1. Igneous Rocks
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2. Objectives Summarize three factors that affect whether rock melts.
Describe how the cooling rate of magma and lava affects the texture of igneous rocks.
Relate cooling time to crystal size
Classify ig rx according to their composition and texture.
Describe intrusive and extrusive ig rx structures.

3. Origin of igneous rocks
Igneous rocks are formed by molten material from a volcano or molten material within Earth
Temps are 1600 at km beneath the surface

4. Why is it so hot? Know these!
The deep rocks are under pressure from the rocks on top
Also … radioactive elements produce heat
Leftover heat from formation of the planet

5. Formation of magma Three factors affect whether rock melts Temperature
Pressure
Presence of fluids

6. This means they crystallize (and melt) at different times
Rocks have different melting points based upon the chemical composition of their minerals
This means they crystallize (and melt) at different times
Remember Bowen's

7. Partial Melting
Different minerals have different melting points, and minerals with lower melting points are the first to melt
This creates lava with a specific composition
As the magma heats up, other minerals melt and the magma’s composition changes

8. Where do Igneous Rocks form?

9. Magma—Inside Earth Lava—Outside Earth

10. Cooling time=crystal size
Magma trapped underground is insulated so it cools slowly
As it cools, the atoms arrange themselves in patterns to form crystals
If the magma cools slowly, the atoms have time to arrange large crystals.
These crystals are called mineral grains.

11. Rocks form when these mineral grains grow together.

12. Intrusive Igneous Rocks
Rocks that form beneath Earth's surface are called intrusive igneous rocks.
They have crystals large enough to see.
We call this texture/grain size: phaneritic
See handout

13. How am I ever going to remember phaneritic?
Think of large “ph”annies

14. Granite

15. Granite is an intrusive igneous rock
It cooled slowly inside the earth
It has a phaneritic texture

16. Diorite—salt and pepper look
Typical Minerals
Na/Ca PLAGIOCLASE & MAFICS (amphibole) about 50/50 Quartz - absent to trace

17. Gabbro
A mafic, phaneritic, igneous rock from the top of Bowen's Reaction Series.

18. Extrusive Igneous Rocks
When rocks form from molten lava above Earth's surface we call them extrusive igneous rocks.
They cool quickly so the atoms don't have time to arrange themselves into patterns to form crystals. They have a fine-grained texture.
This texture is called aphanitic.

19. Rhyolite

20. Rhyolite is an extrusive igneous rock
It cooled quickly,It has small grains
This is called an
aphanitic texture

21. Andesite Porphyry This is an andesite porphyry
A porphyry has big chunks in a matrix
Matrix—fine-grained background

22. Basalt Basalt makes up the ocean floors
It is a mafic igneous extrusive rock

23. Common Igneous rock terms
Type
of lava
Felsic
Intermediate
Mafic
Texture term
Intrusive
Granite
Diorite
Gabbro
Phaneritic
Extrusive
Rhyolite
Andesite
Basalt
Aphanitic
Vesicular-Frothy
Pumice
Scoria
Obsidian
Hyaline-Glassy

24. Cooled slowly cooled quickly
Pairs
Granite Rhyolite
Cooled slowly cooled quickly

25. Others-- Scoria
formed when lava cooled quickly above ground. You can see where little pockets of air had been. Scoria is actually a kind of glass and not a mixture of minerals.
Vesicular—bubbly texture

26. Pumice Vesicular texture Uses
lava soap
Stone-washed jeans
Glass. Pumice has cooled too quickly for minerals to form.
Chemical composition, however, will be from the bottom of Bowen's Reaction series (i.e. felsic).

27. Obsidian Hyaline texture is also called a glassy texture
Obsidian--Volcanic glass
Notice the conchoidal fracture

28. Pegmatite
Rock cools extremely slowly, so very big mineral grains form.

30. Classification of Magmas
Mafic—high in iron and magnesium
Felsic—high in feldspar and silica (quartz)
Mafic—50% silica
Colors-- black, green, red
Intermediate—60% silica
Felsic—70% silica
Colors-- pink, white, light gray
Remember, are granitic rocks mafic or felsic?

31. Composition of Igneous Rocks
Felsic—high in silica (quartz and feldspar)
Light colored
Mafic—high in magnesium and iron
Dark colored
Intermediate
A mixture of the two
In between colored

34. Review
Igneous Rocks

35. Melting point is also affected by two factors
The pressure
As pressure melting point
The water content
High water content=lower melting point
Low water content=higher melting point

36. Remember a rock is made of different minerals
When rocks melt, those with lowest melting points melt first
Partial melting
When rock crystallize, those with highest melting points crystallize first as the magma cools
Fractional crystallization

37. Bowen’s Reaction Series
See handout.

38. Bowen’s Reaction Series

40. Igneous Rock formation beneath the Earth's surface
Pluton-large intrusive igneous rock body
Dike—cuts across rock layers-vertical
Sill—horizontal –parallel to existing rock layers

41. Caldera

42. Pillow Basalt

43. Igneous Rock Structures