GEOLOGICAL TIME Flashcards
1
Q
Geological column
A
Eon
- Phanerozoic
- longest unit of time
Era
- cenozoic
- K/T 66Ma
- mesozoic
-P/T 252Ma - palaeozoic
Period
- Cambrian - 541Ma
- Ordovician
- Silurian
- Devonian
- Carboniferous
- Permian
- Triassic
- Jurassic
- Cretaceous
- Tertiary (palaegone, neogene)
- Quaternary
2
Q
Age of earth theory - Comte de Buffron
A
- 1779
- created a small model earth with similar composition
- measured rate of cooling then scaled up the results
- 75,000 years old
3
Q
Age of earth theory - Lord Kelvin
A
- 1899
- measured rate of cooling of a molten iron ball
- measured time taken for near surface to cool to present temperature
- 20-100 Ma
4
Q
Age of earth theory - Joly
A
- 1899
- assumed oceans started as fresh water
- salt was added by rivers which increased salinity
- 90 Ma
5
Q
Absolute and relative dating
A
- Absolute dating - getting a numerical date for a rock or feature measured in Ma
- relative dating - putting events in order
6
Q
Radiometric dating
A
- uses radioactive decay of isotopes with known half-lives
- half-life - time taken for half of the unstable parent atoms to decay into stable daughter atoms
- shown as a line graph (decay curves)
- if relative amounts of parent and daughter isotopes are measured, we can calculate how many half-lives have passed since the parent was formed
diagram in notes 8
7
Q
Radiometric dating example
A
- Parent - ⁴⁰K
- Daughter - ⁴⁰Ar
- Found in micas and hornblende
- half-life of 1250Ma
- used to date igneous rocks older than 10Ma
8
Q
Radiometric dating limitations
A
- radioactive minerals are scarce
- weathering and erosion releases argon gas, so parent:daughter isotope ratio will be wrong as rock will seem younger (open system)
Igneous
- intrusions take a long time to cool
- different mineral crystallises at different temps
- intrusion will have a range of dates
Metamorphic
- isotopes can be lost/gained while heating
Sedimentary
- date will be the age of the older fragments it is made up from, not the original rock
9
Q
Relative dating
A
10
Q
Relative dating - laws of stratigraphy
A
superposition
- younger beds are deposited on top of pre-existing beds
way-up structures
- shows which way-up structures are facing
- e.g graded bedding, cross bedding, ripple marks, desiccation cracks, flute casts
included fragments
- fragments deposited after the bed
- e.g conglomerates, xenoliths
original horizontality
- beds are deposited horizontal, so deformation happens after deposition
- e.g tilting, folding
cross-cutting relationships
- a rock existed whole before being broken
- e.g dykes, angular unconformities, cleavage, faults
11
Q
Relative dating - biostratigraphy
A
Use zone fossils to correlate rocks of same age in different places
A good zone fossil will be:
- geographically wide spread
- rapid evolution
- easily fossilised
- easily identified
12
Q
Relative dating - lithostratigraphy
A
- uses matching rock types to correlate locations
- lateral variation of beds
- diachronous - same bed has different ages at different places
