Lecture 5

Question 1

Cultural Views of time

Answer 1

• 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

Question 2

Concept of human history

Answer 2

• dates to Herodotus 6.484 BC
• first attempt to research the past
• used a list of pharaohs to do so

Question 3

Biblical time line

Answer 3

CreationEnd of World

-importance was the production of a linear view with unique historical events on the finite time line

Question 4

James Ussher

Answer 4

• 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

Question 5

New Sciences of Earth

Answer 5

• 18th Century
• study of past life-paleontology
• study of Earth physical nature and origin-geology and cosmology
• both quickly outgrow Biblical time scale

Question 6

Benoit de Maillet

Answer 6

• 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

Question 7

de Buffon

Answer 7

• 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

Question 8

Relative time

Answer 8

• time line that represents when something happened in relationship to something else
• measure by relative position, before, after, same time

Question 9

Absolute time

Answer 9

• real time dates
• how long ago something happened
• measured in time units like years

Question 10

Nicholas Steno

Answer 10

• 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

Question 11

Principle of superposition

Answer 11

• 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

Question 12

Superposition applied to relative dating of rocks

Answer 12

• only a few kinds of sedimentary rocks

- sandstone, limestone, shale so it will occur in different ages

Question 13

How to tell ages of rocks

Answer 13

• 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

Question 14

Superposition and strata correlation

Answer 14

• 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

Question 15

James Hutton

Answer 15

• 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

Question 16

Historical data showing age of Earth

Answer 16

• 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

Question 17

Absolute time scale methods

Answer 17

• 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

Question 18

Decay of Radioactive isotopes

Answer 18

• 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

Question 19

Radioactive decay date what?

Answer 19

• only igneous rock

- because only sample where you know the age of all the parts because of measurable parent and daughter isotopes

Question 20

Rate law for radioactive decay

Answer 20

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

Question 21

Radiometric Age Equation

Answer 21

• 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

Question 22

Assumptions for radioactive decay dating

Answer 22

• 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

Question 23

isochron method

Answer 23

• 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

Question 24

Simple accumulation clock

Answer 24

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

Question 25

Dating a sedimentary rock

Answer 25

- 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

Question 26

Carbon 14 dating

Answer 26

- 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

Question 27

Age of earth

Answer 27

- 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