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Not all rocks form the same way; some have a very explosive beginning! In this lesson, we will discover how intrusive igneous rocks come into being and look at some examples. Then, you can test your knowledge with a quiz.

Igneous Rocks

Before we can really look at intrusive igneous rocks, we need to do a little bit of a review. While there are many different types of rocks, igneous rocks are formed from volcanic activity. This means they are either created from cooled magma or cooled lava. The difference between magma and lava is location – interior versus exterior.

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Magma is the molten rock found inside the crust of the earth. Pockets of magma (labeled A here) come up from the mantle and push their way through weak spots in the earth’s crust, often under volcanoes. When magma is pushed up through a volcano that’s erupting, the magma becomes lava as it exits out of the crater (labeled B here).

So why is this distinction important? What does it matter if rocks cool from lava or magma since they’re both volcanic? When we are talking about the formation of igneous rocks, the distinction is very important because the rocks that come from magma become intrusive igneous rocks, and the rocks that come from lava become extrusive igneous rocks.

The difference is due to how fast the material cools.

Cooling Rates

Since magma is located inside the earth, the overlying rock and sediment act as a blanket that keeps the magma warm and allows it to cool slowly. Lava, however, exits the volcano and cools much faster. We can understand cooling rates by using baking as an example. After baking bread, you turn off the oven and allow the bread to cool before slicing. If you leave the bread inside the oven to cool, it does so very slowly. If you take the bread out of the oven and sit it on the counter to cool, it does so more quickly.

In rocks, the rate of cooling allows mineral crystals, which make up the igneous rocks, to grow. Imagine you were in a classroom with 30 other people. Your teacher tells you that when she says ‘go,’ you need to get up and move to form the biggest groups possible. During the first attempt, you have three seconds to move into groups.

The groups would not be very big because you did not have enough time to come together. During the second attempt, you have ten seconds to move into groups. This time, because you were allowed more time, you form bigger groups.The same principle applies to rocks and minerals. Because the magma cools slowly, the ions have time to move around and form mineral crystals. As a result, when you have rocks that cooled from magma, you can easily see the different mineral crystals in the rock.

These rocks are intrusive igneous rocks because they cooled slowly ‘inside’ the earth.Because lava cools more quickly, there is not enough time for mineral crystals to form and grow, so you have rocks with very tiny mineral crystals that cannot be seen without the aid of a magnifier. These rocks are considered extrusive igneous rocks because they cooled quickly ‘outside’ the Earth.

Examples

Let’s take a look at some examples of intrusive igneous rocks.

Granite is the most common intrusive igneous rock as the continental crust is composed of granite. Granite has mineral crystals that can be seen clearly with the naked eye. Here, you can see black crystals, white crystals, and pink crystals, with each color representing a different mineral. Granite is commonly used for countertops, floor tiles, paving stone, and cemetery monuments.

Diorite is another type of intrusive igneous rock. It can look very similar to granite. Here, again, you can see the black and the white crystals without a magnifier. This means the crystals cooled slowly. Diorite can be used to create concrete for construction and roadwork. It can also be cut and polished for stonework you might see on building facings or in foyers.

Gabbro is a very dark and dense intrusive igneous rock. While it can be a little more difficult to see the crystals because they’re similar in color, you can still see them – just not as easily as you can in granite or diorite. Like diorite, gabbro is often used as crushed stone for concrete aggregate, while smaller pieces can be cut and polished for decorative stone.

Peridotite is a very crystalline intrusive igneous rock composed of interlocking green minerals.

Peridotites are economically important because they contain chromite, which is a source of chromium. They can also be source rocks for diamonds! Peridotites can be used in many ways, from industrial manufacturing, decoration, and jewelry-making, to scrubbing greenhouse gases from the atmosphere.

To give you an idea of the difference between intrusive igneous rocks and extrusive igneous rocks, take a look at this example of basalt, an extrusive igneous rock:

You’ll notice that you cannot see the different mineral crystals because this rock cooled from lava much too quickly for mineral crystals to form.

Lesson Summary

In this lesson, we’ve reviewed the fact that igneous rocks are volcanic, forming from either magma or lava. Intrusive igneous rocks cool from magma slowly and have crystals that are easily seen with the naked eye. Examples of intrusive igneous rocks are granite, diorite, gabbro, and peridotite.

Intrusive igneous are used for a variety of purposes, including decoration, building, stonework, and jewelry.

Important Terms ; Facts

igneousrocks
Terms Definitions/Facts
Magma molten rock found inside the crust of the earth
Lava the form that magma takes as it exits the crater of a volcano
Intrusive igneous rocks rocks that come from the magma
Extrusive igneous rocks rocks that come from lava
Granite contains mineral crystals that can be seen clearly
Diorite can be used to create concrete; can also be cut and polished for stonework
Gabbro very dark and dense; smaller pieces can be cut and polished for decorative stone
Peridotites economically important because they contain chromite, a source of chromium
Basalt example of an extrusive igneous rock

Learning Outcomes

Review this video lesson on intrusive igneous rocks so that you will easily be able to:

  • Recall the origins of igneous rocks
  • Compare and contrast intrusive igneous and extrusive igneous rocks
  • Recognize various types of intrusive igneous rocks

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