1. Igneous Rocks

2. Standards Classify matter in a variety of ways
Describe the composition and structure of Earth’s materials, including:
the major rock types (i.e., sedimentary, igneous, metamorphic) and their formation

3. Igneous Rocks Rocks formed from magma
Make up 95% of Earth’s crust, much of which is covered by thin layer of sedimentary rocks

4. Formation of Igneous Rocks
Form from the solidification of magma (molten rock)
Hot temperatures in the aesthenosphere (600  C) cause rock to melt
As rock heats up, it becomes less dense and rises
When it cools and solidifies, either underground or above ground, it becomes igneous rock

5. Classification of Igneous Rocks
Classification is based on:
Where rock is formed
Texture (which is directly affected by where rock is formed)
Mineral content (what minerals are in rock)

6. Environment where Rock is Formed
Two main groups of igneous rocks:
Intrusive (plutonic)
Extrusive (volcanic)

7. Intrusive (Plutonic) Named after Pluto, god of the underworld
Forms from solidification of magma underground
Forms 1 kilometer or deeper below surface of Earth
Ex: granite

9. Granite
Photo:

10. Extrusive (Volcanic) Named after Vulcan, god of fire
Forms from eruption and solidification of magma (lava) on surface of Earth
Forms less than 1 kilometer deep
Ex: basalt

12. Textures
Rock texture refers to the size, shape and arrangement of mineral grains

13. Intrusive Texture
Intrusive magmas cool slowly, over hundreds of thousands of years, due to insulating properties of surrounding rocks.
This allows time for crystals to grow = larger crystals.
These rocks are medium to large grained, called Phaneritic
Ex: granite – most abundant rock in Earth’s crust

14. Intrusive Texture

15. Extrusive Texture Extrusive magmas (lavas) cool rapidly.
There is little time for crystal growth = very small or micro crystals.
These rocks are fine grained, called aphanitic
Ex: basalt

16. Extrusive Texture

17. Extrusive Texture
Some extrusive lavas cool so rapidly there is no time for crystal growth
In this case they have a glassy texture
A glassy texture can be either massive (like obsidian [volcanic glass]) or frothy (like pumice)

18. Obsidian
Pumice

19. Extrusive Texture
Some extrusive rocks can contain holes that are called vesicles.
Vesicles form when gases (air bubbles) escape from the cooling rock leaving behind a cavity.

20. Vesicular Basalt

21. ExtrusiveTexture
Some extrusive rocks form a porphyry (or porphyritic texture) – large crystals, called phenocrysts, are present in a fine-grained groundmass.
Chocolate chip cookie analogy: chocolate chips are like the phenocrysts and cookie dough is like the groundmass.

22. Porphyritic Texture

23. Color Classification
Felsic rocks – igneous rocks that are light colored. They are rich in minerals that contain lots of silica.
Mafic rocks – dark colored igneous rocks. They are rich in iron and magnesium
Intermediate – rocks midway between felsic and mafic.
Ultramafic – very dark rocks, containing very little silica.

24. Igneous Rock Pairs
Igneous rocks are classified in pairs, in which members of the pairs contain identical minerals, but have different textures (intrusive vs. extrusive)
Color Intrusive Extrusive
Felsic Granite Rhyolite
Intermediate Diorite Andesite
Mafic Gabbro Basalt
Ultramafic Peridotite

25. Felsic
Granite
Rhyolite

26. Intermediate
Diorite
Andesite

27. Mafic
Gabbro
Basalt

28. Ultramafic
Peridotite