Igneous Rocks

1. Igneous Rocks Chapter 3.2

2. Igneous Rocks • The word igneous comes from the Latin word igniswhich 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 magmacools 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 nothave 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 crystalsat 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) Texture Aphanitic (Fine-Grained) Texture Vesicular and Glassy Texture Porphyritic 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 Rock Dark-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 Felsic/Granitic- Notice Pink Granite is also Felsic. White is not always the color of rocks with a Felsic Composition. 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

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