Flashcards in absolute (radiomentric) dating Deck (23)
how old is the earth?
4.6 billion years
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
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.
the time it takes for half of the unstable parent isotopes to decay to stable daughter product
they have the same number of protons but different numbers of neutrons
name a mineral and a rock that can be dated using the potassium argon method
mineral= muscovite or biotite mica
what are the best rocks for dating?
muscovite, biotite, potassium feldpar
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
it gives specific dates for rock units or events given in millions of years before (Ma)
putting units or events into sequential order, by saying one event is older or younger than another
when a mineral neither gains nor loses atoms. the higher the temperature more likely exchanges of atoms or ions will be.
what are the most reliable igneous rocks for dating?
shallowly emplaced minor intrusions and extrusive rocks that cool rapidly e.g. lava flows
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
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
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
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
the temperature at which the system has cooled, so there is no diffusion of isotopes in or out of the system
what is the half life of 40^K
What is the half life of 238^U and 235^U?
206 Pb 4470 Ma
207 Pb 704 Ma
what is the half life of 87^Rb?
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.
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.