An Introduction to Geology: 5

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An Introduction to Geology 5
A Bit More Time
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A sedimentary recap
Bedding
3
Sedimentary structures
Cross-bedding
4
Sedimentary structures
Cross-bedding
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Sedimentary structures
Ripple marks
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Sedimentary structures

• Shell beds

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Sedimentary structures
Trace fossils
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SedimentaryBritain
At the surface, most of the UK is sedimentary
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Life layer writing

• Ediacaran
• PALAEOZOIC
• Cambrian
• Ordovician
• Silurian
• Devonian
• Carboniferous
• Permian

• MESOZOIC
• Triassic
• Jurassic
• Cretaceous
• CAINOZOIC
• Palaeogene
• Neogene
• Quaternary

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Made Your Own Mnemonic?

• Fabulous prizes for the best ones!

Fossils
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The Dating Game

• Relative Chronology
• versus
• Absolute Chronology

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Problems of relative dating
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The absolute challenge

• Q. How can we determine how old rocks are?

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Sedimentation rates?

• Phillips (1860) 38-96 million years old

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By heat loss?

• Kelvin (1862-1897) 20-40 million years old

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By ocean saltiness?

• Joly (1897-99) 80-100 million years old

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A. By using isotopes!

• 84 natural elements
• 339 isotopes

Pierre Marie Curie
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Isotopes

• Atoms of a chemical element can have different
  numbers of neutrons

Isotopes of hydrogen
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A. By radioactivity!

• 84 natural elements
• 339 isotopes
• Stable
• Radioactive

70 radioactive isotopes 18 with long half-lives
Pierre Marie Curie
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Half-lives

• Nuclei of radioactive atoms decay
• Parents and daughters
• C-14 decays to N-14
• K-40 decays to Ar-40
• U-238 to Pb-206 (via a decay chain)

Ernest Rutherford
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Half-lives

• Nuclei of radioactive atoms decay

Constant decay rate Half-life time to get
5050 parentdaughter
Ernest Rutherford
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Igneous importance
Decay begins once magma crystallizes
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Holmes radioactive rock detective

• Earth is billions of years old

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Geological application

• U-Pb dating of zircons particularly powerful
• (ZrSiO4)

4.374bn (/- 6 Ma) year-old zircon
crystal, Western Australia
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Combined chronology

• ABSOLUTE
• Radiometric analysis only possible for igneous
  (and some metamorphic) minerals
• Expensive, difficult, slow
• RELATIVE
• Fossils widespread, common, cheap and fast to
  analyse

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Common igneous rocks
Granite Intrusive Coarse-grained Felsic
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Common igneous rocks
Basalt Extrusive Fine-grained Mafic
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Common igneous rocks
Dolerite Hypabyssal Medium-grained Mafic
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Common igneous rocks
Gabbro Intrusive Coarse-grained Mafic
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Common igneous rocks
Rhyolite Extrusive Fine-grained Felsic
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Common igneous rocks
Andesite Extrusive Fine-grained Intermediate
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Completing the Rock Cycle
Tectonics Metamorphism
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Completing to the Rock Cycle
Tectonics Metamorphism