Chapter 5 Igneous Rocks Topics we will cover

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Chapter 5- Igneous RocksTopics we will cover

• 1) The Rock cycle Inter-relationships of
  Earths parts
• 2) Igneous Rocks as the first rocks Timeline,
  formation process
• 3) Intrusions and usefulness of igneous rocks as
  sources of valuable materials.
• 4) Classification of Igneous rocks, and some of
  their uses.
• Pegmatites Dramatic and better known examples
  of igneous rocks characteristics.
• Diamonds jewels, tools, and standards for
  reference.

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The Rock Cycle

• There are 3 types of rocks, depending on
  how/where they are formed. Their inter-relations
  are cyclical The Rock Cycle.
• Igneous rocks solidified molten magma
• Sedimentary rocks Form at or near Earths
  surface, from products of weathering (sediment)
  that undergoes lithification.
• Metamorphic rocks Form below Earths surface,
  with high Temperature, pressure, and/or chemical
  change, but no molting.

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The Rock Cycle
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Igneous Rocks- What are they?

• Igneous ignis fire
• Magma Molten rock below Earths surface
• Lava Magma that flows out onto Earths
  surface
• ? Magma and Lava refer to the same materials.
  The difference is where they are located.
• - Igneous rocks Rocks formed during the
  crystallization of magma.

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Types of Igneous Rocks

• Extrusive igneous rocks Fine-grained igneous
  rocks that cool quickly on Earths surface. (Ext
  external, outside to extrude- thrust out)
  (quickly cooling - small grain)
• Example Rhyolite, obsidian, basalt, pumice.
• Intrusive igneous rocks Coarse-grained igneous
  rocks that cool slowly beneath Earths surface.
  (Int internal, from inside to intrude- to
  thrust in) (slowly cooling - large grain)
• Example Granite, diorite, mica
• ________________________________________
• An analogous process of extrusive igneous rock
  formation is the cooling of wax that has been
  poured as a liquid and allowed to solidify on a
  surface

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Igneous rocks - examples
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Types of Magma - composition

• Magma has the same 8 most abundant elements as
  minerals, in the same ranking of abundance O,
  Si, Al, Fe, Mg, Ca, K, Na.
• SiO2 (Silica) is the most abundant compound in
  magma and used in classification
• Rhyolitic magma 70 SiO2
• Andesitic magma 60
• Basaltic magma 50
• Factors in magma formation
• Temperature- (800-1200 oC before most rocks
  begin melting). Increases with depth in Earths
  crust (geothermal gradient)
• Pressure Increases with depth, due to weight
  of layers above. Higher pressure increases rocks
  melting point (under pressure, they melt at
  higher temperature)
• Water content Higher water content decreases
  melting point.
• Mineral composition Different minerals have
  different melting points

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More on Magma Formation

• Temperature (oT) increases with depth (Geothermal
  Gradient), more rapidly in oceanic crust than
  continental crust. ?
• Pressure also increases with depth.
• Both increased oT and pressure are important
  concerns in mining operations.
• As lava moves up in a volcano, pressure decreases
  (lowering melting point) and oT also decreases
  (alllowing solidification)
• Water content decreases the melting point. Rocks
  with ? higher water content melt at lower oT
  (Granite melts before basalt)

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Igneous rock melting and cooling

• MELTING
• Partial melting Process by which some minerals
  melt at lower oT, and others remain solid.
• - Not all parts of a rock melt at the same time,
  some parts dont melt easily (different minerals
  with varying melting points).
• - As each group of minerals melts, it adds
  elements to the forming molted magma
• - Ice does not show partial melting (it melts
  uniformly - a single mineral changing from solid
  state to liquid)
• COOLING
• Fractional crystallization Process by which
  different minerals form at different oT.
• - The reverse order of melting. The first
  minerals to crystallize from magma are the ones
  that molted last.
• - As each group of minerals crystallizes, it
  removes elements from the remaining molted magma.

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- Bowens Reaction Series - (Formation of
Igneous Rocks as Magma Cools)
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- Bowens Reaction Series - (Formation of
Igneous Rocks as Magma Cools)
What is it? A model of how igneous rocks and
minerals in them form as magma cools down and
solidifies (crystallizes). What are its
parameters? As time passes. 3 things happen 1-
Declining temperature From very hot, to
colder 2- Change of state From all molten rock
(liquid), to partially molten (liquid-solid), to
completely solid. 3- Changing type of Magma For
example, from low Silica content, to high Silica
content.
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Bowens Reaction Series - The Two Branches
Fe Mg- rich Minerals
The Right Branch- Light color minerals (felsic)
The Left Branch- Dark color minerals (Mafic
green, dark and Black)
Sodium- rich
Combination Mixtures with intermediate color
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Bowens Reaction Series

• As magma cools, minerals form in predictable
  patterns (N.L. Bowen, early 1900s).
• As minerals form, they continue to react with the
  molten Magma, and may change from one type to
  another.
• 2 main patterns (branches) of fractional
  crystallization, and thus of mineral composition
• 1) Continuos, gradual change of mineral
  composition in the feldspar group (right branch
  of Bowens model).
• - The first feldspars formed are Ca-rich, those
  later are Na-rich.
• - When magma cools quickly, feldspar crystals
  with distinct zones form (Ca-rich core, Na-rich
  outer layers)
• 2) Abrupt change of mineral type in the
  iron-magnesium groups (left branch of Bowens
  model).
• - The Temperature of molten Magma begins to
  cool-down. Very different minerals form as
  Temperature falls, each time changing the one
  formed before (Olivine, Pyroxene, Amphibole,
  Biotite mica).
• Quartz (SiO2) crystallizes as the last liquid
  portion of magma. This last liquid material
  contains lots of silica and water. It squeezes
  into fractures in previously formed rocks. Thus,
  it is most often seen as veins.

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- Bowens Reaction Series - (Formation of
Igneous Rocks as Magma Cools)
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More on Bowens series

• Crystal separation
• - Sometimes, reactions between formed minerals
  and molten magma stop. Crystals settle to the
  bottom, and distinct igneous types are formed.
• Layered igneous intrusions
• - Minerals may form in distinct bands. These
  layered igneous intrusions can be very valuable
  sources of rare metals.

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Classification of Igneous Rocks by mineral
composition

• Felsic rocks (granite, rhyolite, pegmatite)
• Light color
• High silica content
• Contain quartz and 2 feldspars (orthoclase,
  plagioclase)
• Intermediate (andesite, diorite)
• Medium color
• Mafic rocks (basaltic glass, basalt, gabbro)
• Dark color
• Lower silica content
• Rich in Fe and Mg
• Contain plagioclase, biotite, amphibole,
  pyroxene, olivine
• Ultramafic rocks (peridotite, dunite)
• Mostly green
• Very high content of Fe and Mg
• Low silica content

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(Ultrabasic)
(Basaltic)
(Granitic)
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Grain size and texture

• 1- Coarse grained (phaneritic) When magma cools
  slowly inside Earth (intrusive rocks). The
  majority of crystals are large and of of uniform
  size. (ex. Gabbro)
• 2- Fine grained (aphanitic) When molten
  material cools quicky on or near the surface of
  Earth (extrusive rocks). (ex. Basalt, rhyolite)
• 3- Porphyritic Magma having 2 rates of cooling,
  first fast, then slow (or viceversa), ending in 2
  very contrasting sizes of crystals within the
  same rock large crystals surrounded by smaller
  crystals. (Intrusive) (ex.andesite)
• 4- Glassy Rapid cooling with no time for
  crystals to form. No discernible grain.
  Extrusive. (ex. Obsidian).
• Sometimes, a frothy appearance similar to spun
  glass (ex. Pumice).
• Thus, volcanic glass cannot be classified based
  on mineral percentage. It must be classified on
  the basis of the elements therein, by chemical
  analysis.
• 5- Fragmental The rock contains broken, angular
  fragments of rocky materials produced during an
  explosive volcanic eruption (not as flowing lava)

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Uses of Igneous Rocks

• As building materials
• - Very strong due to interlocking grain texture
  (ex. Granite)
• - Resistant to weathering, thus very durable
  (ex. Quartz, granite)
• - Super-sharp edges for scalpels in surgery (ex.
  obsidian)
• Ore deposits
• - Layered intrusions within igneous rocks.
• - Veins Formed as the last portions of molten
  magma to cool down, and it fills cracks in other
  rocks cooled previously. Often include metals
  (Ag, Au, Pb, Cu) along with silicon and Oxygen.
  The result is quartz veins that contain important
  metals.
• - Pegmatites veins containing elements
  different from metals, often Lithium and
  Beryllium. Beautiful large crystals are frequent.
• - Kimberlites Ultramafic rocks possibly formed
  deep in the crust, at high pressure. Very large
  crystals are formed within pipe-like structures.
  Diamonds are frequently found in kimberlite
  formations.