Geologic Time

Dating the Earth

Relative Dating vs. Radiometric dating

Relative dating places rocks and events in sequence, i.e., from oldest to youngest. The exact age of the object isn't known. With radiometric dating, an age of the object is calculated using the half-lives of radioactive elements to yield numerical dates.

In the example below there are two columns, one for the relative age and one for the numerical age of several wars that have occurred over the last 100 years. In the "Relative Age" column, the wars are simply listed in the order they occured, but the dates for each event is not know. In the "Numerical Age" column, the events are in the same order, but the years when they took place have been added in. Now we can see how close in time two events are.

Geologists use Five Fundamental Geologic Events to help "order" rocks using Relative Dating

1. Intrusion
2. Erosion
3. Deposition
4. Faulting
5. Rock Deformation

Through analysing the rocks, a geologist can determine which event came before another.

Principles of Relative Dating

• Uniformitarianism - “The present is the key to the past”. In other words, geologic processes that occur today have done so in the exact same ways in the past.
  
  
• Superposition - Usually, the rocks at the bottom are the oldest. There are some exceptions to this rule, but it's a good place to start.
  
  
• Original Horizontality - Sediments are usually laid down horizontally, parallel to the surface below them. Then processes like folding, faulting, etc., deform the rocks, changing them from their original horizonal position into a new, non-horizonal possition.
  
  
• Original Lateral Continuity - Sediments are also usually laid down in one contiuous layer. Erosion, folding, and faulting can form breaks in these layers. Take, for example, a canyon. Originally, the layers of rock were unbroken. Then, the river cut down through those layers, producing a gap between one side of the canyon and the other.
  
  
• Cross-cutting Relationships - Any feature that is cutting across another is the younger feature.
  
  
• Inclusions - Inclusions are peices of rock incorporated into another rock. Inclusions are always older that the rock they are embedded in.
  
  
• “Baked” Contacts - Baked contacts form when the hot magma in a magma chamber comes in contact with the cold host rock surrounding it. The host rock will always be older than the magma chamber. It had to have existed before the magma could intrude into the area and 'bake' the rock.
  
  
• Faunal Succession - Faunal succession refers to the first appearance of an organism in the rock record. For example, trilobites first appeared around 520 million years ago during a time called the Cambrian, and when extinct in the Permian period, 250 million years ago. Thus, if a rock has a trilobite fossil in it the rock can only be between 520 - 250 million years old.
  
  
• Unconformities - An unconformity is a break in time in the rock record.
  • Angular unconformity - sedimentary rocks on the bottom are at an angle to the sedimentary rocks on top.
  • Disconformity - all of the sedimentary rock layers are parallel.
  • Nonconformity - igneous or metamorphic rocks on the bottom, sedimentary rocks on the top.