Earth Systems 3209

1. Earth Systems 3209 Reference: Chapters 2, 3, 6, 7; Appendix A & B Unit: 3Earth’s Materials

2. Unit 3: Topic 3.3 Classification of Igneous Rocks (Texture) Focus on . . . • explaining the relationships between geologic environment, rate of cooling and texture. • identifying five common igneous rock textures. • explain how different Igneous rocks can have the same texture.

3. Classification of Igneous Rocks: • Igneous rocks are further classified according to: • 1. Texture • Describes the appearance of an igneous rock, based on the size, shape and arrangement of interlocking crystals. • 2. Mineral Composition • Is the mineral makeup of an igneous rock based on the chemical composition of the magma. Text Reference: pages 61-75

4. Igneous Rock Texture: Texture a. Size b. Shape of interlocking crystals c. Arrangement

5. Igneous Rock Texture: • The texture of an Igneous rock reveals a great deal about the environment in which the rock cooled and solidified. • Crystal size is the most important factor affecting texture and the size of the crystals are determined by cooling rate.

6. Igneous Rock Texture: • Cooling Rate • Molten rock (magma) can cool beneath Earth’s surface (Plutonic environment). • Large crystals form deep within Earth where magmas may take up to tens of thousands of years to cool and crystallize. • Therefore, the slower the magma cools, the larger the crystals. Called a Coarse Texture. • Gabbro, diorite, and granite are examples of intrusive or plutonic igneous rocks Magma Cools Slowly Coarse Texture

7. Igneous Rock Texture: • Cooling Rate • Molten rock (lava) can cool on Earth’s surface. • Fine crystals form on or near Earth’s surface where lava cools quickly in the matter of minutes to hours. • Therefore, the quicker the lava cools, the smaller the crystals. Called a Fine Texture. • Basalt, andesite, and rhyolite are examples of extrusive or volcanic igneous rocks. Lava Cools Quickly Fine Texture

8. Igneous Rock Texture: cooling rate crystal size Slow cooling Larger Crystals Fast Cooling Smaller or No Crystals

9. Orthoclase Feldspar Amphibole Quartz Igneous Rock Texture: • 1. Coarse Grained Texture: • forms deep beneath the surface in a plutonic environment where the molten rock cools slowly. • larger crystals of uniform size. • also called Phaneritic texture. • minerals can be identified withthe unaided eye. Examples: Granite Gabbro Diorite .

10. Igneous Rock Texture: • 2. Fine Grained Texture: • forms at or near the Earth’s surface in volcanic environment where the molten rock cools quickly. • very small crystals (only seen under microscope) • may contain vesicles (gas bubbles) • also called Aphanitictexture. • minerals are not identified with the unaided eye. Examples: Basalt AndesiteRhyolite

11. Igneous Rock Texture: • 3. PorphyriticTexture: • this texture results when magma with crystals already formed escape to the surface and cools quickly forming a fine grained igneous rock with large crystals inside. • this texture is a result of two stages of cooling; • 1) slow cooling forming the larger crystals. • 2) rapid cooling forming the finer crystals. Examples: Andesite Porphyry

12. Igneous Rock Texture: • 4. Glassy Texture: • forms when the ions in the molten rock do not have time to organize into an orderly pattern to form crystals because it cools very, very quickly. • the result is volcanic glass. • minerals can not identified with the unaided eye. Examples: Obsidian

13. Igneous Rock Texture: • 5. Vesicular Texture: • forms when gas bubbles escape from molten rock and are trapped as it cools and crystallizes. • this texture can form near the top of lava flows. • minerals can not identified with the unaided eye. Examples: Scoria

14. Igneous Rock Texture: Coarse-grained Fine-grained Porphyritic Glassy

15. Igneous Rock Texture: Classification Chart

16. Igneous Rock Texture:

17. Example 1: Which rock-forming process will result in a glassy texture? alignment of minerals as a result of stress cementing together of large, angular crystals (C) rapid cooling of lava at Earth’s surface (D) recrystallization of a rock during metamorphism

18. Example 2: An igneous rock is found to contain both large and small crystals, as shown below. What conditions were necessary for this rock to form? Answer: This texture is a result of two stages of cooling; 1) slow cooling forming the larger crystals. 2) rapid cooling forming the finer crystals.

19. Your Turn . . . Take the time and complete the following questions . . .(Solutions to follow) Question: Describe two things that can be determined by examining the crystal size of minerals in igneous rocks.

20. Solutions . . . Question: Describe two things that can be determined by examining the crystal size of minerals in igneous rocks. Crystal size of minerals in igneous rocks can indicate the following: The environment in which the molten rock cooled, intrusive (plutonic) or extrusive (volcanic). The cooling rate of the molten rock. Large crystals suggest a slow cooling rate and small crystals suggest a faster cooling rate.

21. Summary . . . Overview of Points covered: • Texture is controlled by cooling rate, which is determined by the environment in which the rock formed.