absolute (radiomentric) dating Flashcards Preview

Fossils and time > absolute (radiomentric) dating > Flashcards

Flashcards in absolute (radiomentric) dating Deck (23)
Loading flashcards...
1

how old is the earth?

4.6 billion years

2

how did lord kelvin calculate the age of the earth?

He calculated the age of the earth by how much it had cooled. He worked out the geothermal gradient to be 25c/km. Assuming the earth was molten when it formed, he worked out, with about 3000km now solid the earth should be 20-40 Ma

3

why was lord kelvin wrong?

He did not know that radioactive decay within the core heats the earth meaning it has been cooling for way longer.

4

half life

the time it takes for half of the unstable parent isotopes to decay to stable daughter product

5

isotopes

they have the same number of protons but different numbers of neutrons

6

name a mineral and a rock that can be dated using the potassium argon method

mineral= muscovite or biotite mica
rock= basalt

7

what are the best rocks for dating?

muscovite, biotite, potassium feldpar

8

why does the potassium argon method give a margin of error?

loss of argon gas gives a younger age then the rock actually is
loss of parent isotopes due to weathering

9

absolute dating

it gives specific dates for rock units or events given in millions of years before (Ma)

10

relative dating

putting units or events into sequential order, by saying one event is older or younger than another

11

closed system

when a mineral neither gains nor loses atoms. the higher the temperature more likely exchanges of atoms or ions will be.

12

what are the most reliable igneous rocks for dating?

shallowly emplaced minor intrusions and extrusive rocks that cool rapidly e.g. lava flows

13

what is the problem when trying to radiometrically date sedimentary rock?

clasts could have a range of ages, older than the time the rock was formed
more porous and prone to weathering which can alter the ration of parent to daughter isotopes. parent atoms can be added but its more common to lose daughter atoms especially the gas argon

14

what sedimentary rock is an exception when using radiometric dating?

those containing glauconite, a type of mica, as the cementing mineral includes potassium 40 and so can provide a date for the formation of the rock

15

what problems can arise when radiometrically dating metamorphic rocks?

different minerals become closed systems at different temperatures
parent or daughter atoms can be lost during heating events
during a series of metamorphic events the radiometric clock will be reset in some minerals but not in others = a variety of conflicting dates

16

how can you date a metamorphic rock?

dating the minerals with different closure temperatures within the same rock can show the metamorphic history of the rocks

17

closure temperature

the temperature at which the system has cooled, so there is no diffusion of isotopes in or out of the system

18

what is the half life of 40^K

1250 Ma

19

What is the half life of 238^U and 235^U?

206 Pb 4470 Ma
207 Pb 704 Ma

20

what is the half life of 87^Rb?

48800 Ma

21

How are absolute ages measured?

Naturally occurring radioactive isotopes in rocks are unstable and break down at a statistically constant rate. if the relative amounts of parent and daughter atoms can be measured, then we know how many half- lives have passed since the parent was formed.

22

How did Joly try and calculate the age of the earth?

He assumed that the oceans had started as fresh water and the salt must've been added by rivers overtime. so he divided the amount of salt in the oceans by the amount of salt added each year. He estimated that 160 million tons of salt was added per year. With a salt content of 0.35% . he estimated the age of the earth to be 90Ma.

23

what was wrong with Joly's method for calculating the age of the earth?

he hadn't considered evaporation