GEOL 206 Earthquakes, Volcanoes and Plate Tectonics

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GEOL 206Earthquakes, Volcanoes and Plate
Tectonics

• Prof. Chris DeVries
• Fall 2006

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Minerals

• Formal definition
• Minerals must
• be crystalline solids
• occur naturally
• be of definite chemical composition
• be inorganic

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Crystalline Solids?
Eg. Halite, NaCl (table salt) Cubic arrangement
of Na (red) and Cl- (yellow) ions Halite
crystals are cubic in shape.
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Definite Chemical Composition?

• An analysis of the mineral will always produce
  the same ratio of elements.
• E.g. if you analysed NaCl, you would find a 11
  ratio of sodium to chlorine, always.
• E.g. if you analysed 2 different crystals of
  (Mg,Fe)2SiO4 (the mineral olivine), where Mg and
  Fe can substitute for each other in the
  structure, the ratio of Mg Fe to Si would be
  21 in both crystals.

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Inorganic?

• Organic compounds are those that contain carbon
  and hydrogen and are made by living organisms.
• Some organisms can make minerals
• E.g. shell-building organisms, shells of calcite
  or aragonite (CaCO3). Calcite is inorganic.
• Amber (petrified tree sap) is NOT technically a
  mineral, as it is organic.

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Silicate Minerals

• Most common igneous minerals
• Form four major different structures based on the
  silicate tetrahedron (SiO44-)
• To form stable minerals silicate tetrahedra
• Charge-balance with positively-charged ions
• Share oxygen atoms with other tetrahedra

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Form 1 Isolated silicate structure

• Single tetrahedra joined by positively charged
  ions
• E.g. olivine ((Mg,Fe)2SiO4)

Individual SiO44- tetrahedra joined by Mg2 and
Fe2 ions (green balls)
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Form 2 Chain structures

• Single or double chains of tetrahedra joined by
  positively-charged ions

Augite (a pyroxene) is a single-chain silicate
mineral
Single and double chains of silicate tetrahedra
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Form 3 Sheet structures

• Sheets of silicate tetrahedra joined together by
  positively-charged ions

A sheet silicate
Mica is a sheet silicate
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Form 4 Framework structure

• A framework structure contains silicate
  tetrahedra joined in a 3-dimensional array
• Quartz (below), SiO2 is the best example of a
  framework silicate

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Rocks

• Made up of minerals
• Igneous Rocks
• Formed from solidifying magma
• Sedimentary Rocks
• Formed from the lithification (compaction and
  dehydration) of sediments in oceans and lakes
• Metamorphic Rocks
• High pressure and/or high temperature serve to
  re-crystallise igneous, sedimentary or existing
  metamorphic rocks in a process called
  metamorphism
• Contain same chemical elements in same ratios as
  original rock
• Water and other fluids can be lost or added
  however
• Contain different mineral assemblages than the
  parent rock

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The Rock Cycle
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Question There are a few (I wont say how many)
arrows missing from this rock cycle
representation, and perhaps one box. Add them in!
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A more complete cycle!
Shell-forming marine organisms
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Classifying Igneous Rocks

• Common to classify igneous rocks based on their
  silica content
• Felsic rocks have a high (gt65) SiO2 content
• (fel feldspar, a common mineral, sic silica)
• Mafic rocks have a low (50) SiO2 content
• (ma magnesium, f ferric, referring to Fe
  iron)
• Intermediate rocks have a SiO2 content between
  felsic and mafic
• Ultramafic rocks have very low (lt45) SiO2
  content
• Rule of thumb
• The darker an igneous rock is, the less silica it
  has
• There are exceptions!

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Intrusive vs. Extrusive

• Igneous classification based on whether the rock
  solidified below ground, or erupted from a
  volcano
• Intrusive rocks tend to be coarser-grained as
  they cool very slowly underground, giving
  crystals time to form
• Plutons vs. dikes vs. sills (see diagram)
• Extrusive rocks are usually fine-grained, often
  so that you cannot even see individual crystals,
  as they solidify in minutes to hours.
• Any crystals that you do find in extrusive rocks
  will usually have formed before eruption

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The major igneous rock types
Intrusive
Extrusive
Felsic
Mafic
In the volcano section, these terms will be
revisited in greater detail