1. Section 2: Igneous Rocks
Chapter 5: Rocks
Section 2: Igneous Rocks

2. Classifying Igneous Rocks
Origin
Extrusive: formed from lava that erupted onto Earth’s surface (e.g. basalt)
Intrusive: formed when magma hardened beneath Earth’s surface (e.g. granite)

3. Lecture Checkpoint 1
Question: What are the two types of igneous rock and what is the difference between them?

4. Lecture Checkpoint 1
Question: What are the two types of igneous rock and what is the difference between them?
Answer: Extrusive and intrusive. Extrusive rocks are formed from lava that erupted onto Earth’s surface. Intrusive rocks are formed when magma hardens beneath Earth’s surface.

5. Classifying Igneous Rocks
Texture
Depends on the size and shape of its mineral crystals
Intrusive rocks are usually coarse-grained because they cool slowly so their mineral crystals are large
Extrusive rocks are usually fine-grained or glassy (obsidian) because they cool quickly so their mineral crystals are small or non-existent (obsidian)

6. Lecture Checkpoint 2
Question: How is the texture of extrusive rocks different from the texture of intrusive rocks? Why?

7. Lecture Checkpoint 2
Question: How is the texture of extrusive rocks different from the texture of intrusive rocks? Why?
Answer: Extrusive rocks are usually fine-grained because they cool quickly above ground making their mineral crystals comparatively small. Intrusive rocks are usually coarse-grained because they cool slowly below ground making their mineral crystals comparatively large.

8. Classifying Igneous Rocks
Mineral Composition
Determines color (ex. Silica content affects how dark or light)
Geologist make thin slices of a rock and study the crystals under a microscope to determine mineral composition

9. Uses of Igneous Rocks Useful because they are hard, dense, and durable
Building materials (mostly granite)
During ancient times: Egypt, Incas
1800s and early 1900s U.S.: bridges, public buildings, cobblestone
Modern day: countertops, curbstones, floors, gravel (basalt)
Other Uses
Pumice – abrasive for cleaning and polishing
Obsidian – sharp tools for cutting and scraping

10. Lecture Checkpoint 3
Question: Why can’t obsidian be used as an abrasive?

11. Lecture Checkpoint 3
Question: Why can’t obsidian be used as an abrasive?
Answer: Because of its glassy texture.

12. Post-lesson Quiz Which factor is not used to classify igneous rocks?
a. origin
b. size
c. texture
d. mineral composition

13. Post-lesson Quiz
2. Which type of igneous rock is formed from lava that erupted onto Earth’s surface? a. extrusive b. intrusive c. extrinsic d. intrinsic

14. Post-lesson Quiz
3. Which type of igneous rock is formed when magma hardens beneath Earth’s surface? a. extrusive b. intrusive c. extrinsic d. intrinsic

15. Post-lesson Quiz
4. Which type of igneous rock has a coarse-grain? Why? a. extrusive because it cools slowly b. extrusive because it cools quickly c. intrusive because it cools slowly d. intrusive because it cools quickly

16. Post-lesson Quiz
5. Which is not a characteristic of igneous rocks that makes them well-suited to make tools and building materials? a. hard b. dense c. durable d. light