1. IGNEOUS ROCKS Introduction Igneous Compositions Igneous Textures
Classification

2. Igneous Rocks
Introduction

3. Rocks Most are aggregates of mineral crystals
PSCI 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
PSCI 131: Igneous Rocks – Introduction
Rocks
Igneous
Sedimentary
Metamorphic

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

6. Magma’s chemical composition Magma’s cooling rate
PSCI 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

7. Igneous Rocks
Igneous Compositions

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

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

10. GRANITIC 0-25% dark minerals PSCI 131: Igneous Rocks – Compositions
This GRANITE contains about 10% dark minerals
From: geology.com

11. GRANITIC 0-25% dark minerals PSCI 131: Igneous Rocks – Compositions
RHYOLITE is also felsic in composition
From: itc.gsw.edu

12. ANDESITIC 25-45% dark minerals PSCI 131: Igneous Rocks – Compositions
DIORITE: This sample is about 40% dark crystals
ANDESITE: Another intermediate rock
From: nwnature.net
From: geology.com

13. BASALTIC 45-85% dark minerals PSCI 131: Igneous Rocks – Compositions
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

14. Igneous Rocks
Igneous Textures

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

16. Coarse-grained Most crystals visible to naked eye
PSCI 131: Igneous Rocks – Textures
Coarse-grained
Most crystals visible to naked eye
From: geology.com
From: nwnature.net

17. Coarse-grained Slow cooling rate deep within crust
PSCI 131: Igneous Rocks – Textures
Coarse-grained
Slow cooling rate deep within crust
Crystals have time to accumulate more atoms & grow large
From: windows2universe.org

18. Fine-grained Most crystals not visible to naked eye
PSCI 131: Igneous Rocks – Textures
Fine-grained
Most crystals not visible to naked eye
From: geology.com
From: itc.gsw.edu

19. Fine-grained Rapid cooling rate on surface (volcanism)
PSCI 131: Igneous Rocks – Textures
Fine-grained
Rapid cooling rate on surface (volcanism)
Not enough time for crystals to grow large
From: bbc.co.uk

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

21. PORPHYRITIC Two distinct crystal sizes in same rock
PSCI 131: Igneous Rocks – Textures
PORPHYRITIC
Two distinct crystal sizes in same rock
Phenocrysts
Groundmass
From: pitt.edu

22. PORPHYRITIC Two-stage cooling First, slow cooling produces phenocrysts
PSCI 131: Igneous Rocks – Textures
PORPHYRITIC
Two-stage cooling
First, slow cooling produces phenocrysts
Then, rapid cooling of remaining magma produces groundmass

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

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

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

26. Igneous Rocks
CLASSIFICATION

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

28. CLASSIFICATION TABLE PSCI 131: Igneous Rocks – Classification GRANITIC
ANDESITIC
BASALTIC
COARSE-GRAINED
Granite
Diorite
Gabbro
FINE-GRAINED
Rhyolite
Andesite
Basalt
GLASSY
OBSIDIAN
VESICULAR
SCORIA (dark)
PUMICE (light)
PYROCLASTIC
WELDED TUFF
PORPHYRITIC: Used as a modifier for any of the six basic rock names.

29. End of Igneous Rocks