How do intrusive igneous rocks form below Earth’s surface?

Intrusive igneous rocks form when magma is forced into older rocks beneath the Earth's surface and then slowly solidifies. This process occurs at depths within the Earth's crust.1

Magma, which is molten or partially molten rock, originates deep within the Earth's plastic asthenosphere, typically below 60 kilometers (40 miles). Because magma is less dense than the surrounding solid rock, it rises towards the surface. It may become trapped within the crust, where it cools and solidifies over a long period. This slow cooling allows for the growth of larger mineral crystals, resulting in rocks with a phaneritic texture, meaning the minerals are visible to the naked eye.2

These solidified bodies of magma are known as intrusions and can take various forms, including dikes (tabular intrusions that cut across existing rock layers), sills (tabular intrusions parallel to existing rock layers), laccoliths (mushroom-shaped intrusions), lopoliths (saucer-shaped intrusions), and plutons (large, irregularly shaped intrusions like batholiths and stocks). Eventually, these intrusive rocks may be exposed at the surface through erosion or tectonic uplift.32