Download Geological Dating Techniques 2014b

Geological
Time
Geochronology – The Science of
Dating Rocks and Rock Layers
The Geological Time Line
Originally layers of rocks were
defined by the assemblages of
fossils found within them. The
Cambrian, Ordivician,
Devonian and Silurian Eras
were defined by rocks found in
England in the 1800’s. Only
later were these eras dated.
These eras were named for the
ancient tribes that lived in these
areas of Great Britain.
The 4.6 billion years of
Geologic Time broken
into Eons, Eras,
Periods (and subPeriods), Epochs and
Ages.
Notice the
increasingly smaller
increments of time.
We find more
subdivisions in the
Phanerozoic as life
began to flourish and
change.
Phanerozoic Eon
• The Phanerozoic Eon is broken into 3
eras which are further subdivided
into periods and epochs (and more
informally into ages).
How do we date Rocks?
Relative Age Dating – The
ability to determine whether
one layer of rock is older or
younger than other (a
qualitative method)
Absolute Age Dating – The
ability to date rocks
quantitatively. Rock layers are
given fairly exact age
determinations
Geochronolgy – The Science
of Dating Rocks
Relative Age Dating
Techniques:
1) Stratigraphy – layers of rock are
often continuous for kilometers in all
directions. These rock layers can
often be defined by their mineral
composition, fossil assemblages or
other physical features. Generally
deeper layers are older and therefore
the relative ages of rock layers can be
compared. Folding of rocks can put
older layers above younger layers.
Stratigraphy – Rock Layers
• Newer layers of rock are
deposited above older layers.
• Each layer contains a specific
rock type and often a specific
assemblage of fossils.
• Tectonic processes can overturn
rock layers.
Geochronolgy – The Science
of Dating Rocks
Relative Age Dating
Techniques:
1) Stratigraphy continued – A classic
layer called the Gubbio Clay
separates the Cretaceous and the
Tertiary Periods and can be traced
across Europe. This layer had a
higher than normal concentration of
iridium – an element far more
common in meteorites than the
Earth’s crust.
The Gubbio Clay
• The thin black clay layer separates the
white fossil-depleted Tertiary layer
from the reddish fossil-rich Cretaceous
layer below.
• The black clay layer is enriched in the
element Ir (iridium)
Geochronolgy – The Science
of Dating Rocks
Absolute Age Dating Techniques:
1) Dendochronolgy – using tree rings to
date recent geological or archeological
events. This method can be extended
back over a thousand years by
overlapping the rings of trees.
2) Varved Clays in Glacial Lakes Seasonal Climate Changes can be seen
in the sediments of the lake.
Dendrochronolgy
• Rings from many trees or wood planks both young
and old can be overlapped to determine climatic
conditions for as many as a thousand years.
Obviously this method only useful for very modern
(archaeological) studies.
Varved Clays
• The photograph above shows the layers
at the bottom of a glacial lake. Thick
layers form during the summer when the
glacier melts faster.
• Warmer years show the thickest varves.
• Geologist s often drill cores from the
bottom of glacial lakes.
• Again this technique only measures
climate within the Holocene period.
Geochronology – The Science
of Dating Rocks
Absolute Age Dating Techniques:
3) Radiometric dating using radioisotopes:
We can determine the decay rates for many
radioactive isotopes. If the rock contains
these isotopes, then we can determine how
much these isotopes can decay.
4) Fossils (Paleontology) – Individual species
have been dated throughout the fossil
record. Most fossils have time ranges in
which they existed in the fossil record
which can give approximate dates.
However large assemblages of fossils can
help pinpoint dates more exactly
Radiometric Dating
• Most igneous rocks contain atoms which
are unstable. These “radioisotopes”
decay by ejecting particles and releasing
radiation.
• Uranium – 238 goes through a sequence
of decays before eventually turning into
lead – 206. The rate at which this decay
occurs has be en determined scientifically
238U/206Pb
decay chain
Radiometric Dating
• The rate of decay is measured in halflives. In the case of U-238, half of the
material in a rock will decay into Pb-206
in 4.5 billion years. After several halflives the amount of original U- 238
decreases to a half, a quarter, an eighth
and so on…
Radiometric dating
• Scientists must choose an appropriate radioisotope for
each application. Which isotope would be useful for
dating:
a) Rocks that are over 2 billion years old?
b) Dinosaur fossils that are about 200 million years old?
c) The wood from an ancient ship found at the bottom of
the Sea?
Video
• Carbon Dating (How Does it Work?):
http://www.youtube.com/watch?v=ud
kQwW6aLik&feature=related – this
video also discusses paleomagnetism
and 40K/40Ar and 238U/208Pb and Rb/Sr
and thermoluminescence
• Radiocarbon Dating:
http://www.youtube.com/watch?v=2i
o5opwhQMQ&feature=relmfu
• Radiometric Dating (A how-to):
http://www.youtube.com/watch?v=19
20gi3swe4&feature=related
Video
• Radiocarbon Dating Clip: (History
Channel)
http://www.youtube.com/watch?v=10
XD9lJpDAY&list=LPhNseJZQOmkE&in
dex=8&feature=plcp
• Dating The Radiocarbon Way:
http://www.youtube.com/watch?v=xKvq6VLe4s&feature=related (very
clear, very technical)
Radiometric Dating
• Igneous rocks containing
zircon (ZrSiO4) have atoms
of U that substitute for Zr in
the crystal structure.
• Zircons are perfect for
radiometric dating because
they do not erode easily and
they contain radioisotopes.
• Once the zircon crystallizes,
from that point on the ratio
of U-238/Pb-206 begins to
decrease.
238U/206Pb
Age
Dating
• Uranium-lead dating is usually
performed on the mineral zircon
(ZrSiO4), though it can be used on
other durable Zr-containing minerals.
• The zircon mineral incorporates
uranium and thorium atoms into its
crystalline structure, but strongly
rejects lead. Therefore we can assume
that the entire lead content of the
zircon is radiogenic.
238U/206Pb
dating employs a very long
half life (4.5 billion years) and therefore
can be used to date extremely old rocks –
as old as the age of the Earth.
• The amount of U-238 is at its greatest
when the rock solidifies from a magma –
after this point the rock gains no
additional U-238 and this radioisotope
decays from this point onward.
• The other commonly used radioisotope is
40K/40Ar. Potassium is a common element
in feldspars – the most common silicate.
•
238U/206Pb
Age Dating
Carbon-14 Dating
• Carbon-14 Dating is more commonly used by Archeologists.
• Since all living things contain carbon, this method can date
any organic or formerly living thing
• C-14 exists is a rare isotope of carbon, existing as only about
1 part per trillion carbon atoms. Thus a sample of carbon
needs to be reasonably large (up to 1 gram)
Carbon-14 Dating
• Carbon-14 is produced continually in the
atmosphere and becomes part of many CO2 atoms.
• Carbon dioxide is continually taken up by plants
and moves through the carbon cycle.
• Animals and plants continue to ingest new C-14 as
long as they are living. Once an organism dies, it
stops taking up new C-14 and the ratio of C-14/C12 starts to decrease (following the pattern of halflives seen below)
Carbon-14 Dating
• Scientists measure the ratio of C-14/C -12 to
determine how many half lives have occurred and
can date the sample
Radiometric Dating Questions
1) Rn-222 has a half-life of 3.82 days, how
long until only 1/16 of the original
remains?
2) After 24 days, only 2 mg of an original 128
mg sample remains. What is the half life
of this radioisotope?
Radiometric Dating Questions
3) If the half-life of iodine-131 is 8.10 days,
how long will it take a 50.00 g sample to
decay to 6.25 g?
4) The half-life of hafnium-156 is 0.025 s.
How long will it take a 560 g sample to
decay to one-fourth its original mass?
Lets get more technical –
Carbon Dating Equation
Above is the equation for the beta decay of C-14.
14C → 14N + -1 e (neutron is converted into proton and a high
0
energy electron (β-particle)).
• t = age of the fossil (or the date in which the fossil died)
• t1/2 = half-life of C-14 (which equals 5730 years)
• N/NO = relative amount of the radioisotope in a sample in
comparison to the amount of the radioisotope in a “fresh”
sample.
• ln (natural logarithm)
Carbon Dating
• A sample of flesh from the flesh of a Woolly
Mammoth found in the Siberian permafrost
contains 23.2 mg of C-14. An equal mass of the
flesh of a modern animal contains 49.6 mg of C14. The half-life of C-14 is 5730 years. What is
the age of the frozen Mammoth?
Carbon Dating
• N/NO =
• t1/2 =
Carbon Dating
• A sample of wood from a fire found at an
archaeological site containing he bones of a
Neanderthal man contains 7.28 mg of C-14. An
equal mass of a modern piece of wood contains
34.6 mg of C-14. The half-life of C-14 is 5730
years. What is the age of the wood and likely the
Neanderthal man?
Carbon Dating
• N/NO =
• t1/2 =
238U/206Pb
Age Dating
The mathematical expression that relates radioactive decay to
geologic time, is:
•
•
•
•
•
where t = age of the sample
Dt = number of atoms or amount of the radiogenic stable daughter isotope in the
sample at time t
D0 = number of atoms or amount of the daughter stable isotope in the original
composition (at t = 0)
Pt = number of atoms or amount of the parent radioisotope in the sample at time t
λ = the decay constant for the parent radioisotope, equal to ln(2) / t1/2. where t1/2 is
expressed in the same time units as t.
• In its simplest form, this can be rearranged to yield the age t:
t = 1 ln [(Dt – DO) + 1]
λ
Pt
λ = ln 2
t1/2
238U/206Pb
Age Dating
1) An igneous rock contains zircons. U-238 from the zircons
has a mass of 3.25 mg. The mass of Pb-206 from the
zircon has a mass of 1.35 mg. What is the age of the
zircon and therefore the igneous rock?
t = 1 ln [(Dt – DO) + 1]
λ
Pt
λ = ln 2
t1/2
238U/206Pb
Age Dating
2) A volcanic lava flowed over a sedimentary rock contain
trilobite fossils. U-238 from the zircons in the lava has a
mass of 1.17 mg. The mass of Pb-206 from the zircon has
a mass of 0.235 mg. What is the age of the zircon and
therefore the lava flow? How old are the trilobites?
t = 1 ln [(Dt – DO) + 1]
λ
Pt
λ = ln 2
t1/2