1. Chapter 5.2 – Classification of Igneous Rocks
Two main types of igneous rock:
Intrusive rocks form when magma cools and crystallizes below Earth’s surface. Crystals large enough to see without magnification
Extrusive rocks form when magma cools and crystallizes on Earth’s surface (lava flows). Small crystals that need magnification to see

2. Types of Classification
Properties
Grain size
Texture
Mineral composition
Basaltic rocks – mafic rocks, dark-colored, low silica contents, and contain mostly plagioclase and pyroxene
Granitic rocks – felsic rocks, light-colored, have high silica content, and contain mostly quartz and feldspar

3. Types of Classification
Intermediate rocks – rocks that have mineral compositions in between the two others
Ultrabasic – ultramafic, contains rocks with only iron-rich minerals and are always dark
Texture refers to size, shape, and distribution of crystals/grains that make up a rock
Examples: fine-grained, coarse-grained

4. Other Properties
When lava cools on the surface, it cools so quickly that only small crystals form
Sometimes crystals don’t form at all
This results in volcanic glass called obsidian

5. Other Properties
When lava cools slowly under the surface, large crystals form – granite, diorite, gabbro

6. Porphyritic Rocks
Porphyritic texture – large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals
Complex cooling history (example: slow cooling then rapid cooling) causes different crystal sizes

7. Vesicular Rocks
Vesicular texture – Spongy appearance formed when gas is prevented from escaping.
The holes left behind are called vesicles
Examples are pumice and vesicular basalt

8. Thin Sections
A Thin Section is a slice of rock 2 x 4 cm and only 0.03 mm thick
Light is able to pass through this sample
These samples are viewed through a petrographic microscope and geologists look for certain properties
Examples of properties: feldspar grains have banding called “twinning,” quartz grains look wavy as stage is rotated, etc.

9. Igneous Rocks as Resources
Useful in many fields such as
Construction
Energy production
Jewelry making
Ores can be extracted from veins of lava flows
Gold, silver, lead, copper
These elements released at end of magma crystallization process

10. Igneous Rocks as Resources
Pegmatites are igneous rocks made of extremely large-grained materials
Found as igneous intrusions or veins
Rare elements such as Lithium, Beryllium form in these ores
They also produce complex crystals

11. Igneous Rocks as Resources
Kimberlites are ultrabasic rocks that were first found in Kimberly, South Africa
Only formed under very high pressure
Valuable minerals such as diamonds
Most diamonds in the world are found in South African mines. Only one diamond mine in the US in Colorado

12. Igneous Rocks as Resources
Igneous rocks are strong building materials
Durable
Weather-resistant
Countertops, floors, statues, etc.