Lecture 5 Flashcards
1
Q
Cultural Views of time
A
- Greek and Hindu ancient philosophers rejected finite time
- proposed that time is cyclic with universe regenerated in cycles. Things that happen to day is part of the current cycle and one day in a new cycle the same things will happen again
- Europ thought time was linear and finite history was drawn from the bible. Provided the first such time frame for science
2
Q
Concept of human history
A
- dates to Herodotus 6.484 BC
- first attempt to research the past
- used a list of pharaohs to do so
3
Q
Biblical time line
A
CreationEnd of World
-importance was the production of a linear view with unique historical events on the finite time line
4
Q
James Ussher
A
- Bishop to Armagh in 1650
- calculated biblical chronologies
- calculated biblical record of births and deaths
- result was dating the creation making Earth 6000 years old
- Creation October 23, 4004 BC)
- first no qualms with scientists, fossils only then being recognized as former living creatures
- new explanation was that shells and bones found in rocks on mountains were from the flood
5
Q
New Sciences of Earth
A
- 18th Century
- study of past life-paleontology
- study of Earth physical nature and origin-geology and cosmology
- both quickly outgrow Biblical time scale
6
Q
Benoit de Maillet
A
- wrote about mythical philosopher Telliamed who claimed to be telling about what Eastern philosophers thought
- pointed out that places in sea level was going down
- Telliamed calculated how long it would have taken for the mountains to rise from the sea=2mry
7
Q
de Buffon
A
- mid 1700s astronomers begun to realize that the sun and its planets had condensed from a swirling mass of hot gas-how long would it take a planet to cool?
- Buffon tested with hot iron balls and extrapolated getting about 100,000 years
8
Q
Relative time
A
- time line that represents when something happened in relationship to something else
- measure by relative position, before, after, same time
9
Q
Absolute time
A
- real time dates
- how long ago something happened
- measured in time units like years
10
Q
Nicholas Steno
A
- helped with understanding relative time
- recognized and showed that fossils were remains of past life buried before mud became rock (tongue stones=shark teeth)
- came up with general theory of how sediments accumulated
11
Q
Principle of superposition
A
- gives relative age of rocks
- sediments accumulate on horizontal surface
- later different sediments deposited
- ultimately rocks form in layers going from oldest at the bottom and younger above
12
Q
Superposition applied to relative dating of rocks
A
- only a few kinds of sedimentary rocks
- sandstone, limestone, shale so it will occur in different ages
13
Q
How to tell ages of rocks
A
- take information from one location and fuse with information from another and come up with a rock section that you don’t see in any one place, but the order would be correct on earth
- confined to individual strata
- William Smith did this by carefully observing rocks and their fossils
14
Q
Superposition and strata correlation
A
- by use of index fossils in the 19th century used to map strata over wide regions of Europe and America
- also developed relative scale that’s still in use
15
Q
James Hutton
A
- figured out from key geological observation how ancient the world would have to be to account for observation
- saw tilted rock layers below and contact with overlying horizontal layers
- since all rocks are deposited horizontally the rocks were shifted to have new rocks deposited on top
- shifted rocks back to how they would be deposited and makes an order out of it showing oldest to youngest
- saw infinite amount of time
- eventually new horizontal segments will too be bent into mountains and cycle of erosion will begin again
16
Q
Historical data showing age of Earth
A
- buried forest sequences
- Green River formation, remains a huge lake-2000 feet total lake deposited rock
- rates of sediment deposition
- evidence from geology shows sediments deposited over long time scale-no single flood accounts for deposition of Earth’s rocks
17
Q
Absolute time scale methods
A
- 19th C: Lord Kelvin-calculation of life of sun (30-100 million years)
- rate of sedimentation/thickness of rocks (75 million years)
- rate of salt delivery to sea/salinity (100 million years)
- present method: decay of radioactive isotopes
18
Q
Decay of Radioactive isotopes
A
- elements are unstable and decay into radioactive isotopes
- U235 will one day decay into Pb207 and He4
- gives you way to date earth
- half life: always takes same amount of time to decay by half and can be measured
- Pb204 is non radiogenic
- Pb is stable end product
19
Q
Radioactive decay date what?
A
- only igneous rock
- because only sample where you know the age of all the parts because of measurable parent and daughter isotopes
20
Q
Rate law for radioactive decay
A
λ=log_eP_0/P_t
- λ=decay constant
- t=time elapsed
- P_0=amount isotope in time t_0
- P_t=amount isotope at time t
21
Q
Radiometric Age Equation
A
- P_0=P_t+D_t
- D=daughter isotope
- substitute P_t+D_t for P_0 in P_t=P_0e^- λt so you can solve for t=1/ λ*log_e((D_t/P_t)+1)
- all quantities on right hand of equation can be measured in the lab
22
Q
Assumptions for radioactive decay dating
A
- igneous rock
- rock contains measurable parent and daughter isotopes
- half life comparable to age of rock being measured
- closed system
- must be able to estimate amount of daughter isotope at t=0
- assume constant rate of decay over geological time
23
Q
isochron method
A
- done in cases where some daugher isotope might already be present at =0.
- some like Pb 206 and 207 existed when rocks formed
- way of normalizing isotope content against isotopes of same element not part of the decay chain, e.g. Pb 204
- from that the initial content of the decay product can be calculated with great accuracy
24
Q
Simple accumulation clock
A
40K——->40Ar
- done with igneous rocks
- as rock starts molten, no Ar gas present when it hardens and it starts to accumulate at that point
- method useful for rocks of ages from a few to 100,000 years to a few billion
25
Dating a sedimentary rock
- Lava is 435 MY layer (top)
- shale with fossil layer (middle)
- lava 495 MY layer (bottom)
- so the fossil must be between 495 MY and 435 MY old because of the dating of the lava
26
Carbon 14 dating
- special case
- different than radioactive decay dating
- When an individual dies, the C14 is still contained in the bones, however the organism is not still accumulating new C14 and what is there is decaying away
- importance is that it can be applied directly to the subject, not just the surrounding rock layers
- limitation: only young less than about 40,000 year-old specimens can be dated this way
27
Age of earth
- oldest existing rocks dated by radioisotopes go back 4 BY
- meteors are fragments of material left over from formation of planets all dat 4.6 BY=the age of formation of the earth
- geology has destroyed oldest rocks on earth but we do find rocks of that age on the moon
