elements combine to form the common types of silicate minerals, which are the building blocks of igneous rocks. Slow cooling allows a variety of minerals to form and, often, mineral crystals can grow quite large if space allows.

Mineral crystals within intrusive igneous rocks are large enough to see without any magnification. Igneous rocks with large crystals are called phaneritic or course grained. There are many different types of intrusive igneous rocks but granite is the most common type.