1. Geology – Chapter 3 – Igneous Rocks
Magma, Igneous Rocks,
and
Intrusive Activity
Igneous
Rocks

3. The Origin of Magma Questions:
most magma originates in the upper mantle
greatest quantities form at divergent boundaries
small amounts of magma at subduction zones
Questions:
How can magma form from “solid rock ?”
Why is one magma mafic and others felsic?
Why do volcanoes erupt basaltic lava, and
continent volcanoes erupt andesitic/rhyolitic lava ?

4. Magma: The parent material for all rocks
There are three distinct components of magma:
Liquid portion referred to as “melt”
“liquid rock” – low density
composed of: Si, O, Al, Fe, Ca, Mg, Na, K
Solid portion
crystallized portions of the melt as
magma cools – precipitating various
silicate minerals.
Gaseous portion
“volatiles” created from vaporizing
materials at the Earth’s surface
(H2O, CO2 and SO2)

5. Melting solid rock into magma
How would the process of crystallization work?
Rising temperatures within a solid (tightly packed atoms)
creates “vibrating ions” eventually breaking the
chemical bonds and causing ions to roll over each
other producing a liquid.

6. Pressure Extrusive Ig-rocks Lava Surface Intrusive Ig-rocks Magma pool
Cooled solid rock
Droplets rise through
the mantle and collect
to form magma pools
Magma pool
Pressure
150 mi below earth’s surface
partial melting of solid rock
Solid rock
Solid Rock
Solid Rock
Solid Rock

7. Typical temperature depending on chemical composition:
Magma
Typical temperature depending on chemical composition:
7000C (1,2920F) – 13000C (2,3720F)
High in
Si,O
High in
Fe,Mg

8. I Geology , especially igneous rocks!
Describe how solid rock can covert from
the solid phase to liquid rock
Describe the three components to magma
3. What is the temperature range for
high Si and high Fe type magmas?

9. Increasing Temperature:
increasing the temperature within the earth
begins to melt solid rock
Temperatures within the Earth increase as a
function of depth ---- Geothermal Gradient

10. Temperature inside the earth
Geothermal
gradient
500
1000
1500
2000
the rate at which
temperature increases
with depth
Continent gradient
In thicker crust,
gradient increases.
average 7oC/km rate
temperature increases
gently
100
5,000
Depth (km)
200
Pressure (mpa)
10,000
300
Oceanic gradient
Below the ocean floor,
temperature increases
rapidly.
average 130C/km
15,000
400

11. So, why is the mantle still considered solid?
Pressure increases with depth – raising the rocks melting points

12. Decompression Melting:
ascending mantle rock moves into lower pressure zones which lowers
rock melting points generating voluminous magma
Remember: most magma occurs along spreading ridges.

13. Addition of volatiles (water and gases)
addition of water lowers the rocks melting point
“wet” rock drives the melting points downward
mantle rock (peridotite) melting points lower by 1000C/.1%
water!!

14. Magma is generated in three ways:
Increase in temperature causing rocks to exceed
their melting points ---- melting occurs
Decrease pressure (decompression melting), rocks
ascend to low pressure zones
Introduction of volatiles (principally water) lowers
rock melting point.

15. I Geology , especially magma 4. Briefly describe the role of magma
production from the geothermal gradient,
decompression melting, and volatiles.
5. Why is the earth’s mantle solid --- when
it should be “liquid”?

16. Evolving Magmas: A variety of igneous rocks = wide variety of magmas
observations of many igneous compositions from
volcanoes
magma – the parent material of all igneous rocks
investigated by N. L. Bowen (famous geologist)
( ) --- Bowen’s Reaction Series

18. Magmatic Differentiation
the formation of many kinds of igneous rocks from
a single magma
liquid
Simple example
SiO2
SiO2
SiO2 Mg
SiO2 Fe
SiO2
SiO2 Fe
SiO2 Mg
SiO2 Fe
FeSiO2
MgSiO2 Mg
FeSiO2 Fe
MgSiO2
FeSiO2 Mg
SiO2
Cooling
SiO2
SiO2
FeSiO2
MgSiO2
solid
SiO2
Liquid magma
Part liquid/solid
How has the liquid magma changed composition?
As the liquid magma begins to cool, various minerals precipitate as
solids and become separated from the liquid melt. This separation
of various chemistries changes the composition of the original magma.

19. Changing the composition of magmas:
Magmatic Differentiation:
separating the xlized minerals from the melt changing
the overall magma composition
Assimilation and Mixing Magma:
mixing magmas with various compositions
Magmatic
Differentiation
Mixing
Magmas

20. Changing the magma from a basaltic composition to a granitic

21. Composition of magmas – explained by P.T.
The Earth- One big chemistry set!
Basaltic composition
Basalt magma:
Assimilating oceanic plate
material (basalt + basalt =
basalt magma)
Rocks concentrated in mafic
minerals – formed high on BRS
Low in Silica
Granitic composition
Granitic magma:
Assimilating oceanic plate
material (basalt + continental =
granitic magma)
Rocks concentrated in felsic
minerals – formed low on BRS
High in Silica

22. I Geology , especially magma 6. Explain why there are a verity of
igneous rock compositions.
7. Distinguish between
magmatic differentiation
magma assimilation
8. What is Bowen’s reaction series?

23. Solidified above the Earth’s surface
Intrusive Rocks
Intrusive Rocks
Solidified below the Earth’s surface
Plutonic rocks – “the God of lower world”
magma cooled slowly allowing xl growth
coarse-grained textures (can see grains)
large granite mountains
Sierra Nevada Mountains
Mt Rushmore
visible
grains
Extrusive Rocks
Extrusive Rocks
Solidified above the Earth’s surface
Volcanic rocks – “the roman God of fire”
magma cooled fast allowing microscopic
xl growth (can’t see grains)
fine-grained textures (can’t see grains)
Hawaiian Volcanoes, Cascade Range
non-visible
grains

24. When observing “frozen magma” (an igneous rock), how does a geologist
know if the rock is an Extrusive or Intrusive igneous rock????????
Checking the crystal size - does the rock contain visible crystals
or are crystals non-existent?
Igneous Rock Textures
(4) Factors that affect crystal size in an igneous rock
The rate at which magma cools
slow cooling – visible minerals
rapid cooling – nonviable minerals
The amount of silica present
the more silica – more visible minerals
The amount of dissolved gases
abundant dissolved gases – larger crystals
The amount of space for xl growth

25. Types of igneous rock textures
Slow cooling of magma
Phaneritic – xl grains can be seen as an interlocking “mosaic” mass of xls coarse-grained texture
Interlocking
grains

26. Slow cooling of magma
Pegmatitic – xl grains are greater than 1 cm results from late stage magma cooling- charged with volatiles
Large Grains

27. I Geology, especially ig rocks!
9. Briefly describe the textural differences
between intrusive and extrusive ig rocks.
10. What four factors influence igneous rock
textures?
11. Why are pegmatites unique ?

28. No minerals visible xl grains are to small to see with an unaided eye
Fast cooling magma
What do you observe ?
No minerals visible
xl grains are to small to
see with an unaided eye

29. Fast cooling magma
Glassy – xl grains are “frozen” before xlation. Produced from high silica viscous rich magmas that are rapidly quenched

30. Porphyritic – two distinct xl sizes
(same mineral)
Phenocrysts
Groundmass

31. Fast cooling of magma
Pyroclastic – fragmented texture, fragments incorporated into molten lava as it solidifies Tuffacous (tuff) – composed of fragments
Fragments of other rock pieces
(“different pieces other rock)

32. Igneous Rock Textures -

33. I Geology, especially ig rocks
12. Distinguish the textural differences
between porphyritic and fragmental
(tuffaceous) – Draw a picture
13. What type of geological environments
may be responsible for igneous rocks
having a glassy, porphyritic or
fragmental textures?

34. Igneous Rock Composition – What’s in your rock?
Composition is based on the “mineral make-up”
dominantly composed of silicates (Si and O)
Al, Ca, Na, K, Mg and Fe
Ferromagnesian composition (Mafic)
high in Fe, Mg
low in Si, O
Dark colored rocks
Pyroxene
Amphibole
Biotite
Non-ferromagnesian (felsic)
high in Si, O
low in Fe, Mg
light colored rocks
Ca-plagioclase
orthoclase
Muscovite
Quartz

35. Classification of Igneous Rocks
Igneous rocks are classified based on TEXTURE and
COMPOSTION
COMPOSITION
Felsic (light color)
High silica
Intermediate color
Mafic (dark color)
Low silica
phaneritic
Granite
Diorite
Gabbro
TEXTURE
aphanitic
Rhyolite
Andesite
Basalt
Ig rocks have the same chemistry but different textural characteristics
due to the type of geologic environment the rock forms

36. I Geology, especially ig rocks
14. Explain how igneous rocks are classified?
15. Explain the compositional differences
between light and dark colored igneous
rocks.
16. Give examples (rock name) of the following:
light colored intrusive rock
dark colored extrusive rock
intermediate extrusive rock
light colored extrusive rock :

37. What types of features are formed when magma cools below the surface?
Intrusive features

38. Intrusive Igneous Rock Activity
Igneous rocks solidifying below the surface can be
classified as:
tabular or massive (“table-tops” or irregular”)
discordant: cuts across other rock bodies
concordant : parallels other rock bodies

39. Tabular intrusive bodies forming below the earth’s surface
Dikes
Sill
Tabular
Batholith
Tabular intrusive bodies
forming below the earth’s
surface 38

40. Intrusive Bodies
Sill
Loccolith
Dike
Stock
Batholith

45. Intrusive Bodies: Batholith: intrusive body GREATER than 40 mi2
Stock: intrusive body LESS than 40 mi2
Dike: intrusive body cutting across strata
(discordant)
Sill: intrusive/extrusive body parallel to strata
(concordant)
Laccolith: “mushroom-shaped” intrusive body
forming a dome-like structure

46. Sierra Nevada Batholith
Melting magma rises and
mixes with continental
material (high SiO2) and
solidifies beneath the
surface.
Sierra Nevada Batholith
Granite/Diorite 41

48. I intrusive rocks. 17. Given the block diagram below, describe
the following plutonic (intrusive) type
features: