1. Rocks (Igneous)

2. Rocks
Naturally formed, consolidated material composed of the grains or one or more minerals
Three Types of Rocks
Igneous
Sedimentary
Metamorphic

3. Igneous Rocks Igneous rocks form from cooling magma or lava
Magma: molten rock below the Earth’s surface
Lava: magma on the Earth’s surface

4. Classifying Igneous Rock
Igneous rocks can be classified by:
Mineral composition
Texture (crystal size)

5. Types of Igneous Rocks
Intrusive Igneous Rocks: formed from magma cooling
Cool inside the Earth
Large crystals, coarse-grained (most crystals >1mm)
Phaneritic
Cools slowly
Extrusive Igneous Rocks: formed from lava cooling
Cool on the surface
Small crystals, fine-grained (most crystals <1mm)
Aphanitic
Cools quickly
Fine-grained igneous rock
Coarse-grained igneous rock

6. Other Igneous Textures
Porphyritic Texture
Minerals form at different temperatures (1st slow, then fast)
Large crystals are embedded in a matrix of smaller crystals
Glassy Texture
Very rapid cooling of lava
Ex. Obsidian

7. Porphyritic Texture

8. Glassy Texture

9. Other Igneous Textures
Pyroclastic Texture
Composed of fragments ejected during a volcanic eruption
Tuff: ash-sized fragments
Volcanic breccia: particles larger than ash
Vesicular Texture
Volcanic rock having many cavities (vesicles) at the surface and inside

10. Pyroclastic Texture

11. Vesicular Texture

12. Igneous Compositions
Igneous rocks are composed primarily of silicate (SiO2) minerals
Dark (or ferromagnesian) silicates
Olivine, pyroxene, amphibole, and biotite mica
Light (or nonferromagnesian) silicates
Quartz, muscovite mica, and feldspars

13. Igneous Compositions Felsic versus Mafic compositions
Felsic composition
Light-colored silicates
Termed felsic (feldspar and silica) in composition
High amounts of silica (SiO2)
Major constituent of continental crust

14. Igneous Compositions Felsic versus Mafic compositions
Dark silicates and calcium-rich feldspar
Termed mafic (magnesium and ferrum, for iron) in composition
Denser than granitic rocks
Comprise the ocean floor and many volcanic islands

15. Igneous Compositions Other compositional groups
Intermediate (or andesitic) composition
Contains 25% or more dark silicate minerals
Associated with explosive volcanic activity
Ultramafic composition
Rare composition that is high in magnesium and iron
Composed entirely of ferromagnesian silicates

16. Rock Types Naming igneous rocks – Felsic rocks Granite Phaneritic
Over 25% quartz, about 65% or more feldspar
Very abundant - often associated with mountain building
The term granite includes a wide range of mineral compositions

17. Granite

18. Rock Types Rhyolite Naming igneous rocks – Felsic rocks
Extrusive equivalent of granite
May contain glass fragments and vesicles
Aphanitic texture
Less common and less voluminous than granite

19. Rhyolite

20. Pumice is very glassy and sharp, with countless vesicles.
Rock Types
Naming igneous rocks – Felsic rocks
Obsidian
Dark colored
Glassy texture
Pumice
Volcanic
Frothy appearance with numerous voids
Pumice is very glassy and sharp, with countless vesicles.

21. Rock Types Andesite Diorite Naming igneous rocks – intermediate rocks
Volcanic origin
Aphanitic/Porphyritic texture
Diorite
Plutonic equivalent of andesite
Coarse grained- Phaneritic

22. Andesite

23. Diorite

24. Rock Types Basalt Naming igneous rocks – Mafic rocks Volcanic origin
Aphanitic texture
Composed mainly of pyroxene and calcium-rich plagioclase feldspar
Most common extrusive igneous rock

25. Basalt

26. Rock Types Gabbro Naming igneous rocks – mafic rocks
Intrusive equivalent of basalt
Phaneritic texture consisting of pyroxene and calcium-rich plagioclase
Significant % of the oceanic crust

27. Gabbro

28. Andesitic (Intermediate)
COMPOSITION
Felsic
Andesitic (Intermediate)
Mafic
Ultramafic
>66% Silica
54-66% Silica
45-54% Silica
<45% Silica
ORIGIN
TEXTURE
ROCK NAME
Intrusive
Phaneritic
Granite
Diorite
Gabbro
Peridotite
(coarse-grained)
Porphyritic
Extrusive
Aphanitic
Rhyolite
Andesite
Basalt
Komatite (rare)
(fine-grained)
Glassy
Obsidian
Vesicular
Pumice
Scoria
Pyroclastic
Rhyolite Tuff
Andesite Tuff
Basalt Tuff