Section 2: Igneous Rocks

OriginExtrusive: formed from lava that erupted onto

Earth’s surface (e.g. basalt)Intrusive: formed when magma hardened

beneath Earth’s surface (e.g. granite)

Question: What are the two types of igneous rock and what is the difference between them?

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.

TextureDepends on the size and shape of its mineral

crystalsIntrusive rocks are usually coarse-grained because they

cool slowly so their mineral crystals are largeExtrusive rocks are usually fine-grained or glassy

(obsidian) because they cool quickly so their mineral crystals are small or non-existent (obsidian)

Question: How is the texture of extrusive rocks different from the texture of intrusive rocks? Why?

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.

Mineral CompositionDetermines 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

Useful because they are hard, dense, and durableBuilding materials (mostly granite)

During ancient times: Egypt, Incas1800s and early 1900s U.S.: bridges, public buildings,

cobblestone Modern day: countertops, curbstones, floors, gravel

(basalt)Other Uses

Pumice – abrasive for cleaning and polishingObsidian – sharp tools for cutting and scraping

Question: Why can’t obsidian be used as an abrasive?

Question: Why can’t obsidian be used as an abrasive?

Answer: Because of its glassy texture.

1. Which factor is not used to classify igneous rocks?a. originb. sizec. textured. mineral composition

2. Which type of igneous rock is formed from lava that erupted onto Earth’s surface?

a. extrusiveb. intrusivec. extrinsicd. intrinsic

3. Which type of igneous rock is formed when magma hardens beneath Earth’s surface?

a. extrusiveb. intrusivec. extrinsicd. intrinsic

4. Which type of igneous rock has a coarse-grain? Why?

a. extrusive because it cools slowlyb. extrusive because it cools quicklyc. intrusive because it cools slowlyd. intrusive because it cools quickly

5. Which is not a characteristic of igneous rocks that makes them well-suited to make tools and building materials?

a. hardb. densec. durabled. light