Dating (F3) Flashcards
What is relative dating?
putting geological events and rock types in chronological order from oldest to youngest
What is Original Horizontality? (dating)
Most sedimentary rocks are laid down horizontally under marine conditions.
Layers of rock tilted? Assume they have been moved by tectonic forces from their previously horizontal position
What is Lateral Continuity? (dating)
Layers of sediment initially extend laterally in all directions
So rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous
What is the law of superposition?
In a sequence, rocks at the base are oldest and at the top are youngest
(assuming rocks have not been overturned by tectonics)
What is Included Fragments? (dating)
Fragments eroded from older rock found in younger rocks The fragments have to be older than the rock they’re in
What is a Cross Cutting Relationship? (dating)
Features which cut through rocks must be younger than the rocks they cut
Dykes are younger than the rocks they intrude
Faults are younger than the rocks they displace
How are faults and folds formed?
Folds form after the rocks have been laid down
Compressive forces cause crustal shortening
Faults are formed after the rocks have been formed by either tensional, compressional, or shear forces
What are Unconformities?
gaps in geological record where no sediment was being deposited/preserved
They represent a hiatus, a pause, where something is missing… Often occur when older rocks are folded, faulted, uplifted, and eroded before the next episode of sediment deposition takes place
What periods of time are known as the Carboniferous?
Pennsylvanian & Mississippian periods
359.Ma -> 318.Ma
What are the requirements of a GOOD Zone Fossil?
• plentiful with wide-spread distribution
• morphology easily identified
• readily preserved (e.g. rapid burial of hard parts) • rapid evolutionary change
• facies independent (not restricted to one facies).
What is Radiometric Dating?
A method based on the decay of radioactive isotopes
What is the Mass Number and Atomic Number?
Mass Number: total number of protons and neutrons in the nucleus
Atomic Number: number of protons in the nucleus
What is a half life?
Decay is measured in half-lives
• Expressed as the “half life” of the parent isotope
• After one half life has elapsed only half the parent isotopes remain
• By the end of the second half life, half of these will also have changed to stable daughter isotopes
• This leaves only a quarter of the original parent isotopes
• The process continues, halving the number each time, but never
quite eliminating all parent isotopes
• A half life can vary from fractions of a second to billions of years
Which is most reliable for Radiometric Dating? (Lava, Small intrusion, Large intrusion, Metamorphism)
MOST: Lava & Small intrusion
• Fast Cooling
• Little/No weathering (SI)
• Gives minimum age (SI)
LEAST: Metamorphism & Large intrusion
• take millions of years to cool
• resets radiometric clock (M)
• often in many stages (LI)
What is Carbon 14 Dating?
• Different to other radiometric dating methods
• Does not use the Parent-Daughter ratio (C14 to N14)
• C14 method compares the amount of radioactive C14 to non- radioactive C12 and C13 in the sample
• The smaller the proportion of
