1. Igneous Rocks

2. Warm-up Compare and contrast rocks and minerals.
Rocks are made of one or more mineral
A rock is not always a mineral (ex: coal, obsidian)

3. Overview of Igneous Rocks
Koryakskaya
Sopka Volcano,
Eastern Russia
Igneous means from fire
Form when minerals crystallize from magma
Intrusive = cools inside = magma = Plutonic
Extrusive = cools outside= lava = Volcanic
Magmas derived from below the Earth’s surface in the mantle
Magma is hot and buoyant

4. It’s all about heat and density
Heat source??
Geothermal Gradient
Composition
Hot stuff
Heat source = radioactive decay
Geothermal gradient = 25 deg C/km
Composition = heavier elements make heavier minerals/rocks
Hot Stuff = density of a material is dependent on temperature

5. Intrusive Vs. Extrusive
Plutonic
Formed within the Earth
Magma
Reach Surface by uplift and erosion of the Earth’s Crust
Volcanic
Formed at the Surface
Lava

6. Igneous Rock Classification
Composition
Texture
Felsic
Intermediate
Mafic
Ultramafic
Course-Grained
Granite
Diorite
Gabbro
Peridotite
Porphyritic Course-Grained
Porphyritic Granite
Porphyritic Diorite
Porphyritic Gabbro
Fine-Grained
Rhyolite
Andesite
Basalt
Porphyritic Fine-Grained
Porphyritic Rhyolite
Porphyritic Andesite
Porphyritic Basalt

7. Texture Related to the cooling history of the rock Other textures:
Really Fast = no mineral grains glassy
Fast = Small/ microscopic mineral grains Fine-Grained
Slow = large mineral grains Course-Grained
Complex = Mixture  porphyritic
Why??
Other textures:
Vesicular: trapped gases in lava
Pyroclastic: ash and rock fragments formed explosively

8. Texture Fine-Grained Cooled quickly
Peanut Butter or
Sugar Cookie
Fine-Grained
Cooled quickly
Crystallized at the Earth’s surface

9. Fine-Grained--peanut butter cookie

10. Texture Porphyritic Fine-Grained Two stages of cooling
Chocolate Chip Cookie
Porphyritic Fine-Grained
Two stages of cooling
1st cooled slowly within the Earth (larger cyrstals - Phenocrysts)
2nd cooled rapidly on the Earth’s surface (fine-grained matrix)

11. Porphyritic Fine-Grained —Chocolate Chip Cookie

12. Texture Course-Grained Cooled slowly Crystallized within the Earth
Oatmeal Cookie

13. Course-Grained—Oatmeal Cookie

14. Texture Porphyritic Course-Grained Oatmeal Raisin Cookie
Two stages of cooling
1st cooled slowly within the Earth (larger crystals - Phenocrysts)
2nd cooled faster but still slow enough that crystals fully develop – within the Earth (coarse-grained matrix)

15. Porphyritic Course-Grained —Oatmeal Raisin Cookie

16. Texture
Vesicular –voids left by trapped gas

17. Texture Glassy Very rapid cooling
Ions do not have time to from crystalline structures

18. Texture
Pyroclastic – welded shards of rock & ash ejected from a vent during an eruption

19. Composition of Igneous Rocks
Silica (Si02) is primary ingredient of all magmas
Quartz is basically just silicate
Viscosity: Resistance to flow
Affected by 2 main things:
Silica content
temperature
Rocks are made of minerals

20. Composition—Silica Content
Felsic: Feldspar & Silica
>65% silica  High Viscosity
Intermediate:
53-65% silica  Intermediate Viscosity
Mafic: Magnesium and Iron (Ferris)
45-52% silica  Low Viscosity
Ultramafic:
<45% silica  Very Low Viscosity

21. Why does it matter?
Higher viscosity makes a more explosive volcano!

22. Composition
Felsic-rhyolitic:
Light colored
Quartz, feldspar, mica

23. Composition
Mafic-basaltic:
Dark Colored
 pyroxene and olivine

24. Composition Intermediate-andesitic: Salt & Pepper appearance
feldspar, mica, hornblende, and/or pyroxene
Andesite Porphyry
Diorite

25. Volcanic igneous rocks Plutonic igneous rocks
Warm-up: Complete the following table by identifying which of the characteristics in the left-hand column are present in volcanic and/or plutonic igneous rocks by stating yes or no for the appropriate number. One characteristic has been completed as an example.
Characteristic
Volcanic igneous rocks
Plutonic igneous rocks
May form from mafic magma 1. 2.
Form at Earth’s surface 3. 4.
Have texture 
Made of small grains 5. 6.
Granite is an example 7. 8.
Form as a result of melting 9.
10.
Present at Earth’s surface only after erosion
11.
12.
Contains minerals
13.
14.
Classified based on color
15. 16
Dark-colored examples have low silica content 17
18.
Contain visible grains
19.
20.

26. Bowen’s Reaction Series
Melting
Crystallization
Hot
Cold
Different minerals crystallize from magmas at different temperatures
Each mineral has it’s own hardening temp

27. Magmatic Differentiation
Formation of more than one type of magma from a single parent magma
3 ways it can happen
Crystal setting
Assimilation
Magma mixing

28. Magmatic Differentiation
Crystal Settling: crystallized minerals are more dense than the magma and settle to the bottom
Marbles analogy
Because Fe and Mg crystalize first, melt becomes rich in SiO2, Na, and K
Different minerals crystalize based on elements available

29. Magmatic Differentiation
Assimilation: magma reacts with the “country rock” which is adjacent to the magma chamber
“country rock” is the rock surrounding the magma chamber
Magma composition is altered according to the composition of the assimilated country rock
Inclusions are rocks Incompletely melted chunks of country rock

30. Magmatic Differentiation
Magma Mixing: Magmas of different compositions are mixed together
Resulting magma is of a composition intermediate between the parent magmas

31. Magma Mixing

32. Magma Mixing

33. Magma Mixing