Origins of Rocks:

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Origins of Rocks

• Where do rocks come from?
• Why do they form?
• How do they form?
• (Mineralology and Petrology)
• (L2 L3)

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What is a rock?

• A naturally-occurring aggregate of minerals
• CD EM/A,B

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What is a Mineral?

• A naturally occurring, inorganic, crystalline
  solid with a specific chemical composition
• A specific chemical composition means that the
  composition of a mineral can be expressed as a
  chemical formula
• Example halite (salt) is NaCl

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Identify Minerals by Their Physical
Properties See CD Earth Materials
Specific Gravity Color Streak Luster Acid Test
Crystal Habit Cleavage Fracture
Striations Hardness
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What is a Crystal?

• A form of matter which has a regular, repeating
  framework of atoms

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Halite (salt) crystals (NaCl)
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What are atoms?CDSlides 1094-1105 in Geo Time

• The smallest unit of an element that retains the
  elements physical and chemical properties
• An element is a substance that cannot be broken
  down into a simpler substance
• Made up of protons, neutrons and electrons
• examples gold, iron, hydrogen, oxygen, sodium,
  chlorine, carbon, silicon, helium

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Protons, Neutrons, Electrons

• Protons have positive charge mass of 1
• Neutrons have no charge mass of 1
• Protons and neutrons make up the nucleus of an
  atom
• Electrons have a negative charge, NO mass and are
  most important for chemical reactions

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Structure of an Atom
Protons /1 Neutrons 0/1 Electrons -/0
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IONS ISOTOPES

• IONS
• /- CHARGED ATOMS
• FORMERLY NEUTRAL ATOMS WHICH HAVE GIVEN UP () OR
  TAKEN ON AN ELECTRON (-)
• INVOLVED IN IONIC BONDING

• ISOTOPES
• ELEMENTS WITH VARIABLE ATOMIC WEIGHT DUE TO
  VARYING NUMBERS OF NEUTRONS IN THE NUCLEUS.

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Hydrogen atom Mass 2 1 proton, 1 electron
Carbon atom Mass 12 6 protons, 6 neutrons, 6
electrons
Electrons move in orbitals (not orbits!)
forming shell-like spheres around the nucleus.
If atoms loose or gain electrons they have an
electrical charge (-/) and are called ions
also, a group of atoms (molecule) can behave as
an ion if they have too many or too few electrons.
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What holds the atoms togetherin crystals?

• Chemical bonds
• Ionic Bonds
• atoms (ions) held together by their electrical
  charges
• weaker kind of chemical bond
• Covalent Bonds
• atoms held together because they share electrons
• stronger kind of chemical bond

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Example of an ionic bond NaCl
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Example of covalent bonding Carbon
Naturally occurring crystalline carbon is the
mineral diamond Covalent bonding between the
carbon atoms make this the hardest known substance
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WHAT FACTORS DETERMINE WHAT MINERAL WILL FORM?
1 WHAT ELEMENTS ARE AVAILABLE 2 ABUNDANCE OF
EACH ELEMENT 3 SIZE CHARGE OF EACH ELEMENT 4
ENVIRONMENT - SPACE, PRESSURE, TEMPERATURE
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Ionic Radii charge of some geologically
important ions allows ionicsubstitution or
SOLID SOLUTION
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Average Composition of the Continental Crust
O
O
Si
Weight Percent
Volume Percent
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COMMON ROCK-FORMINGMINERAL FAMILIES
NATIVE ELEMENTS - GOLD, COPPER HALIDES -
PRECIPITATES (salts) OXIDES - 1 METAL
OXYGEN SULFIDES - 1 METAL SULFUR SULFATES -
(SO4)2- CARBONATES - (CO3)2- SILICATES - SUPER
STARS!!!
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Silicates

• The largest group of minerals are the silicates
• Silicates are distinguished by being composed of
  the silicate ion (SiO4)4-
• an ion of Silicon has a charge of 4
• four ions of Oxygen have a charge of 8-

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Silicate ions have a tetrahedral shape

• In other words, theyre four-sided pyramids

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Tetrahedrons can link together to make complex
crystals
isolated ring single
chain double chain
sheet
framework--like sheet but in three dimensions
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Where do rocks come from?

• All rocks are made from other rocks!
• Three basic families of rocks based on their
  origin
• Igneous
• Sedimentary
• Metamorphic
• CD EM/C.

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Igneous Rocks

• Igneous rocks formed when their constituent
  minerals crystallized out of molten rock as it
  cooled from a high temperature
• Three ways to melt rocks
• increase in temperature - hot spots
• decrease in pressure - mid-ocean ridges
• add water - subduction zones

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Where do Igneous rocks form?

• Inside the earth (intrusive/Plutonic)
• in cracks dikes
• in-between rock layers sills, laccoliths
• in magma chambers
• magma is molten rock beneath the surface of the
  Earth
• Surface of the earth (extrusive/Volcanic)
• lava flows
• pyroclastics pumice and ash

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Sedimentary Rocks?

• Formed from sediments settling out of a fluid
• sediments are
• particles worn off of other rocks
• tiny mineral grains precipitated out of a fluid
• for example, salt, gypsum, limestone
• created by biologic activity
• Fluid can be either air or water
• mudstones form in water
• petrified sand dunes form in air

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Metamorphic rocks?

• Formed when rocks are exposed for a long time to
  a different (usually higher) pressure /
  temperature than which they formed in
• most minerals are only stable at a particular
  pressure and temperature range
• Examples
• high pressure downgoing slab in subduction
  zones
• high temperature rocks next to magma/lava
• high pressure and temperature roots of mountain
  ranges

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high pressure
high temperature
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The Rock CycleCD Earth Materials, C.

• Since every rock is formed out of pre-existing
  rocks, the evolution of rocks can be shown to be
  a rock cycle, which illustrates how every rock
  type can be formed out of the other two types.

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Igneous RocksCDEM/D

• There are a wide variety of igneous rocks, and
  they can be better understood if we sort them by
  their
• Texture
• size distribution of mineral grains
• Chemical composition
• types of minerals present

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Igneous Rock Textures

• Aphanitic (Fine) Texture
• Very tiny crystals
• Indicates rapid cooling extrusive or volcanic
  rocks
• volcanic rocks form on the surface or shallow
  subsurface
• Phaneritic (Coarse) Texture
• Large crystals
• Indicated slow cooling intrusive or plutonic
  rocks
• plutonic rocks form underground
• Porphyritic (Mixed) Texture
• Large crystals set in a matrix of tiny crystals
• Indicates slow then fast cooling

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Aphanitic Texture (Fine)
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Phaneritic Texture (Large)
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Porphyritic Texture (Mixed)
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Igneous Rock Chemistry

• Most igneous rocks are made of
• Oxygen (O)
• Silicon (Si)
• Aluminum (Al)
• Iron (Fe)
• Magnesium (Mg)
• Calcium (Ca)
• Sodium (Na)
• Potassium (K)

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Igneous Rock Chemistry

• All igneous rocks can be described as mafic,
  intermediate or felsic
• Mafic Igneous Rocks
• Dominated by dark-colored mineral grains
• High Fe, Mg, Ca
• Low Si
• Felsic Igneous Rocks
• Dominated by light-colored mineral grains
• Low Fe, Mg
• High Si

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Igneous Rock Chemistry

• Finally, igneous rocks can be further subdivided
  on the basis of how much Sodium (Na) and
  Potassium (K) they contain
• If they have more Na and K than Ca, they are
  considered to be alkaline igneous rocks (Felsic)

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Chemistry and Minerals

• The kind of minerals an igneous rock contains
  results from the chemical composition of the
  parent magma, the molten rock from which the rock
  formed
• The chemistry of the parent magma is the result
  of two processes
• Partial melting of a source rock
• Fractional crystallization of the magma as it
  cools

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Partial Melting

• Any given rock is made of several different
  minerals with different melting temperatures
• When a rock begins to melt, some minerals will
  melt before others
• This process results in a melt which has a
  different chemical composition than the rock from
  which it is forming

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Fractional Crystallization

• As a magma cools, crystals will start to grow
  within it
• These crystals remove elements from the magma,
  changing the chemical composition of the
  remaining melt
• This remaining melt can then grow crystals
  different from those that have already grown from
  it

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Bowens Reaction Series
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Classification of Igneous Rocks
volcanic
felsic intermediate mafic
ultramafic
plutonic
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Hawaiian Igneous Rocks

• It takes a great many steps of fractional
  crystallization to make a felsic rock out of a
  mafic one
• In Hawaii, most of the magma was erupted too
  quickly for it to be anything other than a mafic,
  aphanitic rock a basalt
• Therefore, the most common minerals found in
  Hawaiian igneous rocks are olivine and Ca-rich
  plagioclase

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Another way of looking at it...
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Magma Plate Tectonics

• Ultramafic magmas - mantle source
• Mafic magmas - partial melt of mantle
• Intermediate magmas - of oceanic crust
• Felsic magma - partial melt of continental crust
• Dont forget that the mother of all rock is
  PERIDOTITE.

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Assignment V6