Intrusion facts for kids

Devils Tower National Monument in Wyoming. This huge rock was formed when melted rock pushed up from below and then the softer rock around it wore away.

An intrusion happens when magma (melted rock) pushes its way into cracks and spaces deep inside the Earth's crust. This magma then cools down and becomes solid rock, all while still underground. Think of it like liquid candy filling a mold, but the mold is made of other rocks!

This process can take millions of years. As the magma cools slowly, tiny parts of it crystallize into different minerals. Because the cooling is slow, these crystals usually grow quite large. This is different from extrusion, where magma erupts onto the surface (like a volcano) and cools quickly, forming much smaller crystals. Both intrusive and extrusive rocks are types of igneous rock.

Many large landforms, like the Sierra Nevada mountains in California, are made mostly of intrusive rock. These are often huge granite formations.

Types of Intrusions

Intrusions come in many shapes and sizes. They can be as big as a mountain range or as thin as a vein filling a small crack.

Batholiths and Plutons

• Batholiths are very large intrusions, often covering hundreds of square kilometers. They are like the huge core of a mountain range. When the softer rocks above them wear away over time, these massive batholiths can be seen on the Earth's surface.
• Plutons are simply large bodies of magma that solidify underground before they ever reach the surface. Batholiths are a type of pluton.

Sills and Dykes

Intrusions can also form in specific ways related to the layers of existing rock:

• A sill is an intrusion that spreads out horizontally between layers of older rock. Imagine a flat pancake of rock pushed between two slices of bread.
• A dyke is an intrusion that cuts vertically or at an angle across older rock layers. Think of a wall of rock pushing straight up through different layers.

Related pages

• Volcanic plug

Images for kids

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  A pink monzonite pluton (an intrusive rock) that pushed up below gray sedimentary rocks. This was later lifted up and can be seen near Notch Peak, Utah.

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  This is a laccolith (a dome-shaped intrusion) on top of a huge pluton system near Sofia. It was formed by syenite and diorite rocks and later pushed upwards.

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  This image shows how heat spreads out from an intrusion into the surrounding rocks over time.