1. Section 2: Classification of Igneous Rocks
Classification of igneous rocks is based on mineral composition and texture. K
What I Know W
What I Want to Find Out L
What I Learned

2. Essential Questions
What are the different types and textures of igneous rocks?
How do cooling rates affect the grain sizes in igneous rocks?
What are some of the uses of igneous rocks?
Classification of Igneous Rocks
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3. Vocabulary Review New fractional crystallization intrusive rock
extrusive rock
basaltic rock
granitic rock
texture
porphyritic texture
vesicular texture
pegmatite
kimberlite
Classification of Igneous Rocks
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4. Mineral Composition of Igneous Rocks
When magma cools and crystallizes below Earth’s surface, intrusive rocks form.
Magma that cools and crystallizes on Earth’s surface forms extrusive rocks.
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Classification of Igneous Rocks

5. Mineral Composition of Igneous Rocks
Igneous rocks are classified by their mineral compositions.
Basaltic rocks, also called mafic rocks, are dark-colored, have lower silica contents, and contain mostly plagioclase and pyroxene.
Granitic rocks, also called felsic rocks, are light-colored, have high silica contents, and contain mostly quartz and feldspar.
Classification of Igneous Rocks
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6. Mineral Composition of Igneous Rocks
Rocks that have a composition of minerals that is somewhere between basaltic and granitic are called intermediate rocks.
A fourth category, called ultrabasic, or ultramafic, contains rocks with only iron-rich minerals such as olivine and pyroxene and are always dark.
Classification of Igneous Rocks
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7. Mineral Composition of Igneous Rocks
Rock type can be determined by estimating relative percentages of minerals in the rocks.
Classification of Igneous Rocks
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8. Texture
Texture refers to the size, shape, and distribution of the crystals or grains that make up a rock.
Classification of Igneous Rocks
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9. Texture Crystal size and cooling rates
When lava flows on Earth’s surface, it cools quickly and there is not enough time for large crystals to form. The resulting extrusive igneous rocks have crystals so small that they cannot be seen without magnification.
Classification of Igneous Rocks
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10. Texture Crystal size and cooling rates
When lava cools so quickly that crystals do not form, volcanic glass is the result.
When magma cools slowly below Earth’s surface, there is sufficient time for large crystals to form. Thus, intrusive rocks can have crystals larger than 1 cm.
Classification of Igneous Rocks
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11. Texture Porphyritic rocks
A porphyritic texture is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals.
Classification of Igneous Rocks
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12. Texture Vesicular rocks
The spongy appearance that results from gas bubbles trapped in lava is called vesicular texture.
Classification of Igneous Rocks
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13. Thin Sections
Scientists use a special microscope, called a petrographic microscope, to help them identify minerals by examining the mineral grains in sections of rock thin enough for light to pass through.
Classification of Igneous Rocks
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14. Igneous Rocks as Resources
Veins
Valuable ore deposits often occur within igneous intrusions. At other times, they may occur as veins in the rocks surrounding intrusions.
Classification of Igneous Rocks
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15. Igneous Rocks as Resources
Pegmatites
Veins of extremely large-grained minerals that can contain ores are called pegmatites.
Classification of Igneous Rocks
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16. Igneous Rocks as Resources
Kimberlites
Diamonds and other minerals that can form only under very high pressure are found in rare, ultrabasic rocks known as kimberlites.
Classification of Igneous Rocks
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17. Igneous Rocks as Resources
Igneous rocks in construction
The interlocking grain textures and the resistance to weathering of many of the minerals present in igneous rocks make them especially useful as building materials.
Classification of Igneous Rocks
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18. Add link to Animation from page 68 here.
Ionic Bonds
Concepts In Motion
FPO
Add link to Animation from page 68 here.
Classification of Igneous Rocks
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19. Review Essential Questions Vocabulary
What are the different types and textures of igneous rocks?
How do cooling rates affect the grain sizes in igneous rocks?
What are some of the uses of igneous rocks?
Vocabulary
intrusive rock
extrusive rock
basaltic rock
granitic rock
texture
porphyritic texture
vesicular texture
pegmatite
kimberlite
Classification of Igneous Rocks
Copyright © McGraw-Hill Education