1. Igneous
Rocks

2. Igneous Rocks

3. Igneous Rocks
FIGURE 5.13 Decompression melting creates magma at seafloor spreading centers.

4. Igneous Petrotectonic Assemblages
Igneous Rocks
Igneous Petrotectonic Assemblages

5. Igneous Rocks Divergent Plate Boundary: Continental Rifts
Magma origin:
Partial melting of mantle rocks
Decompression melting due to rise of material in a hot spot (?)
Magmatic processes:
Initial mantle melting produces mafic magma (basaltic)
Magma accumulates in chambers
Fractional crystallization, contamination produce intermediate (andesite) and feslic (rhyolite) magmas.
Rocks
Highly varied
Dominated by bimodal volcanic activity
Early: basalt lava flows, shield volcanoes, cinder cones
Later: rhyolite flows and calderas (ignimbrites)
Also mafic through felsic plutons

6. Igneous Rocks Divergent Plate Boundary: Mid-Ocean-Ridges Magma origin:
Partial melting of mantle rock
Decompression melting due to mantle convection
Magmatic processes:
Mantle melting produces dominantly mafic magma (basaltic)
magma may be modified slightly by fractional crystallization
Rocks
“Ophiolite suite”
Cool and crystallize in magma chamber
Erupted onto sea floor as effusive lava flows or pillows

7. Igneous Rocks Ophiolites oceanic crust preserved on land and
these are the main units comprising all oceanic crust

8. Igneous Rocks at Convergent Plate Margins

9. Igneous Rocks Convergent Plate Boundary: Island Arc Magma origin:
Water released from subducting slab due to T-P increase
Flux melting (partial) of overlying mantle rocks
Magmatic processes:
Initial melting produces mafic magma (basalt)
fractional crystallization and melting modifies composition
Rocks
Dominantly andesite stratovolcanoes (“Ring of Fire”)
Mafic to intermediate batholiths (Gabbro, diorite)

10. Igneous Rocks Convergent Plate Boundary: Continental Arc Magma origin:
Water released from subducting slab due to T-P increase
Flux melting (partial) of overlying mantle rocks
Magmatic processes:
Initial melting produces mafic magma (basalt)
Basalt magma rises into crust; fractional crystallization, melting and assimilation of crust modifies composition (intermediate to felsic)
Rocks
Dominantly andesite to rhyolite stratovolcanoes
Rhyolite calderas and ignimbrites (tuffs)
Vast intermediate to felsic batholiths (granodiorite, granite)

11. Igneous Rocks Convergent Plate Boundary: Continental collision
Magma origin:
Crustal thickening due to thrust faulting during collision
Causes partial meting of crustal rocks (anatexis)
Magmatic processes:
Magma composition depends largely on the source rocks that were melted
Dominantly felsic composition
Rocks
Extensive batholiths and plutons of mostly felsic composition (granite)
Compression and thick crust prevent eruption – volcanoes rare

12. Water is often implicated in helping rocks melt

13. Subduction example

19. Igneous Rocks Hot Spots Magma origin: Magmatic processes: Rocks
Partial melting of mantle rocks
Decompression melting due to rise of hot material
Magmatic processes:
Initial mantle melting produces mafic magma (basaltic)
Heat from mafic magma can melt overlying crustal rocks, producing felsic magmas
Rocks
Initial flood basalts erupted from fissures (associated dikes)
Oceanic: basalt lava flows, shield volcanoes, cinder cones
Continental: bimodal (basalt-rhyolite) volcanic activity; rhyolite calderas and ignimbrites
Also mafic through felsic plutons

20. SUMMARY
Chapters 4 & 5

21. Summary Minerals & Rocks
Inorganic
Crystalline
Solid
Naturally occurring
definite chemical composition
Elements
Atom
Nucleus
Protons
neutrons
electrons
Ion
cation
anion
complex ion
Bond
Ionic
Covalent
Metallic
Hydrogen
Van der Waal’s
Crystal structure
Mineral formation
Crystallization from a liquid
Precipitate from a solution
Physical properties of Minerals
Chemical composition
Iso-structure
polymorphism
Identify using
Color
Hardness
Luster
Streak
Cleavage
Density
Miscellaneous Properties

22. Summary Minerals & Rocks
Rock-Forming Minerals
Element abundances: O (46.6%), Si (27.7%), Al (8.1%), Fe (5.0%), Ca (3.6%), Na (2.8%), K (2.6%), and Mg (2.1%).
Mineral classification: major “building block” in the chemical structure
Silicates (SiO44-)
Oxides (O22-)
Sulfides (S2-)
Sulfates (SO42- )
Carbonates (CO32-)
Halides (Cl1- , F1- Br1- )
Native Elements (single element
Silica Tetrahedron
fundamental building block
4 oxygen ions surrounding a much smaller silicon ion
joining silicate structures
Independent tetrahedra
Single chains
Double chains
Sheets
3-D framework
mafic minerals
sheets or networks of silica tetrahedrons
bonded to aluminum, potassium, and sodium
generally light colored
less dense
felsic minerals
single silica tetrahedrons or chains
bonded to iron, magnesium, and calcium
generally dark colored
more dense
Use of Minerals
Gems
Ore minerals

23. Summary Minerals & Rocks
Naturally occurring solids containing one or more minerals that have been:
Melted and cooled together,
Cemented together, or
Squeezed and heated together
Three major rock groups:
Igneous (“born of fire”); originally molten
Sedimentary; originally particulate material or produced from precipitation out of water
Metamorphic; pre-existing rocks modified by pressure or temperature
Rock Cycle
the continuous changing of rocks from one kind to another over long periods of time.
has no definite sequence.
can follow many different pathways
Mineral resources
Ore - rock that contains useful metallic minerals that can be mined at a profit.
Geochemically abundant elements
Geochemically scarce elements
Concentration necessary for profitable mining
Process that concentrate elements
Igneous Processes
Hydrothermal
Magmatic
Sedimentary Processes
Mechanical
Chemical
Metamorphism Processes
Groundwater Processes

24. Summary Igneous Rocks What is magma and where does it form?
Igneous Rocks: crystallization from a magma
Magma
silicate liquid
gases mostly H2O & CO2
some crystals
has wide range of chemical composition
with high T and properties of a liquid
physical properties controlled by
temperature
SiO2 content
dissolved gas content
less dense thus rises, two major mechanisms:
Brittle: fractures and dikes
Ductile: diapirism
formed by partial melting of solid rocks of the crust and mantle due to:
raising a rock's temperature,
a decrease in pressure, or
the introduction of volatiles (water).
types of magmas
mafic (basaltic or gabbroic)
intermediate (andesitic or dioritic)
felsic (rhyolitic or granitic)

25. Summary Igneous Rocks How do igneous rocks differ from one another?
Two main types of igneous rocks:
intrusive
cool and crystallize inside the earth
cool slowly
grow large crystals
coarse-grained (phaneritic)
extrusive
lava flows
form at the surface
cool rapidly
grow small crystals or form glass
fine-grained (aphanitic)
porphyritic- bimodal grain size distribution
glassy- no crystals formed (extremely rapid cooling)
pyroclastic rocks
fragmental rocks
characterized by size
ash < 2mm rock is tuff
lapilli 2-64 mm rock is lapilli tuff
bombs > 64 mm rock is breccias, agglomerate
Igneous rocks textures
Phaneritic: corse-grained
Porphritic: large crystals in a fine grained (glassy) matrix
Aphanitic: fine-grained
Glassy: no crystals seen, like a glass
Vesicular: contain vesicles (holes)
Pyroclastic: fragmental

26. Summary Igneous Rocks How do igneous rocks differ from one another?
Magmatic differentiation (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
Bowen’s reaction series: as a magma cools, minerals crystallize in a systematic fashion based on their melting points.
Composition►
Texture▼
Felsic
(light color)
Intermediate
Mafic
(dark color)
Ultramafic
Phaneritic
Granite
Diorite
Gabbro
Peridotite
Aphanitic
Rhyolite
Andesite
Basalt
Vesicular
Pumice
Scoria
Glassy
Obsidian
Classification of Igneous Rocks

27. Summary Igneous Rocks Forms of Igneous rocks Extrusive rocks
Eruption style
lava flows
explosive eruption
control on eruption style
temperature
composition: SiO2 content
gas content
Lava flows: gentle molten rock flows along surface and solidifies as it cools Aa
Pahoehoe
Obsidian
Vesicular
Landforms formed by Volcanoes
basaltic fissural eruption forms plateau
central vent eruptions
Shield volcanoes
Cinder cones
Lava (volcanic) domes
Stratovolcanoes
Question -  Why do we see intrusive igneous rocks at the surface of the Earth?
Answer   - They are exposed by erosion which has removed all of the material above the intrusion
Intrusive bodies are defined by size, shape and relationship to country rock.
Volcanic Necks
Dikes
Sills
Lacoliths
Stocks
Batholiths

28. Summary Igneous Rocks Plate tectonics & Magmatism Plate boundaries
Divergent
Convergent
Within plate
Hot spots
oceanic
continental