An Introduction to Geology: 5
A Bit More Time
A sedimentary recap
Bedding
Sedimentary structuresCross-bedding
Sedimentary structuresCross-bedding
Sedimentary structures
Ripple marks
Sedimentary structuresShell beds
Sedimentary structuresTrace fossils
SedimentaryBritain
At the surface, most
of the UK is sedimentary
Life layer writing
Ediacaran
PALAEOZOIC:
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Permian
MESOZOIC:
Triassic
Jurassic
Cretaceous
CAINOZOIC
Palaeogene
Neogene
Quaternary
Made Your Own Mnemonic?
Fabulous* prizes for the best ones!
*Fossils
The Dating Game
Relative Chronologyversus
Absolute Chronology
Problems of relative dating
The absolute challenge
Q. How can we determine how old rocks are?
Sedimentation rates?Phillips (1860) = 38-96 million years old
By heat loss?Kelvin (1862-1897) = 20-40 million years old
By ocean saltiness?Joly (1897-99) = 80-100 million years old
A. By using isotopes!
Pierre & Marie Curie
84 natural elements
339 isotopes
IsotopesAtoms of a chemical element can have
different numbers of neutrons
Isotopes of hydrogen
A. By radioactivity!
Pierre & Marie Curie
84 natural elements
339 isotopes– Stable– Radioactive
70 radioactive isotopes
18 with long half-lives
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
Half-lives
Nuclei of radioactive atoms decay
Ernest Rutherford
Constant decay rate:
Half-life = time to get 50:50 parent:daughter
Igneous importanceIgneous importance
Decay begins once Decay begins once magma crystallizesmagma crystallizes
Holmes: radioactive rock detective
Earth is billions of years old
Geological applicationU-Pb dating of zircons particularly powerful
(ZrSiO4)
4.374bn (+/- 6 Ma)year-oldzircon crystal,Western Australia
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
Common igneous rocks
Granite
Intrusive
Coarse-grained
Felsic
Common igneous rocks
Basalt
Extrusive
Fine-grained
Mafic
Common igneous rocks
Dolerite
Hypabyssal
Medium-grained
Mafic
Common igneous rocks
Gabbro
Intrusive
Coarse-grained
Mafic
Common igneous rocks
Rhyolite
Extrusive
Fine-grained
Felsic
Common igneous rocks
Andesite
Extrusive
Fine-grained
Intermediate
Completing the Rock Cycle
Tectonics & Metamorphism
Completing to the Rock Cycle
Tectonics & Metamorphism