How to Learn About Igneous Rocks

Four Methods:Knowing How Igneous Rock is FormedIdentifying Igneous RockClassifying Igneous RockMaking an Edible Igneous Rock

There are hundreds of types of igneous rock. These rocks form when magma cools and solidifies. This process can happen below or above the surface of the earth. You can identify an igneous rock by examining the texture and color of the rock.

Method 1
Knowing How Igneous Rock is Formed

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    Understand how igneous rock is made. Igneous rock forms from magma (i.e. molten rock). Magma is a liquid that is under the earth. The magma moves to a cooler part of the earth, loses its heat, and crystallizes into an igneous rock.
    • Magma is thick and is composed of very hot elements and compounds.
    • Magma can cool both inside the earth and out onto the surface of the earth.[1]
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    Know what an intrusive igneous rock is. This is formed when magma cools beneath the surface of the earth. The magma is insulated by the surrounding rock, and cools slowly. This allows the rock to form large crystals.[2]
    • Another name for intrusive rock is plutonic rock.[3]
    • You can often see the crystals of intrusive rocks with your naked eye.
    • Examples of intrusive rock include diorite, gabbro, granite, pegmatite, and peridotite.[4]
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    Know what an extrusive igneous rock is. When magma erupts onto the earth's surface, it becomes lava in a volcanic eruption. Lava cools and turns to rock relatively quickly. The crystals in the rock don't have much time to form, creating fine-grained rocks with small crystals.[5]
    • You can hardly see the crystals at all with the naked eye. You will need a microscope or a hand lens to see the crystals.
    • Another name for extrusive rock is volcanic rock.[6]
    • Examples of extrusive rock include andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff.[7]
    • Some extrusive rocks cool very quickly and do not form crystals at all. These rocks are considered volcanic glass. Obsidian and pumice are examples of volcanic glass.[8]
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    Know what pyroclastic igneous rock is. When there's an explosive volcanic eruption, ash is thrown high into the air. When this ash falls and turns into rock, it's called pyroclastic rock. The minerals are broken into small pieces in this type of rock by the explosive eruption.[9]
    • Examples of pyroclastic rock include serpentinite, diabase, spilite, greisen and skarns.

Method 2
Identifying Igneous Rock

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    Determine the color and texture. When you pick up a rock, examine the color of the rock. Is it light? Is it dark? Is it a mixture of colors? Once you identify the color, begin to examine the texture.
    • If you can see the crystals, you most likely have a coarse-grained/phaneritic rock.
    • If there are no crystals, you may have a glassy texture rock or a pyroclastic rock.
    • If you notice big crystals and small crystals, you may have a porphyritic texture.
    • Take notes as you examine the rock to help you identify it.
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    Compare your findings to an igneous rock chart. There are many rock charts available online.[10][11][12] Type "igneous rock chart" into your search engine to find one. Find the intersection on the chart where the texture and mineral composition meet.
    • For example, a light colored, coarse-grained rock may be granite.
    • A glassy looking, dark colored rock may be obsidian.
    • A dark, fine-grained rock may be basalt.
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    Visit a museum or park. The best way to learn about igneous rocks is to touch them in person. This will help you identify texture and color. Visit museums that have geology exhibits. Many national parks also provide opportunities to learn about rocks.
    • The National Park Service website maintains a list and allows you to find a park in your state.[13]
    • You can also order an igneous rock collection online or from a local science store.
    • If you are unable to visit a park or order a rock collection, look up pictures of the rocks online or in a book so that you can see the differences in color and texture.

Method 3
Classifying Igneous Rock

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    Examine the texture of the rock. Texture describes the way a rock looks. Grain size is the main texture you look for when classifying igneous rock. Grain size is the size of the individual mineral crystals in the rock.[14]
    • Coarse-grained (phaneritic) describes rocks with large crystals. Most intrusive rocks are considered coarse-grained because of their large crystals.
    • Fine-grained (aphanitic) describes rocks with crystals that are too small to see with the naked eye. Most extrusive rocks are considered fine-grained rocks.
    • Rocks that cool quickly may have trapped bubbles of gas called vesicles. The texture of these rocks is vesicular.
    • Porphyritic texture rocks have both fine-grained and coarse-grained crystals.[15]
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    Examine the chemical composition. You can examine the chemical composition of a rock just by looking at it. Minerals vary in color, and the color tells you what chemicals are in the rock. The most common chemicals are silica, iron, and magnesium.[16]
    • Light colored minerals (often called felsic) have more silica in them and are often white, gray, pink, or colorless.
    • Dark-colored minerals (often called mafic) have more iron and magnesium in them are usually black, brown, dark gray, or green.
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    Know the four classifications of igneous rock. Igneous rock can be classified as felsic, intermediate, mafic, and ultramafic. These classifications apply to both extrusive and intrusive rocks.[17] Felsic rocks are the lightest, and ultramafic rocks are the darkest.[18]
    • Felsic rocks have the highest silica content (65% +). Common examples of felsic rocks are rhyolite (extrusive rock) and granite (intrusive rock).
    • Intermediate rocks have about 55-65% silica content and are a little darker than felsic rocks. Common examples of intermediate rocks are andesite/dacite (extrusive rock) and diorite/granodiorite (intrusive rock).
    • Mafic rocks are low in silica (45-55%) and are dark in color. Common examples of mafic rocks are basalt (extrusive rock) and gabbro (intrusive rock).
    • Ultramafic rocks have the lowest silica content (less than 45%) and may have a green tint. Common examples of ultramafic rocks are peridotite (intrusive rock) and komatiite (extrusive rock).

Method 4
Making an Edible Igneous Rock

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    Get your supplies. You can use food to learn about how igneous rocks are formed. Remember that these rocks are formed when magma cools and forms crystals. You can use food items to simulate this process. You may need a parent to help you.The supplies you need include:[19]
    • A bag of white chocolate chips
    • A bag of milk chocolate chips
    • A piece of wax paper
    • A cookie sheet
    • A spoon
    • A microwave safe bowl
    • A microwave
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    Heat the white and milk chocolate chips. Place the chips in a microwave safe bowl and notice how different they look. Each of these pieces represents the individual minerals in a rock. Heat the chips until they melt.
    • The chips melting is similar to the earth melting the rock.
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    Blend the melted chips. Once the chips have been heated, use your spoon to blend the chips together. Stir until you cannot see the individual chips anymore. This liquid mixture represents magma[20].
    • When the earth melts the rock and forms magma, the individual minerals of the rock cannot be seen anymore. They blend together, just like the chocolate chips.
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    Pour the liquid onto the baking sheet. Place a piece of wax paper on the cookie sheet, and pour the melted mixture onto the sheet. This represents that magma coming out of the earth as lava. As the meted chocolate cools, it will become hard. This is the same way that magma turns into igneous rock.[21]
    • When magma cools, it forms igneous rock.
    • Allow the chocolate to cool and do not touch it.
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    Break up the cooled chocolate. Once the chocolate has cooled on the sheet, break it and notice what it has become. The white and milk chocolate chips have formed a new piece of chocolate that looks completely different from the original pieces of chocolate.[22]
    • When magma cools, the rock becomes a new igneous rock.

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