Time & Change

Relative Dating

Radiometric Dating

Geological Column

Law of Cross-cutting Relationships

Law of included fragments

Law of Superposition

Youngest rocks on top of sequence

Cross-cutting rocks are younger

Younger rock includes fragments of older rock below

Zone fossils

Rapid Evolutionary Change

Principals

Unstable chemical elements change or decay

Potassium-Argon Dating

Samarium-Neodymium Dating

Potassium 40 decays into Argon 40

Rare elements

Large population so they are common

Wide geographical distribution

Swimming organisms such as graptolites make best zone fossils

Ready preservation after death

Facies independence

Easy identifiable with distinct features

Rate are extremely variable

Over time they decay into more stable elements by emission of quantities of the sub-atomic partilces

Number of techniques developed

Unstable elements contain too many neutrons

The stable end product is referred to as the daughter

Radioactive decay can be used to establish age of rocks as rate of decay is constant

The element that changes or decays is known as the parent

Quantities of parent and daughter atoms in minerals can be measured and then the age can be calculated

Rate of decay is exponential and measured as Half-life

Comparing K40 to Ar40, and knowing decay rate can give age of rock

Potassium= one of most abundant elements in crust- 2.4% by mass

Half-life K40= 1.26 billion years

Used to date rocks 20,000 to 4.6 billion years

When rocks are heated Ar40 is released into atmosphere as a gas

When the rock recrystallises it becomes impermeable to gasses

Limitations

As k40 decays into Ar40, the gas is trapped in the rock

Metamorphism can contaminate rocks with more K40 and make the rock seem younger

Weathering/ Erosion releases the Argon gas

Occur naturally in 7 different isotopic forms (5 stable and 2 radioactive)

More useful than K-Ar

Based on Sm147, Nd143 and Nd144

Half-life= 106 billion years so best suited for rocks that are billions of years old

Not affected by Metamorphism or Weathering

Sm 147 decays into Nd143

Compariosns

K-Ar

Sm-Nd

Parent K40

Parent Sm147

Daughter Ar40

Half life= 1.26 billion years

100,000 yrs + age for dating

Feldspar in igneous rocks is dated

Daughter Nd147

Not affected by Metamorphism and Weathering

Half-life= 106 billion years

Feldspars and Pyroxenes are dated

Ar40 is gas

Can be affected by Metamorphism and Weathering

Can be used to date all rocks

10 million yrs + age for dating

Mesozoic

Palaeozoic

Cenozoic

Quaternary

Cretaceous

Permian

Tertiary

Jurassic

Triassic

Carboniferous

Cambrian

Devonian

Silurian

Ordovicain

Pre-Cambrian