II. DETERMINING ABSOLUTE AGE
- absolute age – the numeric age of an object or event, often stated in
years before the present, as established by an absolute-dating process
such as radiometric dating
A. Absolute Dating Methods
1. Rates of Erosion
a. rate at which a stream erodes its bed
b. measuring rate of erosion can lead to an estimated age of the
stream
c. practical only for features formed within the past 10 000 to
20 000 years ago (i.e. Niagara Falls)
d. method is less reliable for features formed over a long period
of time (i.e. Grand Canyon)
2. Rate of Deposition
a. in general, about 30 cm of sedimentary rock are deposited
over a 1000 year period
b. this method is not always accurate, it merely provides an
estimate
1b. floods can deposit many meters of sediment in just one
day
2b. rate of deposition may change over time
3. varve count
a. tree’s age can be estimated by counting growth rings
b. varve – a banded layer of sand and silt that is deposited
annually in a lake, especially near ice sheets or glaciers can
be used to determine absolute age
1b. generally from in glacial lakes
2b. consists of light-colored band of coarse particles and a
dark band of fine particles
aa. coarse particles deposited in summer
bb. fine particles settle during winter
c. one varve represents one year
B. Radiometric Dating
1. rocks generally contain small amounts of radioactive material
a. isotope – atoms of the same element that have a different
atomic mass
b. radioactive isotope – isotopes whose nuclei emits particles
and energy at a constant rate regardless of surrounding
conditions
2. scientists use the natural breakdown of isotopes to measure
absolute age
3. radiometric dating – a method of determining the absolute age
of an object by comparing the relative percentages of a
radioactive isotope and a stable isotope
a. parent isotope – the original radioactive isotope that is going
through radioactive decay
b. daughter isotope – the stable isotope that results from the
breakdown of the parent isotope
4. scientists use the decay rate and compare the percentages of the
parent and daughter isotope to determine absolute age
5. Half-Life
a. radioactive decay happens at a relatively constant rate that
is not changed by temperature, pressure or other
environmental conditions
b. half-life – the time required for half of a sample of a
radioactive isotope to break down by radioactive decay to
form a daughter isotope
c. the greater the amount of the daughter isotope, the more
half-lives and therefore the older the rock
6. Radioactive Isotopes
a. accuracy of radiometric dating depends on two items
1a. if too little time has gone by, there may not be enough
of the daughter isotope
2a. if too much time has gone by, there may not be enough
of the parent isotope
b. radiometric dating uses a ratio of the parent and daughter
isotopes
c. uranium-238 is most useful for dating features that are more
than 100 million years old
d. potassium-40 is used to date rocks that are between 50 000
and 4.6 billion years old (Table 1 page 195)
C. Carbon Dating
1. younger rock layers may be dated indirectly by dating organic
material found within the rock
2. only accurate for layers of rock that are less than 70 000 years
3. carbon-14 dating (radiocarbon dating) – the use of carbon-14
found in once living organisms and comparing it to carbon-12
a. half-life of carbon-14 is 5730 years
b. scientists first determine the ratio of C-14 to C-12
c. compare that ratio to the ratio of C-14 to C-12 in living
organisms