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