1. Chapter 6 Rocks
6.2 Igneous Rocks

3. Igneous Rock Formation
Form from the cooling of molten rock
Extrusive (volcanic)- form at or near the surface from lava
Intrusive (plutonic) – form under the surface from magma

4. Igneous Rock Formation
Igneous rocks are classified by mineral composition (mafic or felsic) and texture
Texture
is the size, shape and arrangement of grains (crystals)

5. Texture Grain or crystal size depends on: Rate of cooling
Fast = small
Slow = large
Dissolved gasses
More = larger
Order of solidification

6. The Starting Material
Magma is Classified as Felsic, Mafic or and intermediate
Felsic
Light colored
High silica magma with abundance of quartz and feldspar.
Thick slow flowing in molten state.
Most intrusive rocks.
Acidic

7. The Starting Material Mafic Dark colored,
Low silica ferromagnesian magma
More fluid than felsic magma.
Most extrusive igneous rocks.

8. Underground Magma
Magma can cool slow or fast depending on where cooling occurs
Intrusive rocks cool very slowly underground
Result of slow cooling
Coarse (large) grains

9. INTRUSIVE – large grains
GRANITE
GABBRO

10. At the Surface Magma pouring onto the surface is called lava
Extrusive igneous rocks cool rapidly
Microscopic grains or none at all

11. EXTRUSIVE – fine grains
OBSIDIAN
BASALT

12. At the Surface Porphyry
Rocks that cool both underground and at the surface
Results in large and small grains

13. PORPHYRY – large grains embedded in fine grains
RHYOLITE PORPHYRY

14. Igneous families Grouped according to mineral composition
Texture is determined where it cools – intrusive or extrusive
Each family can have coarse grained, fine grained or glassy textures
Also can have vesicles
Vesicles are gas cavities within the rock

15. Igneous Families Granite family Gabbro Family Felsic magma
Light colored
Quartz and Feldspar
Coarse
Granite
Fine
Rhyolite
Glassy
Pumice and Obsidian
Gabbro Family
Mafic magma
Dark colored
Pyroxene and Olivine
Coarse
Gabbro
Fine
Basalt
Vesicular
Scoria

16. Igneous Families Diorite family Intermediate Coarse Fine Diorite
Andesite

17. IGNEOUS INTRUSIONS
PLUTONS – A ROCK MASS THAT FORMS WHEN MAGMA COOLS UNDER GROUND
DIKES
SILLS
LACOLITHS
VOLCANIC NECKS
BATHOLITHS – LARGEST OF ALL PLUTONS, FORM THE CORES OF MOST MOUNTAINS

18. Laccolith
Volcanic neck
Stock
Dike
Sill
Batholith

19. DIKE

20. SILL

21. LACOLITH

22. VOLCANIC NECK

23. BATHOLITH