1. Types of Rock include Igneous, Sedimentary and Metamorphic
Igneous Rocks Lecture 3
Marble demo, rock specimens, Petrographic Microscope, Olivine Porphyry or Gabbro
Trays of mafic and felsic minerals

2. Characteristics of magma
Igneous rocks form as molten rock cools and solidifies
Characteristics of magmas (molten rock) depend on parent material and where they crystallize
Where determines speed of crystallization
At surface, fast cooling makes small crystals

3. Geothermal Gradient Hot Cool
Silica-rich rocks (with Quartz, K-feldspar) melt at cooler temperatures.
Melts are viscous
Silica-poor rocks (with Olivine, Pyroxene,
Ca-feldspar) melt at higher temperatures
Melts are very fluid
Hot

4. Characteristics of magma
General Characteristics of molten rock
Forms from partial melting of rocks inside the Earth
Rocks formed from lava at the surface are classified as extrusive, or volcanic rocks
Rocks formed from magma that crystallizes at depth are termed intrusive, or plutonic rocks

5. Two Geologic Environments Where Igneous Rocks Form
Formed in Rift
Igneous Rocks
Formed Above
Sinking Plate
Both melts are "Basaltic" i.e. Olivine, Pyroxene and Ca-Feldspars

6. Extrusive Igneous Rock - Lava (Hawaii)

7. Intrusive Igneous Rock (Granite) – This granite cooled 30 kilometers under the surface
Plagioclase
Feldspar
Quartz Amphibole K Feldspar

8. Characteristics of magma
Three parts:
Liquid portion, called melt, that is
mobile ions
Solids, if any, are silicate minerals
already crystallized from the melt
Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)

9. Characteristics of magma
Crystallization of magma
Cooling of magma results in the systematic arrangement of ions into orderly patterns
The silicate minerals resulting from crystallization form in a predictable order
Rock-forming minerals crystallize with increasing complexity as the magma cools. The most complex 3-D minerals crystallize last. The hottest magmas can only crystallize Olivine (Independent Tetrahedra), but as the magma cools, more complex minerals can form.

10. Bowen’s Reaction Series
Molten- VERY Hot
No solids
First mineral to crystallize out
Molten- Not so hot
100% Solid

11. Fine crystals
Need a microscope
Low silica, HOT, fluid
Intermediate
High silica, warm, viscous
Course crystals
Easily seen

12. Granite Hand Sample Order of Crystallization Granite Thin Section
Microscope
Demo
Bowens reaction series says: as a granitic melt cools, Biotite Mica and Plagioclase Feldspar crystallize out before Quartz
Granite
Hand Sample
Order of
Crystallization
We can see the order of crystallization under the microscope
Granite
Thin Section

13. Crystals can react with the melt if they touch it
If the first formed crystals of Calcium-rich (Ca) Plagioclase touch the melt they will react with it,
and will become more sodium-rich on their outer rims
Zoned feldspar (plagioclase) showing change in
composition with time in magma chamber
(calcium-rich in core to sodium-rich at rim)

14. However, if early crystals are removed, the melt becomes richer in Silica
Fe, Mg, Ca
Some Si
Left with
K and Al
Most of Si
You can start with a
Mafic (silica-poor) magma
and end up with some
Felsic (silica-rich)
Granites.
Marble Demo
A melt will crystallize its mafic components first, and the remaining melt may be granitic

15. Characteristics of magma
Igneous rocks are typically classified by both:
Texture
Mineral composition
Texture in igneous rocks is determined by the size and arrangement of mineral grains

16. Igneous textures Most important is crystal size
Factors affecting crystal size
Rate of cooling
Slow rate promotes the growth of fewer but larger crystals
Fast rate forms many small crystals
Very fast rate forms glass

17. Types of Igneous textures
Aphanitic (fine-grained) texture
Rapid rate of cooling of lava or magma
Microscopic crystals
May contain vesicles (holes from gas bubbles)
Phaneritic (coarse-grained) texture
Slow cooling
Crystals can be identified without a microscope

18. Fine grained because it cooled quickly at the surface
Aphanitic texture
Fine grained because it cooled quickly at the surface

19. Coarse crystals cooled slowly at great depth
Phaneritic texture
Coarse crystals cooled slowly at great depth

20. Igneous textures Types of igneous textures Porphyritic texture
Minerals form at different temperatures as well as differing rates
Large crystals, called phenocrysts, are embedded in a matrix of smaller crystals, called the groundmass
Glassy texture
Very rapid cooling of molten rock
Resulting rock is called obsidian

21. Porphyritic texture
Granite
Two-stage cooling?

22. Glassy texture
Obsidian
Fast cooling

23. More types of Igneous textures
Pyroclastic texture
Various fragments ejected during a violent volcanic eruption
Textures often appear to more similar to sedimentary rocks

24. Pyroclastic Rock - Superheated Flows

25. Naming igneous rocks – pyroclastic rocks Composed of fragments ejected during a volcanic eruption Varieties Tuff – ash-sized fragments Volcanic breccia – particles larger than ash

26. Ash and pumice layers

27. Still more types of Igneous textures
Pegmatitic texture
Exceptionally coarse grained crystals
Form in late stages of fractionation of magmas
This is often what prospectors are looking for
A Pegmatite with Feldspar and Zircon
Zircon is very good for obtaining radiometric ages

28. Show tray of Mafic Minerals
Igneous Compositions
Igneous rocks are composed primarily of silicate minerals that include:
dark (or ferromagnesian) colored silicates
Olivine
Pyroxene
Amphibole
versus …
“MAFIC” Magnesium and Iron

29. Show tray of Felsic Minerals
Igneous Compositions
Igneous rocks also contain light colored silicate minerals that include:
Quartz
Muscovite mica
Feldspars
“FELSIC” Feldspar and Silica

30. Igneous Rock Classification- Bowen’s Reaction Series on its side
Note Minerals in
Note Minerals in
Felsic rocks crystallize from warm melts
Mafic from hot melts

31. Igneous compositions Naming igneous rocks – granitic (felsic) rocks
Granite
Phaneritic
Over 20 percent quartz, about 25 percent or more feldspar (usually much more feldspars).
Plagioclase is Sodium-rich
Abundant and often associated with mountain building
The term granite covers a wide range of mineral compositions

32. Igneous compositions Naming igneous rocks – granitic (felsic) rocks
Rhyolite
Extrusive equivalent of granite
May contain glass fragments and vesicles
Aphanitic texture (means fine grained minerals)
Less common and less voluminous than granite
Phenocrysts can include quartz and feldspar
fine grained because extruded, so crystallized quickly

33. Igneous compositions Basaltic composition can be fine or coarse
Composed of dark Olivine and Pyroxene and grey calcium-rich plagioclase feldspar
No Potassium-rich feldspar (no K-spar ‘Microcline’)
Designated as being mafic (magnesium and ferrum, for iron) in composition
Much denser than granitic rocks - sinks
Comprises the ocean floor as well as many volcanic islands such as Hawaii. Also rift valley lavas

34. Igneous compositions Basalt
Naming igneous rocks – basaltic (mafic) rocks: Fine-grained
Basalt
Volcanic origin
Aphanitic texture
Composed mainly of pyroxene, some olivine and also calcium-rich plagioclase feldspar
Most common extrusive igneous rock

35. Scoria type Basalt: note Gas Bubble Pits

36. Igneous compositions Gabbro
Naming igneous rocks – basaltic (mafic) rocks: Coarse Grained
Gabbro
Intrusive equivalent of basalt
Phaneritic texture consisting of pyroxene and calcium-rich plagioclase
Makes up a significant percentage of the oceanic crust, beneath the basalt pillow lavas.

37. Igneous compositions Other compositional groups
Intermediate (or andesitic) composition
Contain at least 25 percent dark silicate minerals
Associated with explosive volcanic activity
Often gray

38. Igneous compositions Intermediate rocks Andesite Volcanic origin
Aphanitic texture
Often resembles rhyolite
Intermediate silica content
Frequent composition in volcanoes above subduction zones, e.g. in Andes Mountains

39. Igneous compositions Extrusive products can include: Pumice Volcanic
Glassy texture, very light weight, mostly air
Frothy appearance with numerous voids (extrusive foam)
Forms when lavas have a lot of water and other volatiles
Common with intermediate compositions

40. Igneous compositions Intermediate rocks Diorite
Plutonic equivalent of andesite
Coarse grained
Intrusive
Composed mainly of intermediate feldspar and amphibole

41. Silica Content Silica content influences a magma’s behavior
Granitic magma
High silica content
Extremely viscous
Liquid exists at temperatures as low as 700oC
Huge explosion if it erupts (Yellowstone, Toba)
Plutonic
“Granite”
When Yellowstone explodes, half of Wyoming will perish
Volcanic
“Rhyolite”

42. Silica Content Silica content influences a magma’s behavior
Basaltic magma
Much lower silica content
Fluid-like behavior
Crystallizes at higher temperatures
Gurgles when it erupts (Hawaii)

43. Origin of Magma Role of Pressure
Reducing the pressure lowers the melting temperature – the rock probably melts
RIDGE: When confining pressures drop, decompression melting occurs

44. Role of volatiles - WATER
Origin of Magma
Role of volatiles - WATER
Volatiles (primarily water) cause rocks to melt at lower temperatures
This is particularly important where oceanic lithosphere descends into the mantle in a subduction zone

45. Assimilation and magmatic differentiation
Show Samples

46. Basalts forming in rifts and MORs
Decompression Melting:
Magma under lithosphere heats and cracks it. Mantle rock is exposed to low pressures – it partially melts

47. Origin of Andesite & Diorite: intermediate silica content
Basaltic here
Good diagram for the Andes Mountains
Small blobs, not much heat in them
Assimilate some crust, fractionate

48. Plate Tectonics- Andesite Line
Andesites form above the deep portions of a subduction zone

49. Origin of Granitic Rocks
Huge blobs under thick part of continent w/ low temps but lots of magma, fractionation & assimilation => Granite Batholiths
Can also get small amounts of granites from deep felsic rock passed by ascending magma

50. Some intrusive igneous structures