Intrusive Rocks: Definition & Examples

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  • 0:01 What Are Intrusive Rocks?
  • 1:05 How to Identify…
  • 3:33 Unearthing Buried Rocks
  • 4:35 Lesson Summary
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Lesson Transcript
Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

If you've ever looked up at Half Dome in Yosemite National Park, Mt. Rushmore in South Dakota, or Stone Mountain in Georgia, you were looking at intrusive rocks. We're going to investigate how these rocks form and how you can identify them in the field.

What Are Intrusive Rocks?

Intrusive rocks are a type of igneous rock. Igneous rocks are rocks that form from cooled magma. Intrusive rocks are igneous rocks that form from crystallized magma beneath the earth's surface. The other main category of igneous rocks is extrusive rocks, which are igneous rocks formed on the surface.

Half Dome at Yosemite National Park is made of intrusive rock. Despite its current solid state, this formation was once a large body of magma beneath the earth.
Picture of granite at Yosemite

If you have trouble remembering the definition of intrusive rocks, remember that intrusive rocks form inside the earth. You'll also see intrusive igneous rocks referred to as plutonic rocks, named after the Roman God of the underworld, Pluto.

Intrusive rocks form from cooled magma within the crust. This illustration shows how magma bodies, shown in red, may look within the crust. Because we cannot directly observe magma in the crust, there is still much we do not know about the storage and behavior of magma below the surface of the earth.
illustration of magma inside crust

How to Identify Intrusive Rocks

Intrusive igneous rocks have a characteristic appearance that makes them easy to identify. We'll cover some key features of these rocks and how they are formed.

All igneous rocks are distinguished from other rock types by their interlocking texture. Interlocking texture means that there is no space between individual crystals, because the crystals grew into one another. During cooling, crystals begin to form in the magma, much like ice crystals in a partially frozen drink. As cooling progresses and the crystals grow larger, only small pockets of liquid remain. The final crystallization of these small pockets of liquid results in the interlocking crystal texture.

Since intrusive rocks are cooling underground, the surrounding rock insulates the magma allowing it to cool very slowly. This slow cooling allows the crystals to become a much larger size than if they were cooled quickly on the surface. Thus, you can identify intrusive rocks by their large crystals, which can be seen with the naked eye.

Observing the crystals in this rock can tell you two things about its origin. First, the interlocking crystals show that this is an igneous rock formed from the crystallization of a magma. Second, the large crystal size tells us that this is an intrusive rock that was created beneath the earth
close-up of intrusive rock

Intrusive igneous rocks can have a range of crystal sizes. Those with roughly the same crystal size are called phaneritic.

This rock is clearly an intrusive rock because the crystals are easily visible with the naked eye. Also, the rock has a phaneritic texture because all the crystals are about the same size.
Granite

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