1. IGNEOUS ROCKS Introduction Igneous Compositions Igneous Textures
Rocks and the rock cycle
Igneous rocks
Igneous Compositions
Igneous Textures
Classification
Bowen’s Reaction Series
Origin of Magmas
Evolution of Magmas

2. Igneous Rocks
Introduction

3. Rocks Most are aggregates of mineral crystals
GEOL 131: Igneous Rocks – Introduction
Rocks
Most are aggregates of mineral crystals
A few kinds of rocks aren’t made of minerals
Granite is made of mineral crystals
Obsidian (volcanic glass) has no mineral crystals
Images from: geology.com

4. Rocks Igneous Sedimentary Metamorphic
GEOL 131: Igneous Rocks – Introduction
Rocks
Igneous
Sedimentary
Metamorphic

5. The Rock Cycle GEOL 131: Igneous Rocks – Introduction
From: mnh.isu.edu

6. Igneous Rocks Form by cooling and crystallization of magma
GEOL 131: Igneous Rocks – Introduction
Igneous Rocks
Form by cooling and crystallization of magma
From: en.wikipedia.org
From:meteorites.wustl.edu

7. Magma’s chemical composition Magma’s cooling rate
GEOL 131: Igneous Rocks – Introduction
Igneous Rocks
What kind of igneous rock will form when a magma crystallizes?
Depends on:
Magma’s chemical composition
Magma’s cooling rate

8. Igneous Rocks
Igneous Compositions

9. Igneous Rock Compositions
GEOL 131: Igneous Rocks – Compositions
Igneous Rock Compositions
What minerals the rock is made of
Orthoclase
Quartz
Hornblende
From: geology.com

10. Igneous Rock Compositions
GEOL 131: Igneous Rocks – Compositions
Igneous Rock Compositions
Percentage of dark silicate minerals
From: ux1.eiu.edu
From: geology.com
From: nwnature.net

11. FELSIC (“granitic”) 0-25% dark minerals
GEOL 131: Igneous Rocks – Compositions
FELSIC (“granitic”)
0-25% dark minerals
This GRANITE contains about 10% dark minerals
From: geology.com

12. FELSIC (“granitic”) 0-25% dark minerals
GEOL 131: Igneous Rocks – Compositions
FELSIC (“granitic”)
0-25% dark minerals
RHYOLITE is also felsic in composition
From: itc.gsw.edu

13. INTERMEDIATE (“andesitic”)
GEOL 131: Igneous Rocks – Compositions
INTERMEDIATE (“andesitic”)
25-45% dark minerals
DIORITE: This sample is about 40% dark crystals
ANDESITE: Another intermediate rock
From: nwnature.net
From: geology.com

14. MAFIC (“basaltic”) 45-85% dark minerals
GEOL 131: Igneous Rocks – Compositions
MAFIC (“basaltic”)
45-85% dark minerals
This GABBRO contains about 70% dark minerals
BASALT: Light silicate crystals are present, but are too small to see without magnification
From: ux1.eiu.edu
From: geology.com

15. ULTRAMAFIC 85-100% dark minerals
GEOL 131: Igneous Rocks – Compositions
ULTRAMAFIC
85-100% dark minerals
PERIDOTITE: This sample contains olivine and pyroxene, both dark silicate minerals
From: newark.osu.edu

16. Igneous Rocks
Igneous Textures

17. IGNEOUS TEXTURES Visual appearance, not feel
GEOL 131: Igneous Rocks – Textures
IGNEOUS TEXTURES
Visual appearance, not feel
Usually refers to size of mineral crystals
Determined by magma’s rate of cooling

18. PHANERITIC “Coarse-grained” Most crystals visible to naked eye
GEOL 131: Igneous Rocks – Textures
PHANERITIC
“Coarse-grained”
Most crystals visible to naked eye
From: geology.com
From: nwnature.net

19. PHANERITIC Slow cooling rate
GEOL 131: Igneous Rocks – Textures
PHANERITIC
Slow cooling rate
Crystals have time to accumulate more atoms & grow large
INTRUSIVE cooling
From: windows2universe.org

20. PEGMATITIC Very coarse-grained GEOL 131: Igneous Rocks – Textures
From: marlimillerphoto.com

21. APHANITIC “Fine-grained” Most crystals not visible to naked eye
GEOL 131: Igneous Rocks – Textures
APHANITIC
“Fine-grained”
Most crystals not visible to naked eye
From: geology.com
From: itc.gsw.edu

22. APHANITIC Rapid cooling rate
GEOL 131: Igneous Rocks – Textures
APHANITIC
Rapid cooling rate
Not enough time for crystals to grow large
EXTRUSIVE (volcanic) cooling
From: bbc.co.uk

23. PORPHYRITIC Two distinct crystal sizes in same rock
GEOL 131: Igneous Rocks – Textures
PORPHYRITIC
Two distinct crystal sizes in same rock
Phenocrysts
From: geo1.tcu.edu

24. PORPHYRITIC Two distinct crystal sizes in same rock
GEOL 131: Igneous Rocks – Textures
PORPHYRITIC
Two distinct crystal sizes in same rock
Phenocrysts
Aphanitic matrix
From: pitt.edu

25. PORPHYRITIC Two-stage cooling First, slow cooling produces phenocrysts
GEOL 131: Igneous Rocks – Textures
PORPHYRITIC
Two-stage cooling
First, slow cooling produces phenocrysts
Then, rapid cooling of remaining magma produces smaller crystals

26. OTHER TEXTURES GLASSY Very fast cooling
GEOL 131: Igneous Rocks – Textures
OTHER TEXTURES
GLASSY
Very fast cooling
No time for any crystals to form

27. OTHER TEXTURES VESICULAR Vesicles (bubbles)
GEOL 131: Igneous Rocks – Textures
OTHER TEXTURES
VESICULAR
Vesicles (bubbles)
Gas trapped in rapidly cooling magma
From: texasbeyondhistory.net

28. OTHER TEXTURES PYROCLASTIC Ash and rock fragments
GEOL 131: Igneous Rocks – Textures
OTHER TEXTURES
PYROCLASTIC
Ash and rock fragments
Violent volcanic eruptions
Rock fragments
Ash matrix

29. Igneous Rocks
CLASSIFICATION

30. CLASSIFICATION Why classify? Accurate communication
GEOL 131: Igneous Rocks – Classification
CLASSIFICATION
Why classify?
Accurate communication
Names contain information about how rocks formed

31. CLASSIFICATION Based on texture and composition
GEOL 131: Igneous Rocks – Classification
CLASSIFICATION
Based on texture and composition
Some rocks can be classified based only on texture

32. CLASSIFICATION FELSIC INTERM MAFIC PHANERITIC Granite Diorite Gabbro
GEOL 131: Igneous Rocks – Classification
CLASSIFICATION
FELSIC
INTERM
MAFIC
PHANERITIC
Granite
Diorite
Gabbro
APHANITIC
Rhyolite
Andesite
Basalt
PORPHYRITIC: Used as a modifier for any of the six basic rock names.

33. CLASSIFICATION (expanded)
GEOL 131: Igneous Rocks – Classification
CLASSIFICATION (expanded)
FELSIC
INTERM
MAFIC
ULTRA MAFIC
PHANERITIC
Granite
Diorite
Gabbro
Peridotite
APHANITIC
Rhyolite
Andesite
Basalt
Komatiite (rare)
GLASSY
OBSIDIAN
VESICULAR
SCORIA (dark)
PUMICE (light)
PYROCLASTIC
WELDED TUFF
PORPHYRITIC: Used as a modifier for any of the six basic rock names.

34. Igneous Rocks
ORIGIN OF MAGMAS

35. All magmas form in the crust or mantle
GEOL 131: Igneous Rocks – Origin of Magmas
All magmas form in the crust or mantle
From: facweb.bhc.edu

36. Processes That Create Magmas
GEOL 131: Igneous Rocks – Origin of Magmas
Processes That Create Magmas
“Simple” heating
Shallow crust
Decompression
Upper mantle
Hydration
From: buzzle.com

37. Pressure and Melting Point
GEOL 131: Igneous Rocks – Origin of Magmas
Pressure and Melting Point
Higher pressure = higher melting point
Rocks under high P must get hotter before they will melt

38. “Simple” Heating Heating and melting of pre-existing rock
GEOL 131: Igneous Rocks – Origin of Magmas
“Simple” Heating
Heating and melting of pre-existing rock
Only occurs in shallow crust
Top few kilometers
Deeper rock is under too much pressure
Only heat source hot enough to melt rock is another magma

39. GEOL 131: Igneous Rocks – Origin of Magmas
From: geochemistry.usask.ca

40. Decompression Melting
GEOL 131: Igneous Rocks – Origin of Magmas
Decompression Melting
Occurs without an increase in temperature
Occurs because of drop in pressure

41. GEOL 131: Igneous Rocks – Origin of Magmas
Hydration Melting

42. Hydration Melting Occurs without an increase in temperature
GEOL 131: Igneous Rocks – Origin of Magmas
Hydration Melting
Occurs without an increase in temperature
Occurs because of addition of water to solid rock above a subducting crustal plate

43. Bowen’s Reaction Series
Igneous Rocks
Bowen’s Reaction Series

44. Bowen’s Reaction Series
GEOL 131: Igneous Rocks – Bowen’s Reaction Series
Bowen’s Reaction Series
Describes sequence in which minerals form as a magma crystallizes
Different minerals crystallize at different temperatures
First crystals begin to form at about 1200°C

45. GEOL 131: Igneous Rocks – Bowen’s Reaction Series

46. Igneous Rocks
EVOLUTION OF MAGMAS

47. GEOL 131: Igneous Rocks – Evolution of Magmas
Chemical changes after magma forms, and before it crystallizes
Most magmas begin with mafic or ultramafic compositions
Lower in silica, rich in Fe and Mg

48. Processes That Change Magmas
GEOL 131: Igneous Rocks – Evolution of Magmas
Processes That Change Magmas
Crystal settling/partial melting
Assimilation
Mixing

49. Crystal Settling MAGMA (liquid)
GEOL 131: Igneous Rocks – Evolution of Magmas
Crystal Settling
Early-formed crystals settle to bottom
MAGMA (liquid)

50. GEOL 131: Igneous Rocks – Evolution of Magmas
Crystal Settling
Early crystals use up Fe and Mg, but little silica (Si and O)
As crystals form, magma becomes depleted in Fe and Mg and enriched in silica
Last minerals to form have no Fe/Mg and are silica-rich

51. Animation of crystal settling
GEOL 131: Igneous Rocks – Evolution of Magmas
Crystal Settling
Animation of crystal settling

52. Assimilation Magma melts host rock as it migrates through crust
GEOL 131: Igneous Rocks – Evolution of Magmas
Assimilation
Magma melts host rock as it migrates through crust
From: wikipedia.org
From:
jan.ucc.nau.edu

53. Mixing Magma A mixes into magma B within the crust
GEOL 131: Igneous Rocks – Evolution of Magmas
Mixing
Magma A mixes into magma B within the crust
Composition of magma B changes

54. End of Igneous Rocks chapter