Igneous Rocks Lecture 3

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Igneous RocksLecture 3
Types of Rock include Igneous, Sedimentary and
Metamorphic

Marble demo, rock specimens, Petrographic
Microscope, Olivine Porphyry or Gabbro Trays of
mafic and felsic minerals
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Characteristics of magma

• Igneous rocks form as molten rock cools and
  solidifies
• Characteristics of magmas (molten rock) depend on
  parent material and where they crystallize
• Where determines speed of crystallization
• At surface, fast cooling makes small crystals

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Geothermal Gradient
Cool
Silica-rich rocks (with Quartz, K-feldspar) melt
at cooler temperatures. Melts are viscous
Silica-poor rocks (with Olivine, Pyroxene,
Ca-feldspar) melt at higher temperatures Melts
are very fluid
Hot
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Characteristics of magma

• General Characteristics of molten rock
• Forms from partial melting of rocks inside the
  Earth
• 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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Two Geologic Environments Where Igneous Rocks Form
Igneous Rocks Formed in Rift
Igneous Rocks Formed Above Sinking Plate
Both melts are "Basaltic" i.e. Olivine, Pyroxene
and Ca-Feldspars
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Extrusive Igneous Rock - Lava (Hawaii)
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Intrusive Igneous Rock (Granite) This granite
cooled 30 kilometers under the surface
Plagioclase Feldspar
Quartz Amphibole
K Feldspar
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Characteristics of magma

• Three parts
• Liquid portion, called melt, that is
• mobile ions
• Solids, if any, are silicate minerals
• 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
• Rock-forming minerals crystallize with increasing
  complexity as the magma cools. The most complex
  3-D minerals crystallize last. The hottest magmas
  can only crystallize Olivine (Independent
  Tetrahedra), but as the magma cools, more complex
  minerals can form.

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Bowens Reaction Series
Molten- VERY Hot No solids
First mineral to crystallize out
Molten- Not so hot
100 Solid
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Fine crystals Need a microscope
Low silica, HOT, fluid
High silica, warm, viscous
Intermediate
Course crystals Easily seen
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Bowens reactrion series says as a granitic melt
cools, Biotite Mica and Plagioclase
Feldsparcrystallize out before Quartz
Microscope Demo
Granite Hand Sample
Order of Crystallization
We can see the order of crystallization under the
microscope
Granite Thin Section
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Crystals can react with the melt if they touch it
If the first formed crystals of Calcium-rich (Ca)
Plagioclase touch the melt they will react with
it, and will become more sodium-rich on their
outer rims
Zoned feldspar (plagioclase) showing change in
composition with time in magma chamber
(calcium-rich in core to sodium-rich at rim)
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However, if early crystals are removed, the melt
becomes richer in Silica
Remove Fe, Mg, Ca Some Si
Left with K and Al Most of Si
You can start with a Mafic (silica-poor)
magma and end up with some Felsic
(silica-rich) Granites.
Marble Demo
A melt will crystallize its mafic components
first, and the remaining melt may be granitic
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Characteristics of magma

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

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Igneous textures

• Most important is crystal size
• Factors affecting crystal size
• Rate of cooling
• Slow rate promotes the growth of fewer but larger
  crystals
• Fast rate forms many small crystals
• Very fast rate forms glass

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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
Fine grained because it cooled quickly at the
surface
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Phaneritic texture
Coarse crystals cooled slowly at great depth
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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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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

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Pyroclastic Rock - Superheated Flows
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Naming igneous rocks pyroclastic
rocksComposed of fragments ejected during a
volcanic eruptionVarietiesTuff ash-sized
fragmentsVolcanic breccia particles larger
than ash
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Ash and pumice layers
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Still more types of Igneous textures

• Types of igneous textures
• Pegmatitic texture
• Exceptionally coarse grained crystals
• Form in late stages of fractionation of magmas
• This is often what prospectors are looking for

A Pegmatite with Feldspar and Zircon Zircon is
very good for obtaining radiometric ages
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Igneous Compositions
Show tray of Mafic Minerals

• Igneous rocks are composed primarily of silicate
  minerals that include
• dark (or ferromagnesian) colored silicates
• Olivine
• Pyroxene
• Amphibole
• versus

MAFIC Magnesium and Iron
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Igneous Compositions
Show tray of Felsic Minerals

• Igneous rocks also contain light colored silicate
  minerals that include
• Quartz
• Muscovite mica
• Feldspars

FELSIC Feldspar and Silica
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Igneous Rock Classification- Bowens Reaction
Series on its side
Note Minerals in
Note Minerals in
Felsic rocks crystallize from warm melts
Mafic from hot melts
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Igneous compositions

• Naming igneous rocks granitic (felsic) rocks
• Granite
• Phaneritic
• Over 20 percent quartz, about 25 percent or more
  feldspar (usually much more feldspars).
• Plagioclase is Sodium-rich
• Abundant and often associated with mountain
  building
• The term granite covers a wide range of mineral
  compositions

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Igneous compositions

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

fine grained because extruded, so crystallized
quickly
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Igneous compositions

• Basaltic composition can be fine or coarse
• Composed of dark Olivine and Pyroxene and grey
  calcium-rich plagioclase feldspar
• No Potassium-rich feldspar (no K-spar
  Microcline)
• Designated as being mafic (magnesium and ferrum,
  for iron) in composition
• Much denser than granitic rocks - sinks
• Comprises the ocean floor as well as many
  volcanic islands such as Hawaii. Also rift valley
  lavas

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Igneous compositions

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

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Scoria type Basalt note Gas Bubbles
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Igneous compositions

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

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Igneous compositions

• Other compositional groups
• Intermediate (or andesitic) composition
• Contain at least 25 percent dark silicate
  minerals
• Associated with explosive volcanic activity
• Often gray

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Igneous compositions

• Intermediate rocks
• Andesite
• Volcanic origin
• Aphanitic texture
• Often resembles rhyolite
• Intermediate silica content
• Frequent composition in volcanoes above
  subduction zones, e.g. in Andes Mountains

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Igneous compositions

• Extrusive products can include
• Pumice
• Volcanic
• Glassy texture, very light weight, mostly air
• Frothy appearance with numerous voids (extrusive
  foam)
• Forms when lavas have a lot of water and other
  volatiles

Common with intermediate compositions
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Igneous compositions

• Intermediate rocks
• Diorite
• Plutonic equivalent of andesite
• Coarse grained
• Intrusive
• Composed mainly of intermediate feldspar and
  amphibole

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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
• Huge explosion if it erupts (Yellowstone, Toba)

Plutonic Granite
When Yellowstone explodes, half of Wyoming will
perish
Volcanic Rhyolite
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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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Origin of Magma

• Role of Pressure
• Reducing the pressure lowers the melting
  temperature the rock probably melts
• RIDGE When confining pressures drop,
  decompression melting occurs

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Origin of Magma

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

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Assimilation and magmatic differentiation
Show Samples
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Basalts forming in rifts and MORs
Decompression Melting Magma under lithosphere
heats and cracks it. Mantle rock is exposed to
low pressures it partially melts
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Origin of Andesite Diorite intermediate silica
content
Basaltic here
Good diagram for the Andes Mountains
Small blobs, not much heat in them Assimilate
some crust, fractionate
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Plate Tectonics- Andesite Line
Andes
Andesites form above the deep portions of a
subduction zone
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Origin of Granitic Rocks
Huge blobs w/ low temps but lots of magma,
fractionation assimilation gt Granite Batholiths
Can also get small amounts of granites from deep
felsic rock passed by ascending magma