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