2. Hutton - unsure but probably hundreds of millions of
years. (abbrev: Ma = mega annum = million years)
3. Darwin - probably hundreds of Ma.
4. John Joly (1890's) - Salt = 90 Ma.
total ocean salt (kg)
Age of the earth = -----------------------
rate of salt input (kg/yr)
- Why was he wrong? Is total wrong? Is rate wrong?
5. Lord Kelvin - 40-100 Ma, MAXIMUM based on heat flow
in the earth.
1896 Henri Becquerel discovered radioactivity
- isotopes of elements =
- radioisopes are unstable
- Parent isotopes decay
1903 Marie and Pierre Curie - radioactive isotopes produce Heat as well as particle emission.
1907 Bertram Boltwood - 206Pb is an end product of decay of 238U.
Radioactivity solved two problems:
1) Heat nullified Kelvin -
2) Radioactive decay could be used to date rocks containing radioactive
isotopes.
Decay occurs at an exponential rate, Not linear.
Over a given period of time, a percentage of the radioactive parent
isotopes will decay to form daughters.
Half-Life =
*** ILLUSTRATION : Two Pizza Browsers ***
- Two trays Tombstone Pizza.
- Linear browser takes 10 samples, returns in 10 minutes and takes
10 more off the tray until in 40 minutes they are all gone.
- Exponential browser takes HALF the samples, comes back in 10 minutes
and takes HALF AGAIN off the tray, and again until in 40 minutes they are
down to one. Then he continues to take only half of what is left,
never finishing the pizza, eventually only leaving smaller and smaller
crumbs.
*** ILLUSTRATION : Roll the dice ***
1. Each student in the classroom picks a random number from 2-12 (representing
pseudo-random dice roll).
2. Teacher: "How many students picked the number X?" where X
is any random number of the teacher's choice.
3. An X-Y graph on the board is begun, showing number of students in
the class on the Y axis and number of rolls of the dice on the Y-axis.
After seeing that 3 (or 5 or however many) students picked the number X,
the teacher marks a dot on the X-Y graph for the number of remaining students
(ex. 62-4=58) at roll #1.
4. Repeat steps 1-3 until it becomes clear that randomly decimating
the population of radioactive atoms yields an exponential decay pattern
-- the number of decaying atoms per unit time is a function of how large
the population of atoms is.
How To Do Radiometric dating (2 ways, of many related ways...)
1. measure amounts of Parent and Daughter isotopes and compare.
ex. 238U => 206Pb (4.5
Ga half-life) measure U and Pb, determine ratio.
2. measure amounts of Parent and a stable isotope companion, and compare.
ex. 14C => 14N (5730 year
half-life) measure 14C and 12C, determine ratio.
Lots of 14C = young, lots of 12C = old.
? How far back can we date with 14C?
? Why don't we measure parent and daughter?
Some Useful radioisotopes for Radiometric dating:
useful for older rocks, common in igneous and metamorphic minerals
238U => 206Pb (4.5 Ga
half-life)
235U => 207Pb (713 Ma
half-life)
232Th => 208Pb (13.9 Ga half-life)
40K => 40Ar
(1.5 Ga half-life)
Useful for young sediments containing organic carbon.
14C => 14N (5730 year half-life)
Tree-Ring dating. Tree rings come from Seasonality: slow
in winter, fast in summer. Take trees, correlate to older trees,
wood from dwellings, etc. These records go back about 14,000 years.
Correlate well to climate patterns too. Provide a precise time frame
to calibrate 14C dating and a verification of the 14C method.
Read Perspective 3.1 (page 67) - change in length of day through
time by counting daily growth rings in old corals.
If a radioactive element has a half-life of 2 Ma, what fraction of the original amount of parent material will remain after 8 Ma of decay?
a) 1/32 b) 1/16
c) 1/8 d) 1/4
e) 1/2
Why?
Which scientist argued on the basis of the current temperature of the earth's interior that the earth was less than 100 Ma in age?
a) Hutton b) Lyell
c) Joly d) Ussher
e) Kelvin
Why?
Unconformities: erosional surfaces.
- Disconformity -
- Angular Unconformity -
- Nonconformity -
Hiatus - time missing at an unconformity.