Igneous Rocks

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Igneous Rocks

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Origin of Magma

• Highly debated topic
• Generating magma from solid rock
• Produced from partial melting of rocks in the
  crust and upper mantle
• Role of heat
• Temperature increases within Earths upper crust
  (called the geothermal gradient) average between
  20oC to 30oC per kilometer

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Approximate temperatures in the crust and mantle
Deep Potosi Mine Anecdote
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If geothermal gradient is 25 degrees C per km,
and temperature at surface is 10 degrees C,What
is temperature at a depth of 30 km?
Clicker Question 4.1

• 660 deg C
• 760 deg C
• 860 deg C
• 960 deg C
• 1060 deg C

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Origin of Magma

• Role of heat
• Rocks in the lower crust and upper mantle are
  near their melting points
• Any additional heat (from rocks descending into
  the mantle or rising heat from the mantle) may
  induce melting

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Origin of Magma

• Role of pressure
• An increase in confining pressure causes an
  increase in a rocks melting temperature or
  conversely, reducing the pressure lowers the
  melting temperature
• When confining pressures drop, decompression
  melting occurs

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Decompression melting
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Origin of Magma

• Role of volatiles
• Volatiles (primarily water) cause rocks to melt
  at lower temperatures
• This is particularly important where oceanic
  lithosphere descends into the mantle

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Evolution of magmas

• Processes responsible for changing a magmas
  composition
• Magmatic differentiation
• Separation of a melt from earlier formed crystals
  to form a different composition of magma
• Assimilation
• Changing a magmas composition by the
  incorporation of foreign matter (surrounding rock
  bodies) into a magma

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Assimilation and magmatic differentiation
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• Which process is likely to change a
• magmas chemical composition to a
• chemistry more similar to the host rock?
• Assimilation
• Differentiation
• Crystallization
• None of above

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Evolution of magmas

• Processes responsible for changing a magmas
  composition
• Magma mixing
• Involves two bodies of magma intruding one
  another
• Two chemically distinct magmas may produce a
  composition quite different from either original
  magma

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Evolution of magmas

• Partial melting and magma formation
• Incomplete melting of rocks is known as partial
  melting
• Formation of basaltic magmas
• Most originate from partial melting of ultramafic
  rock in the mantle
• Basaltic magmas form at mid-ocean ridges by
  decompression melting or at subduction zones

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Evolution of magmas

• Partial melting and magma formation
• Formation of basaltic magmas
• As basaltic magmas migrate upward, confining
  pressure decreases which reduces the melting
  temperature
• Large outpourings of basaltic magma are common at
  Earths surface (Deccan Traps)
• The presence of water promotes partial melting of
  the mantle to create basalt

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Evolution of magmas

• Partial melting and magma formation
• Formation of andesitic magmas
• Interactions between mantle-derived basaltic
  magmas and more silica-rich rocks in the crust
  generate magma of andesitic composition
• Andesitic magma may also evolve by magmatic
  differentiation

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Evolution of magmas

• Partial melting and magma formation
• Formation of granitic magmas
• Most likely form as the end product of
  crystallization of andesitic magma
• Granitic magmas are higher in silica and
  therefore more viscous than other magmas
• Because of their viscosity, they lose their
  mobility before reaching the surface
• Tend to produce large plutonic structures (Pikes
  Peak, Rocky Mtn Natl Park)

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Characteristics of magma

• Igneous rocks form as molten rock cools and
  solidifies
• Characteristics of magma (molten rock)
• Parent material of igneous rocks
• Forms from partial melting of rocks inside the
  Earth
• Magma that reaches the surface is lava

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Geologic Environments Where Igneous Rocks Form
Igneous Rocks Formed in Rift
Igneous Rocks Formed Above Sinking Plate
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Clicker Question 4.2 How do you melt rock?
A) Lower pressure B) Raise temperature C) Add
H20 D) B C E) A, B, C
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Characteristics of magma

• General Characteristic of magma
• 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

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• Is this an extrusive, or intrusive igneous rock?
• Extrusive
• Intrusive

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Intrusive Igneous Rock (Granite) - Sorry no
pictures available from 25 kilometers down!
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Characteristics of magma

• The nature of magma
• Consists of three components
• A liquid portion, called melt, that is composed
  of mobile ions
• Solids, if any, are silicate minerals that have
  already crystallized from the melt
• Volatiles, which are gases dissolved in the melt,
  including water vapor (H2O), carbon dioxide
  (CO2), and sulfur dioxide (SO2)

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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

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Olivene sand on the worlds only Green Sand
beach on the southern Big Island - Hawaii.
Olivene is derived from erosion of olivene
crystals in volcanic rock.
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Crystallization of minerals in magma bodies

• Bowens reaction series and the composition of
  igneous rocks
• N.L. Bowen demonstrated that as a magma cools,
  minerals crystallize in a systematic fashion
  based on their melting points

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Bowens Reaction Series
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Granite Hand Sample
Order of Crystallization
Granite Thin Section
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Zoned feldspar (plagioclase) showing change in
composition with time in magma chamber (calcic
in core to sodic at rim)
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• Which mineral crystallized first?
• Orthoclase
• Quartz
• Olivene

• Which mineral do you think crystallized first?
• Feldspar (Plagioclase)
• Quartz
• Hornblende

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Characteristics of magma

• Crystallization of magma
• Texture in igneous rocks is determined by the
  size and arrangement of mineral grains
• Igneous rocks are typically classified by both
• Texture
• Mineral composition

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Igneous textures

• Texture is used to describe the overall
  appearance of a rock based on the size, shape,
  and arrangement of interlocking minerals
• Factors affecting crystal size
• Rate of cooling
• Slow rate promotes the growth of fewer but larger
  crystals

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Igneous textures

• Factors affecting crystal size
• Rate of cooling
• Fast rate forms many small crystals
• Very fast rate forms glass
• Amount of silica (SiO2) present
• Amount of dissolved gases

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Hint What are the small grey minerals? Qtz,
Plag or Biotite? The big one is Plagioclase

• Assuming little change in bulk chemistry or
  dissolved volatiles, what does the texture of
  this rock tell you about its cooling history?
• It cooled evenly deep in the Earth
• It cooled slowly first (deep), and much faster
  later (shallow)
• It first cooled quickly, then cooled slowly
• It was heated and never cooled
• None of the above

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Types of Igneous textures

• 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

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Aphanitic texture
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Phaneritic texture
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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

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Porphyritic texture
Granite
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Glassy texture
Obsidian
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Yet more types of Igneous textures

• Types of igneous textures
• Pyroclastic texture
• Various fragments ejected during a violent
  volcanic eruption
• Textures often appear to more similar to
  sedimentary rocks
• Pegmatitic texture
• Exceptionally coarse grained
• Form in late stages of crystallization of
  granitic magmas

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Pyroclastic Rock - Superheated Superfast Flows
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A beautiful pegmatite vein of orthoclase and
quartz
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Igneous Compositions

• Igneous rocks are composed primarily of silicate
  minerals that can include
• Dark (or ferromagnesian) silicates
• Olivine
• Pyroxene
• Amphibole
• Biotite mica

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Igneous Compositions

• Igneous rocks are composed primarily of light
  silicate minerals that include
• Quartz
• Muscovite mica
• Feldspars

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Igneous compositions

• Granitic versus basaltic compositions
• Granitic composition
• Composed of light-colored silicates
• Designated as being felsic (feldspar and silica)
  in composition
• Contains high amounts of silica (SiO2)
• Major constituents of continental crust

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Igneous compositions

• Naming igneous rocks granitic (felsic) rocks
• Granite
• Phaneritic
• Over 65 percent quartz, about 25 percent or more
  feldspar
• Abundant and often associated with mountain
  building
• The term granite covers a wide range of mineral
  compositions

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Granite
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Igneous compositions

• Naming igneous rocks granitic (felsic) rocks
• Rhyolite
• Extrusive chemical equivalent of granite
• May contain glass fragments and vesicles
• Aphanitic texture
• Less common and less voluminous than granite
• Phenocrysts can include quartz and feldspar

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Rhyolite
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Igneous compositions

• Granitic versus basaltic compositions
• Basaltic composition
• Composed of dark silicates and calcium-rich
  feldspar
• Designated as being mafic (magnesium and ferrum,
  for iron) in composition
• More dense than granitic rocks
• Comprise the ocean floor as well as many volcanic
  islands

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Igneous compositions

• Naming igneous rocks basaltic (mafic) rocks
• Basalt
• Volcanic origin
• Aphanitic texture
• Composed mainly of pyroxene and calcium-rich
  plagioclase feldspar
• Most common extrusive igneous rock

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Basalt
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Basalt (scoria with gas bubbles)
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Igneous compositions

• Naming igneous rocks basaltic (mafic) rocks
• Gabbro
• Intrusive equivalent of basalt
• Phaneritic texture consisting of pyroxene and
  calcium-rich plagioclase
• Makes up a significant percentage of the oceanic
  crust

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Gabbro - a mafic igneous rock
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Igneous compositions

• Other compositional groups
• Intermediate (or andesitic) composition
• Contains at least 25 percent dark silicate
  minerals
• Associated with explosive volcanic activity
• Ultramafic composition
• Rare composition that is high in magnesium and
  iron
• Composed entirely of ferromagnesian silicates

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Igneous compositions

• Naming igneous rocks intermediate rocks
• Andesite
• Volcanic origin
• Aphanitic texture
• Often resembles rhyolite

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Andesite
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Igneous compositions

• Naming igneous rocks intermediate rocks
• Diorite
• Plutonic (intrusive) equivalent of andesite
• Coarse grained
• Composed mainly of intermediate feldspar and
  amphibole

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Diorite
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Name the aphanitic volcanicrock that is probably
basalt.
Clicker Questions 4.3
C)
A)
D)
B)
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Name the phaneritic intrusive igneous rock that
is granite.
A)
C)
D)
B)
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Name the aphanitic volcanicrock that is probably
rhyolite.
C)
A)
D)
B)
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Igneous compositions

• Silica content influences a magmas behavior
• Granitic magma
• High silica content
• Extremely viscous
• Liquid exists at temperatures as low as 700oC
• Goes Boom when it erupts (Yellowstone)

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Igneous compositions

• Silica content influences a magmas behavior
• Basaltic magma
• Much lower silica content
• Fluid-like behavior
• Crystallizes at higher temperatures
• Gurgles when it erupts (Hawaii)

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Comparison of Igneous Rocks
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Summary - Igneous Rock Classification
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Igneous Textures

• Naming igneous rocks granitic (felsic) rocks.
  Extrusive products can include
• Obsidian
• Dark colored
• Glassy texture
• Pumice
• Volcanic
• Glassy texture
• Frothy appearance with numerous voids (extrusive
  foam)

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Obsidian
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Pumice
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Frothy appearance with numerous voids (extrusive
foam)
Guinness at Night
Head of gas bubbles on a pint of Guinness is
a perfect analog for pumice (and it tastes a lot
better!)
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Clicker Question
B)
Name the intrusive igneous rock
Name the following rock types.List two key
characteristics that identify each specimen
A)
Gabbro Obsidian Pumice
C)
D) A) and B)
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Igneous compositions

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

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Ash and pumice layers
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Summary - Igneous Rock Texture and Composition