1. Igneous Rocks
2/22/2019

2. Igneous Rocks Rock heated to liquid (molten) state is called magma.
When magma cools and hardens it becomes Igneous rock.
Different minerals solidify (or melt) at different temperatures
Usually degrees Celsius
2/22/2019

3. Igneous Rocks  On planet Earth where does magma cool slowly?
Slowly cooling magma forms rock with large mineral crystals
Ex. Granite
 On planet Earth where does magma cool slowly?
2/22/2019

4. Igneous Rocks  On planet Earth, where does magma cool rapidly?
Rapidly cooling magma forms rock with small mineral crystals
Often too small to see
Ex. basalt
 On planet Earth, where does magma cool rapidly?
2/22/2019

5. Igneous Rocks  Where might obsidian form?
When magma cools very fast crystals have no time to form
Ex. Obsidian
 Where might obsidian form?
2/22/2019

6.  Igneous Rock Formation:
Crystal Size vs. Cooling Rate of Magma
Draw a line!
Rate of Cooling
Fast Slow
Tiny Large
Crystal Size
2/22/2019

7. Intrusive Igneous Rocks
Are formed deep underground
Usually have large, well formed crystals
Also called plutonic igneous rocks
Example: granite
2/22/2019

8. Extrusive Igneous Rocks
Are formed at or near the Earth’s surface
Usually have small mineral crystals
Also called volcanic igneous rocks
Example: obsidian
2/22/2019

10.  Igneous Rock Formation:
Crystal Size vs. Depth beneath Earth’s surface
Draw a line!
Depth below Earth’s surface
Deep Shallow
Tiny Large
Crystal Size
2/22/2019

11. Igneous Rock Characteristics
Do not usually contain layers
Varied in color
Color depends on minerals present
Color and crystal size are used to identify igneous rocks
2/22/2019

12.  Describe a situation in which both small and large crystals would be found in the same igneous rock.
2/22/2019

13. Igneous Rocks - Color
Light colored igneous rocks are usually rich in feldspar (fel) and silica (s)
Called felsic
Dark colored igneous rocks are usually rich in magnesium (Ma) and iron (Fe)
called Mafic igneous rocks
Exceptions
Obsidian is felsic but appears dark in color
2/22/2019

14. 2/22/2019

15. 2/22/2019

16. Mafic Igneous Rocks Known as basaltic Dense Dark colored
Make up Earth’s ocean floors
2/22/2019

17. Felsic Igneous Rocks Known as granitic
Less dense than mafic igneous rocks
Light colored
Form Earth’s continents
2/22/2019

18.  2 examples:  2 examples: Cooling Magma • volcanic or extrusive
Rapid cooling
Slow cooling
• volcanic or extrusive
• crystal size is small
- fine texture
 2 examples:
• Plutonic or intrusive
• crystal size is large
- coarse texture
 2 examples:
2/22/2019

19. 3 steps for identifying igneous rocks
Determine texture (you have to look at the rock!)
Determine color
Determine what minerals are present, if you can see them
2/22/2019

20. Igneous Rock Identification--step 1
Determine the texture (crystal size):
Can I see the crystals by eye?
yes no
fine textured
coarse textured
2/22/2019

21. Igneous Rock Identification--step 2
Determine the relative color:
light
colored
(felsic)
 Example:
In-between
 Example:
dark
colored
(mafic)
 Example:
2/22/2019

22. Igneous Rock Identification--step 3
Determine mineral composition - what minerals are in the rock?
 Use physical characteristics and special properties of minerals such as:
2/22/2019

23. 2/22/2019