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In Geo-science, How is time measured?
A. Backwards from the present
B. Forwards from the past
C. With a give year
Time is measured A. Backwards from the present.
For example: at approximately 2.4 Ga (2.5 billion years ago) the atmosphere became oxygenated.
What is Uniformitarianism ?
- The main principal of Uniformitarianism is "the present is the key to the past". Present day natural forces and processes operated in the past to form the rocks found in the geological record.
- ie. the rules of chemistry and physics have always been the same.
State the major principles of relative time? and give an explanation of each.
- 1. Original horizontality - (sedimentary rocks) produces horizontal beds
- 2. Superposition - (sedimentary rocks) Horizontal beds are laid sequentially on top of each other; like a layer cake. Can also show the the younging direction.
- 3. Cross - cutting relationships younger rocks cut across older rocks. For example magmatic intrusion intrudes older horizontal strata.
- 4. Inclusions Older rocks occur as inclusions in younger rocks
State the relative time methods and give an explanation of each.
- Stratigraphic correlation - Identification of time-equivalent rock layers at places distant from each other
- Biostratigraphy - Correlation using fossils
- Seismic stratigraphy - Using arrival times of seismic waves reflected off rock layers, can 'follow' buried rock layers.
- magnetostratigraphy - Earth's magnetic field polarity reverses on an irregular basis. This 'bar code' of changes can be used for correlation.
When was the Earth formed?
The Earth was formed 4.56 Ga (billion years) ago.
What are the two different types of geological chronology ?
Relative and absolute chronology.
How many years do the following time units indicate?
- Ka = one thousand = 1,000 years
- Ma = one million = 1,000,000 years
- Ga = one billion = 1,000,000,000
Who was William Smith ?
- William Smith (1769 -1839)
- Worked as a canal surveyor during the industrial revolution.
- He formed the concept of biostratigraphy using marker fossils.
- He used this concept to singlehandedly make the first geological map; of England and Wales.
What was the old western view on the age of the Earth ?
The now dated western view of the age of the Earth was heavily influenced by religion. When bishop Ussher in 1654 used the book of Genesis to date the Earth and come to the conclusion that the Earth was created at 9am on the 26 October 4004. It become very widely excepted. Thus continued the western belief that the Earth was indeed still quite young and made by god
Who discovered radiometric dating ?
Henri Becquerel in 1895 discovered radioactivity: the spontaneous decay of one element into another element.
However, it was Ernest Rutherford in 1905 that suggested that it could be used to date rocks.
What is radioactive decay?
Is the decay of one parent Isotope into another daughter Isotope.
What is half life?
Half life is the time taken for half the parent ISOTOPE to decay.
What are the four natural radioactive decay parent : daughter relationships?
- Potassium - Argon
- 40K - 40Ar
- Rubidium - Strontium
- 87Rb - 87Sr
- Uranium - Lead
- 238U - 206U
- 235U - 207U
What is the formula for radioactive decay ?
- need :
- >rate of decay (lambda) or half life (t1/2)
- >amount of parent (P) present day
- >amount of daughter (D) present day
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