Transcript Chapter 2: Rocks

Section 2: 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)
 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.
 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)
 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 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
 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
 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. origin
b. size
c. texture
d. mineral composition
2. Which type of igneous rock is formed from lava that
erupted onto Earth’s surface?
a. extrusive
b. intrusive
c. extrinsic
d. intrinsic
3. Which type of igneous rock is formed when magma
hardens beneath Earth’s surface?
a. extrusive
b. intrusive
c. extrinsic
d. intrinsic
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
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