Dr Ian Clark

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Introductory Soil Science

• Lecture 3
• The Rock Cycle

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Last time

• Cycles in the Earth system
• The changes and balancing take place via cycles
• Chemical elements such as oxygen and carbon
  circulate through the Earth by photosynthesis and
  respiration
• These are called geochemical cycles or
  biogeochemical cycles
• The systems can be viewed as reservoirs and the
  cycles trace the flux or flow between them

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Hydrological Cycle
Atmosphere
transpiration
evaporation
precipitation
evaporation
Ocean
Surface water
infiltration
Groundwater
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Earth System
Interactions between Sub-systems Energy in from
Sun Energy out
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Energy sources

• Two energy sources drive the cycles in the Earth
  System
• Sun
• Drives surface processes like weather
• Internal Energy
• Drives processes like volcanic eruptions,
  earthquakes etc

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The Rock Cycle

• Driven by internal and external energy sources
• Leads to the continuing differentiation of the
  solid Earth materials

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References

• All introductory geology books deal with this
  topic, for example
• Murck B., Skinner B Porter S 1996 Environmental
  Geology J Wiley Sons New York
• pp 34-35 46-53
• Press F Siever R 1997 Understanding Earth WH
  Freeman Co New York
• pp 26-45

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What do you need to know about the rock cycle

• Recognise that earth materials cycle between
  reservoirs
• Reservoirs are rocks
• Cycle is driven by internal and external energy
  sources
• Rock cycle is unique to Earth in our Solar System
  and explains the constantly changing morphology
  of the Earths surface

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Structure of the Earth
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The Rock Cycle

• Driven by both energy sources
• Cycles materials which make-up solid Earth
• Material cycled between three main reservoirs
• Igneous rocks
• Sedimentary rocks
• Metamorphic rocks
• Other minor reservoirs are assoc. with these
• Soil is one important minor reservoir that we
  will deal with in detail.

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Rock Cycle
Igneous Rock
magma
Metamorphic Rock
sediment
Sedimentary Rock
soil
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Rock Cycle

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Characteristics of main rock groups

• Igneous Rocks
• Form by crystallisation of molten material called
  magma
• Crystalline
• Form on or below the Earths surface
• On extrusive (Volcanic)
• Below intrusive (Plutonic)
• Examples
• Granite, basalt

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Characteristics of main rock groups

• Sedimentary Rocks
• Form at the Earths surface from pre-existing
  rock
• Composed of fragments or chemical precipitate
• Most form in shallow seas
• Can form anywhere on Earth
• Examples
• Sandstone, limestone

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Characteristics of main rock groups

• Metamorphic Rocks
• Form by recrystallisation of existing rocks as
  the result of extreme heat and pressure
• Crystalline

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Characteristics of main rock groups

• Metamorphic Rocks (cont)
• Form at depth below the Earths surface or
  adjacent to igneous intrusions
• Analogous to making pots from clay and heating
  them in a kiln
• Examples
• Quartzite, marble, slate, schist gneiss

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Minerals

• Minerals make-up rocks
• Some rocks are composed of only one type of
  mineral (mono-minerallic)
• Others are composed of several different kinds of
  minerals
• There are more than 3000 different minerals
• Only a few are common

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What do you need to know about minerals

• Recognise that rocks are composed of minerals
• Composition of minerals is related to the
  relative abundance of the elements that make-up
  Earth
• Only a small number of minerals make-up rocks
• Different groups of minerals characterise the
  three main groups of rocks
• Properties of minerals are related to their
  chemistry

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Minerals

• Minerals are
• Naturally occurring
• Solid
• Inorganic
• and must have
• a specific chemical composition
• a regular atomic arrangement (crystalline)

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Chemical composition of minerals

• Most minerals are composed of compounds of
  silicon and oxygen (called silicates)
• Why?
• Because silicon oxygen make-up 74 of the crust
• These with 6 other elements make-up more than 99
  of the crust

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Commom rock forming minerals

• 11 minerals make up most rocks

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What do you need to know about Silicate Minerals

• Silicon oxygen are joined together in minerals
  by chemical bonds. Because of their relative
  sizes they form tetrahedra
• Relative sizes of the atoms controls the silicate
  structure
• Different silicate frameworks are formed by
  sharing different numbers of oxygen atoms between
  adjacent silica tetrahedra
• Families of minerals have similar properties
  because they have similar silicate frameworks

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Silicate Minerals

• Silicon fits neatly into the hole made when 4
  oxygens arrange themselves as a tetrahedron

because size (charge) is important atoms of
similar size can substitute for silicon eg. Al
Fe.
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The Silicate Tetrahedron

• Complex anion of silicon (4) and oxygen (2-)
• (Si04)4-
• Regular tetrahedron

O
Si
O
O
O
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Silicate Minerals

• Silica tetrahedron is the basic building block of
  the silicate minerals
• Tetrahedra can join by sharing one or more oxygen
  atoms with adjacent tetrahedra
• Possible to share 0,1,2,3 or 4
• Each represents a different group of silicate
  minerals.

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Isolated Tetrahedra eg. olivine Mg2SiO4
Bonds between Oxygen and magnesium atoms
Tetrahedra pointing up Tetrahedra pointing
down
Magnesium
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Single Chain Structures (SiO3)2-

• Each tetrahedron shares two oxygen atoms
• Pyroxene group
• e.g. Diopside CaMg(SiO3)2

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Double Chain Structures (Si4O11)6-

• Two single chains joined
• Amphibole group
• e.g. Tremolite Ca2Mg5(Si4O11)2(OH)2

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Sheet Structures (Si4O10)4-

• Tetrahedra linked by three oxygen atoms
• Micas,Chlorite,Clays
• Al, K etc. bonded to oxygen atoms to hold the
  sheets together

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Silicate Minerals
The basic tetrahedral silicate structure can be
built into 5 different structures by sharing
0,1,2,3,or all 4 oxygen atoms between adjacent
tetrahedra.

• sharing 0 island silicates olivine
• sharing 1 single chain pyroxene
• sharing 2 double chain amphibole
• sharing 3 sheet structure micas, clays
• sharing 4 3-D structure quartz

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Concept list

• Earth
• Closed System
• Cycles
• Energy
• Internal
• External
• Hydrosphere
• Lithosphere
• Atmosphere
• Biosphere

• Crust
• Minerals
• Rocks
• Silicon
• Oxygen
• Igneous
• Sedimentary
• Metamorphic