1. Igneous rocks – rocks formed by the crystallization of magma

2. How are igneous rocks formed? How does magma differ from lava?
Guide questions
How are igneous rocks formed?
How does magma differ from lava?
What two criteria are used to classify igneous rocks?
How does the rate of cooling of magma influence the crystal size of minerals in igneous rocks?
How is the mineral makeup of an igneous rock related to Bowen’s reaction series?
In what ways are granitic rocks different from basaltic rocks?
How are economic deposits of gold, silver, and many other metals formed?

3. molten rock found ~200km beneath the surface consists primarily of:
Igneous rocks
Introduction
Magma
molten rock found ~200km beneath the surface
consists primarily of:
elements found in silicate minerals
gases (volatiles, water vapor) – confined within the magma by surrounding rocks
volatile = substance that occurs as gas at Earth’s surface temperature and pressure

4. less dense than surrounding rock and so works upward
Introduction
Magma
less dense than surrounding rock and so works upward
breaks through as lava in volcanic eruptions
explosions are caused by escaping volatiles
blockage of the vent, surface water seepage into the magma chamber to produce steam can aggravate explosions
eruptions produce ejected rock fragments and extensive lava flows

5. magma that has reached the surface
Introduction
Lava
magma that has reached the surface
extrusive or volcanic rocks – formed as lava solidifies
intrusive or plutonic rocks – formed as magma crystallizes at depth

6. contains suspended crystals and a gaseous component
Crystallization of magma
Magma
hot fluid
contains suspended crystals and a gaseous component
the liquid portion (melt) is composed of ions that move about freely (recall kinetic molecular theory)
Magma with crystals (green bars) and gas bubbles (white dots)

7. Crystallization of magma
as magma cools, the ions lose kinetic energy
the ions slow down and begin to form orderly bonds
the process is called crystallization

8. Disintegration of crystals ions in crystals form orderly bonds
Crystallization of magma
Disintegration of crystals
ions in crystals form orderly bonds
they vibrate in place
increasing heat = stronger movements (higher KE)
ions acquire enough KE to slide past each other
crystal disintegrates into a liquid whose ions move about randomly
the reverse process is crystallization

9. Crystallization of magma

10. small crystals form throughout (embryo crystals)
Crystallization of magma
Embryo crystals
in crystallization, not all of the molten material crystallizes at once
small crystals form throughout (embryo crystals)
ions are systematically added as the embryo crystals grow
the liquid is then transformed into a solid mass of interlocking crystals

11. Crystallization of magma

12. few and large embryo crystals
Crystallization of magma
Rate of cooling
slow cooling
few and large embryo crystals
rock is made of large crystals (mm to meters in diameter)
rapid cooling
small and numerous embryo crystals
rock is made of small, intergrown crystals (often microscopic)

13. liquid magma is quenched instantly
Crystallization of magma
Formation of glass
liquid magma is quenched instantly
this leaves no time for the ions to form orderly bonds
rock consists of a solid mass of unordered ions (glass)

14. consists of the eight primary constituents of silicate minerals
Crystallization of magma
Composition of magma
consists of the eight primary constituents of silicate minerals
silicon
oxygen
aluminum
sodium
potassium
calcium
iron
magnesium
volatiles (water vapor, CO2)

15. Crystallization of magma silicon-oxygen tetrahedra form first
the tetrahedra join with each other and with other ions to form embryo crystals
embryo grows as ions are added to the crystalline network

16. Crystallization of magma
certain minerals crystallize at higher temperatures than others
hence, magma often consist of solid crystals surrounded by molten material

17. Factors that influence the crystallization of magma rate of cooling
mineral composition of magma
volatile components
This results to physical and mineral differences among igneous rocks.

18. size and arrangement of mineral grains (crystals)
Crystallization of magma
Igneous rocks are therefore classified according to texture and mineral composition.
texture
size and arrangement of mineral grains (crystals)
reflects environment of crystallization
mineral composition
proportion of the eight constituents of silicate minerals
volatile components

19. Igneous rock textures texture overall appearance of the rock
based on size and arrangement of interlocking crystals
important field characteristic
rock’s origin can be inferred
when equipment is not available

20. Factors influencing texture rate of cooling of magma aphanitic
Igneous rock textures
Factors influencing texture
rate of cooling of magma
aphanitic
phaneritic
porphyritic
glassy
composition of magma
basaltic magma (very fluid)
granitic magma (more viscous)
pyroclastic texture

21. Rate of cooling of magma may be:
Igneous rock textures
Rate of cooling of magma may be:
slow (thousands of years) – magma chambers deep within the crust
rapid (hours) – thin lava flows
instantaneous (seconds) – small molten blobs ejected into the air

22. Aphanitic rocks – fine-grained texture
Igneous rock textures
Aphanitic rocks – fine-grained texture
forms at the surface or as small masses within the upper crust
crystal grains are indistinguishable by the unaided eye
mineral identification not possible
often contains vesicles (openings produced by expanding gas bubbles)

23. Classes of aphanitic rocks
Igneous rock textures
Classes of aphanitic rocks
light aphanitic rock – primarily composed of light-colored nonferromagnesian silicates
intermediate aphanitic rock
dark aphanitic rock – primarily composed of dark-colored ferromagnesian silicates

24. Phaneritic rocks – coarse-grained texture
Igneous rock textures
Phaneritic rocks – coarse-grained texture
from crystallization of large masses of magma far below the surface
mass of intergrown crystals
equal in size
large enough to be identified by the naked eye
exposed only through erosion

25. from eruption of magma containing large crystals
Igneous rock textures
Porphyritic rocks – large crystals embedded in a matrix of small crystals
from eruption of magma containing large crystals
phenocrysts – large crystals
groundmass – matrix of smaller crystals
porphyry – rock that has porphyritic texture

26. Glassy rocks – no crystal formation
Igneous rock textures
Glassy rocks – no crystal formation
from ejected molten rock which cools rapidly in the atmosphere
ions did not have time to form embryo crystals

27. Igneous rock textures

28. Igneous rock textures

29. Igneous rock textures

30. basaltic magma (very fluid) – usually forms crystalline rocks
Igneous rock textures
Composition of magma
basaltic magma (very fluid) – usually forms crystalline rocks
granitic magma (more viscous) – usually forms glassy rocks
pyroclastic texture – composed of rock fragments

31. Igneous rock composition
ultimately determined by the chemical composition of the original magma
Question: Are there as many types of magma as there are igneous rocks?
Evidence: a volcano often produces different lava flows and pyroclastic material
Hypothesis: Could a single magma type produce rocks of varying mineral content?

32. Bowen’s Reaction Series N.L. Bowen (1900 to 1925) Key points
Igneous rock composition
Bowen’s Reaction Series
N.L. Bowen (1900 to 1925)
Key points
minerals with higher melting points crystallize first
during crystallization, the composition of the melt (liquid portion of magma) continually changes
minerals react with melt to produce next set of minerals (hence, reaction series)
minerals that form in the same temperature regime are found in the same igneous rock

33. Bowen’s Reaction Series discontinuous reaction series
Igneous rock composition
Bowen’s Reaction Series
discontinuous reaction series
each mineral has a different crystalline structure
does not normally run to completion (all reactants are not consumed)
continuous reaction series
minerals become progressively rich in certain ions (does not change crystalline structure)
crystals typically have cores and outer zones with varying ionic content

34. Igneous rock composition

35. assimilation – magma melts surrounding bedrock
Igneous rock composition
Magmatic differentiation – the process of developing more than one rock type from a common magma (parent magma)
crystal settling – dense minerals settle at the bottom of the magma chamber
assimilation – magma melts surrounding bedrock
magma mixing – happens as two rising magma bodies merge and mix by convection

36. Naming igneous rocks Three main types basaltic andesitic granitic
Classification closely corresponds to Bowen’s reaction series
Gradations among the types exist
Silica (SiO2) content plays an important role in classification
Different textures result to different rocks

37. primarily composed of potassium feldspar and quartz
Naming igneous rocks
Granitic rocks
primarily composed of potassium feldspar and quartz
felsic – rich in feldspar and silica (quartz)
light in color

38. Some granitic rocks Granite Rhyolite Obsidian Pumice
Naming igneous rocks
Some granitic rocks
Granite
Rhyolite
Obsidian
Pumice

39. Naming igneous rocks

40. primarily made of amphibole and the intermediate feldspars
Naming igneous rocks
Andesitic rocks
intermediate rocks containing minerals found near the middle of Bowen’s series
primarily made of amphibole and the intermediate feldspars

41. Naming igneous rocks
Some andesitic rocks
Andesite
Diorite

42. primarily composed of calcium feldspar, pyroxene, and olivine
Naming igneous rocks
Basaltic rocks
primarily composed of calcium feldspar, pyroxene, and olivine
these minerals are high in iron, magnesium or calcium, and low in silicon
mafic – rich in magnesium and ferric (iron)
dark colored and slightly denser than most other igneous rocks

43. Naming igneous rocks
Some basaltic rocks
Basalt
Gabbro

44. form from fragments ejected during volcanic eruptions
Naming igneous rocks
Pyroclastic rocks
form from fragments ejected during volcanic eruptions
tuff (welded tuff)
volcanic breccia
these names do not denote mineral composition

45. Naming igneous rocks

46. Mineral resources & igneous processes
Mineral resources and igneous processes
Mineral resources & igneous processes
some of the most important accumulations of metals (gold, silver, copper, mercury, lead, platinum, nickel) are produced by igneous processes
such processes are
magmatic differentiation
hydrothermal solutions
vein deposits
disseminated deposits
surface deposits

47. References http://community.weber.edu/museum/Philliteweb.jpg