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Igneous Rocks Chapter 3.2. Igneous Rocks  The word igneous comes from the Latin word ignis which means “fire”.  The reason why fire is a fitting name.

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Presentation on theme: "Igneous Rocks Chapter 3.2. Igneous Rocks  The word igneous comes from the Latin word ignis which means “fire”.  The reason why fire is a fitting name."— Presentation transcript:

1 Igneous Rocks Chapter 3.2

2 Igneous Rocks  The word igneous comes from the Latin word ignis which means “fire”.  The reason why fire is a fitting name for igneous rocks is because igneous rocks come from hot molten magma and lava.

3 The Two Types of Igneous Rocks  Intrusive Igneous Rocks – Also called Plutonic Igneous Rocks.  Intrusive Igneous Rocks are rocks that form under the ground when magma cools and crystallizes.  Magma is what we call rock when it is melted.  Extrusive Igneous Rocks- Also called Volcanic Igneous Rocks.  Extrusive Igneous Rocks are rocks that form above the ground when magma comes to the surface.  When magma reaches the surface it is called lava.

4 Igneous Rocks

5 Classification of Igneous Rocks  The way Geologists classify Igneous Rocks is by their texture and composition.  First let’s explain texture.  The texture of an igneous rock is determined by how quickly the rock cooled.  If an igneous rock cooled quickly, then it did not have time to form large crystals.  If an igneous rock formed more slowly, then it usually has larger crystals.  So to summarize, the slower the rock forms, the larger the crystal size.  Bigger crystals means more time. Smaller crystals means less time.

6 Igneous Rock Textures  Coarse-Grained Texture – the term Geologists use to describe Igneous Rocks with a Coarse-Grained Texture is Phaneritic.  Fine-Grained Texture – the term Geologists use to describe Igneous Rocks with a Fine-Grained Texture is Aphanitic.  Vesicular and Glassy – Some Igneous Rocks form so quickly they don’t form crystals at all. Sometimes they form bubbles or air pockets. Other times they form a smooth glass.  Porphyritic Texture – If an Igneous Rock has a Fine- Grained Texture with some large crystals mixed in.

7 Igneous Rock Textures Phaneritic (Coarse-Grained) TextureAphanitic (Fine-Grained) Texture Porphyritic Texture Vesicular and Glassy Texture

8 Igneous Rock Compositions  Generally speaking, you can tell an Igneous Rock’s Composition, or what it is made of by its color.  Almost all of Igneous Rocks are made up of Silicates.  The lighter color Silicates are referred to as Felsic or Granitic.  The darker color Silicates are referred to as Mafic or Basaltic.  Also as a general rule, the lighter the color, the lighter the minerals in the rock.  The darker the minerals, the heavier the minerals in the rock.

9 Felsic (Granitic) vs. Mafic (Basaltic) Light-Colored Felsic/Granitic RockDark-Colored Mafic/Basaltic Rock

10 Igneous Rock Compositions  You can also have an Igneous rock that is an intermediate or in between a mafic and felsic composition.  We refer to these rocks as andesitic, dioritic, or intermediate composition.  You can also have an Igneous Rock that is extremely Mafic.  We refer to these rocks as Ultramafic.  Ultramafic rocks often have an olive/green coloration.  The reason why they are that color is because they contain the mineral Olivine.  Also, some Felsic/Granitic rocks are colored pink.  The reason why is because they contain the mineral orthoclase feldspar.

11 Other Compositions Andesitic/Dioritic- Intermediate Color Notice it is Gray. A mix between the two compositions Ultramafic- Notice the Olive/Green Coloration that comes from the mineral Olivine Felsic/Granitic- Notice Pink Granite is also Felsic. White is not always the color of rocks with a Felsic Composition.

12 Igneous Rock Compositions  And to make matters even more complicated and confusing, the rocks that cool so quickly and don’t have time to form crystals are usually not classified by composition.  You can not determine their composition by color alone.  For example, Obsidian which is extremely dark black would seem to be mafic because it is dark.  But actually, Obsidian is considered to be a felsic rock.

13 Other Igneous Compositions: None of these rocks have a determined composition

14 Classification of Igneous Rocks


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