1. Magma and Igneous Rocks
Chapter 6

2. Outline
• Igneous rocks
-What are they? Basics: Magma vs. Lava, intrusive vs. extrusive
• Magma
-Why does it form?
-Melting mechanisms (pressure release, heat transfer, volatiles)
-Components (solids, melts, gas)
• Magma composition
-4 major types of magma (based on silica content)
-Controls eruptive style (explosive vs. non-explosive)
-Variability due to source, partial melting, assimilation, fractional
crystallization
• More details….
-Magma migration, extrusive vs. intrusive environments
-Intrusive forms: sills, dikes, plutons, influence on landscape
-Igneous textures, classification, global distribution of magmatism
Chapter 6

3. Igneous Rocks
• Defined: rock solidified from the melt
• Freezes at temperatures of 1,100degreesC
• Composition dependent
• Earth is mostly igneous rock
• Magma – subsurface melt
• Lava – melt at the surface
• Magma erupts via volcanoes
Chapter 6

4. Igneous Rocks
• Melted rock can cool above or below ground.
• Intrusive igneous rocks – Cool slowly underground
• E.g. granite - yosemite
• Extrusive igneous rocks – Cool quickly at the surface
-E.g. basalt – hawaii
-lava
-flowing, cooling molten rock
Pyroclastic debris
-cooled rock frgaments
-e.g. ash, fragmented lava
Many types of igneous rocks!
Chapter 6

5. Outline
• Igneous rocks
-What are they? Basics: Magma vs. Lava, intrusive vs. extrusive
• Magma
-Why does it form?
-Melting mechanisms (pressure release, heat transfer, volatiles)
-Components (solids, melts, gas)
• Magma composition
-4 major types of magma (based on silica content)
-Controls eruptive style (explosive vs. non-explosive)
-Variability due to source, partial melting, assimilation, fractional
crystallization
• More details….
-Magma migration, extrusive vs. intrusive environments
-Intrusive forms: sills, dikes, plutons, influence on landscape
-Igneous textures, classification, global distribution of magmatism
Chapter 6

6. Magma Formation
• Why does magma
form?
• -earth is hot inside
• Why?
• 1. Earth formation
• -planetesimal and
meteorite accretion
• -differentiation
• 2. Existence of
radioactive decay
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7. Magma Formation
• Melting (partial) in
crust/upper mantle.
• Melting mechanisms:
• 1 pressure release
• 2 heat transfer
• 3 volatile addition
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8. Magma Formation
• Earth is hot inside, thus a geothermal gradient.
• -crustal temperature (T) increases 25degreesC/km depth
• -base of crust T is 1,280degreesC
• GEOTHERMAL GRADIENT VARIES FROM PLACE TO PLACE
Chapter 6

9. Magma Formation
• A mechanism for melting: pressure release.
• Base of crust is hot enough to melt mantle rock
• But due to high pressure, the rock does not
• A drop in pressure initiates “decompression melting”
• A  B:
-Big change in pressure, little in
Temperature
Vocab:
-Solidus  melting begins
-Liquidus  No solid left
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10. Magma Formation
• A mechanism for
melting: Heat transfer.
Chapter 6

11. Addition of Volatiles
• A mechanism for melting: introduction of volatiles
• -volatiles decrease rock melting temp
• -water
• -carbon dioxide
• Adding volatiles to hot, dry rocks initiates melting
• Important in subduction process
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12. What Is Magma Made of?
• Magmas have 3 components (solid, liquid, and gas).
• Solid – solidified minerals carried by the liquid
• Liquid – melt itself comprised of mobile ions
Mostly si and o; some ca, fe ,mg, al, na , k
Other ions to a lesser extent
Different mixes of elements
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13. What Is Magma Made of?
• Gas – volatiles dissolved in the melt.
• Dry magma – no volatiles
• Wet magma – up to 15% volatiles:
-water vapor (h2o)
-carbon dioxide (co2)
-sulfur dioxide (so2)
Chapter 6

14. Outline
• Igneous rocks
-What are they? Basics: Magma vs. Lava, intrusive vs. extrusive
• Magma
-Why does it form?
-Melting mechanisms (pressure release, heat transfer, volatiles)
-Components (solids, melts, gas)
• Magma composition
-4 major types of magma (based on silica content)
-Controls eruptive style (explosive vs. non-explosive)
-Variability due to source, partial melting, assimilation, fractional
crystallization
• More details….
-Magma migration, extrusive vs. intrusive environments
-Intrusive forms: sills, dikes, plutons, influence on landscape
-Igneous textures, classification, global distribution of magmatism
Chapter 6

15. Types of Magma (composition)
• 4 major types based on % silica (SiO2).
• Felsic (Feldspar and silica) 66 to 76% silica.
• Intermediate 52 to 66% silica.
• Mafic (Mg and Fe-rich) 45 to 52% silica.
• Ultramafic 38 to 45% silica.
Chapter 6

16. Magma Composition -> Eruptive Style
• Composition controls density, T, and viscosity.
• Most imp - is the silica (SiO2) content.
• Silica-rich magmas  thick and viscous
• Silica-poor magmas  thin and flow easily (less viscous)

TypeThese characteristics govern eruptive
Density Temperature style: Viscosity
Felsic Very low Very low (600 to 850°C) Very High: Explosive eruptions.
Intermediate Low Low High: Explosive eruptions.
Mafic High High Low: Thin, hot runny eruptions.
Ultramafic Very high Very high (up to 1,300°C) Very low
Chapter 6

17. Magma Composition Variation
• Why various magma compositions?
• Due to:
-intital source rock composition
-partial melting
-assimilation
-frctional crystallization
Chapter 6

18. Magma Composition Variation
• Source rock dictates initial magma composition.
• Mantle source – ultramafic and mafic magmas
• Crustal source – mafic, intermdiate, felsic magmas
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19. Partial Melting
• Upon heating, silica-rich minerals melt first.
• Thus, partial melting yields a silica-rich magma
• Removing a partial melt from its source creates:
• -felsic magma
• -mafic residue left behind
Chapter 6

20. Assimilation
• Magma melts the country rock it passes through
• Assimilated materials change maga composition
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21. Magma Mixing
• Different magmas may blend in a magma chamber
-result combines characteristics of both
-mixing often incomplete, resulting in blobs of one type
suspended w the other
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22. Fractional Crystallization
• As magma cools, early formed crystals settle by gravity.
• Melt compostion changes as a result
-fe, mg, ca is removed in early formed solids
-si, al, na, and k remain in melt
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23. Fractional Crystallization
• Felsic
magma can
evolve from
mafic
magma.
• By
progressive
removal of
mafic
minerals.
Chapter 6

24. Outline
• Igneous rocks
-What are they? Basics: Magma vs. Lava, intrusive vs. extrusive
• Magma
-Why does it form?
-Melting mechanisms (pressure release, heat transfer, volatiles)
-Components (solids, melts, gas)
• Magma composition
-4 major types of magma (based on silica content)
-Controls eruptive style (explosive vs. non-explosive)
-Variability due to source, partial melting, assimilation, fractional
crystallization
• More details….
-Magma migration, extrusive vs. intrusive environments
-Intrusive forms: sills, dikes, plutons, influence on landscape
-Igneous textures, classification, global distribution of magmatism
Chapter 6

25. Magma Migration
• Magma is less dense than rock, so it rises.
• Magma moves by…
-injection into cracks
-melting overlying rocks
-pressure decrease with upward migration releases
volatiles (bubbles), thereby decreasing density
Chapter 6

26. Magma Migration
• Viscosity depends on temp, volatiles, and silica.
• Temp:
• Hot  low viscosity (flows well)
• Cooler  high viscosity (flows poor)
• Volatile content:
• More low vsc
• Less  high visc
• Silica (SiO2) content:
• Less (Mafic)  low visc
• More (Felsic)  high visc
Chapter 6

27. Igneous Environments
• 2 major categories - based on cooling site.
1. Extrusive settings – Cool at or near the surface.
-cool rapidity
-chill too fast-only small crystals form
2. Intrusive settings – Cool at depth.
-cool slowly
-crystals grow large
-most mafic magmas extrude
-most felsic magmas do not
Chapter 6

28. Extrusive Characteristics
• 1. lava flows – sheets of cooled lava
• 2. lava flows exit volcanic vents and flow outward
• 3. lava cools as it flows, eventually solidifies
• 4. low viscosity lava (basalt) can flow long distances
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29. Extrusive Characteristics
• Explosive ash eruptions.
-high viscosity felsic magma builds up pressure
-violent eruptions yield huge volumes of volcanic ash
-ash can cover large regions
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30. Intrusive Characteristics
• Intrusive rocks cool at depth, they don’t surface.
• Magma invading colder country rock initiates…
-thermal heat metamorphism and partial melting.
-Inflates fractures, pushing rock aside
-incorporation of country rock fragments (xenoliths)
-hydrothermal (hot water) alteration
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31. Intrusive Characteristics
• Intrusive contacts preserve evidence of high heat.
-baked zone: rim of heat altered country rock
-chill margin: magma at contact that cooled rapidly
Xenolith: country rock fragment in magma
-thermally altered
-magma cooled before xenolith
• Xenolith - Country rock fragment in magma.
Chapter 6

32. Intrusive Activity
• Magma intrudes into rocks in 2 main ways:
-as planar, tabular bodies (dikes, sills)
-as balloon-shaped blobs (plutons)
Size varies widely: plutons can be massive
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33. Tabular Intrusions
• Tend to have a uniform thickness.
• Can be traced laterally.
2 subdivisions:
1. Sill – parallels rock fabric
2. Dike – crosscuts rock fabric
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34. Example Large Sill
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35. Plutons
• Most magma is emplaced at depth within Earth.
-a large deep igneous body (blob) is called a pluton
Plutonic intrusions modify the crust
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36. Plutons  Batholith
• Plutons may coalesce to form
a larger batholith
-plutons are created at
subduction zones
-magma generation may occur
for 10s of myrs.
-long subduction history linked
to large batholiths
Chapter 6

37. Intrusive and Extrusive
• Intrusive & extrusive rocks commonly co-occur.
• Magma chambers feed overlying volcanoes
• Magma chambers can cool – become plutons
• Many igneous geometries are possible
• Dikes
• Sills
• Laccoliths
• Plutons
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38. Influence on Landscape
• Deeper features are exposed by uplift and erosion.
-intrusive rocks are resistant to erosion
-intrusive rocks often stand above the landscape
Exposing intrusive rocks by erosion takes a long time
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39. Rate of Cooling
• How fast is heat lost?
• Depth: Deep is hot, shallow
is cool.
-deep plutons cool slowly
-shallow flows cool rapidly
Shape: Surface to volume
ratio.
-spherical bodies cool slowly
-tabular bodies cool faster
• Ground water.
Chapter 6

40. Igneous Textures
• Size, shape, and arrangement of the minerals.
-glassy: solid glass or glass shards
-interlocking crystals: minerals that fit like jigsaw pieces
-fragmental: pieces of pre-existing rocks
-texture directly reflects magma history
 Texture directly reflects magma history.
Chapter 6

41. Glassy Textures
• Form by very rapid cooling of lava in water or air.
-basalts may quench into blobs of lava called pillows in
water
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42. Crystalline Textures
• Texture reveals cooling history.
• Aphanitic (small crystals – too hard to see).
-rapid cooling: extrusive
-crystals: no time to grow
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43. Crystalline Textures
• Texture reveals cooling history.
• Apharitic (small crystals – too hard to see)
-rapid cooling: extrusive
-crystals: no time to grow
• Phaneritic (large cryst
• als – easy to see).
• Slow cooling: intrusive
• Crystals have a long
• Time to grow
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44. Crystalline Textures
• Texture reveals cooling history.
• Porphyritic – A mixture of coarse and fine crystals.
-indicates a 2-stage history
-initial slow cooling creates large phenocrysts
-subsequent eruption
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45. Igneous Rock Classification
• Based on composition and texture.
• Composition (silica) – Felsic, intermediate, mafic, ultramafic.
• Texture - Fine (aphanitic), coarse (phaneritic).
Type Aphanitic (fine) Phaneritic (coarse)
Felsic Rhyolite Granite
Intermediate Andesite Diorite
Mafic Basalt Gabbro
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46. Igneous Rock Classification
• Composition.
• Texture.
• Grain size.
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47. Igneous Activity Distribution
• Igneous activity tracks tectonic plate boundaries.
• Also in plate interiors – hot spots (Hawaii)
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48. Igneous Activity Distribution
• Igneous activity tracks tectonic plate boundaries.
-convergent boundaries: felsic igneous activity
-divergent boundaries: mafic igneous activity
-hot spots: mafic volcanic activity
-NOT at continental transform boundaries
Chapter 6