1. Dating rocks Relative dating
Using a set of principles to put rocks in their proper sequences of formation
Absolute dating
Using radioactive decay to determine the exact age of rocks

2. Absolute Dating: any method of measuring the age of an event or object in years Most common:based on Radioactive Decay
Parent
daughter
Why does it work?
1. The decay rate is CONSTANT,
independent of external conditions in the earth.
2. The daughter/Parent ratio can be precisely measured.

3. Radioactive Decay
The process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element.
Sounds great but what is an isotope?
An isotope is an atom that has the same number of protons (atomic #) as other atoms of the same element do but that has a different number of neutrons (and thus a different atomic mass)

4. Isotopes
Most isotopes are stable, meaning they stay in their original form
But some are unstable
Unstable isotopes are radioactive
Radioactive decay is the process in which a radioactive isotope tends to bread down into a stable isotope of the same or another element

5. How does it work? What does this have to do with the age of rocks?
An Unstable isotope is called a parent isotope
The stable isotope produced by radioactive decay is the daughter isotope.
Decay is constant
The more daughter isotope- the older the rock!

6. Radiometric dating
A method of determining the age of an object by estimating the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope
Ratio or parent material to daughter material

7. Absolute dating Helps scientists determine actual age of fossils
Rocks near fossils contain radioactive elements – unstable elements that break down into different elements
Half-life of a radioactive element is the time it takes for half of the atoms in a sample to decay
Scientists compare the amount of radioactive element in a sample to the amount of the element into which it breaks down
Scientists use this info to calculate the age of the rock, which then tells the age of the fossil

8. Half-life – the time required for one-half of the radioactive nuclei in a sample to decay

9. Dating with carbon-14 (radiocarbon dating)
Carbon is normally found in three forms: stable C-12, stable C-14 & radioactive C-14
All combine with oxygen to form CO2
Half-life of only 5730 years
Used to date very recent events
Carbon-14 is produced in the upper atmosphere
Useful tool for anthropologists, archeologists, and geologists who study very recent Earth history

10. Carbon-14
Carbon-14 is continuously created in the atmosphere by cosmic radiation.
There is one atom of radioactive C-14 for every trillion atoms of C-12 in the atmosphere
Plants absorb C-14 directly through their leaves in the form of carbon dioxide
Animals take in C-14 indirectly when they eat plants
Although C-14 disintegrates at a constant rate, it is continuously renewed as long as an organism remains alive.
When an organism dies, it stops absorbing new C-14 and its radiocarbon “clock” is set.

11. Types of radiometric dating
Potassium-argon method
K-40 half-life 1.3 billion years
Decays into argon and calcium
Used to date rocks older than 100,00 years old
Uranium-lead method
U-238 half-life 4.5 billion years
Decays into lead-206
Used for rocks more than 10 million years old
Rubidium-strontium method
Rb-87 half-life 49 billion years
Decays into strontium-87
Carbon-14 method

12. Relative-Dating Principles
Lateral continuity
sediment extends laterally in all directions until it thins and pinches out or terminates against the edges of the depositional basin
Cross-cutting relationships
an igneous intrusion or a fault must be younger than the rocks it intrudes or displaces

13. Cross-cutting Relationships
A dark-colored dike has intruded into older light colored granite: the dike is younger than the granite
North shore of Lake Superior, Ontario Canada

14. Cross-cutting Relationships
A small fault displaces tilted beds: the fault is younger than the beds
Templin Highway, Castaic, California

15. Back to Steno

16. Why are layers tilted? Deformation of rocks Folding Faulting
Occurs after they are deposited
Important factor in relative dating
Folding
Anticlines, synclines
Rock bends, but does not break
Faulting
Normal, reverse, transform
Rock breaks

17. Folding

18. Faulting

19. The Map That Changed the World
Smith's map of Scotland, England, and Wales

20. Relative Geologic Time Scale
The relative geologic time scale has a sequence of
eons
eras
periods
epochs
E-or played at Epcot! (Eon, Era, Period, and Epoch!)
but no numbers indicating how long ago each of these times occurred

21. Geologic Time Scale
Large divisions based on characteristics of fossils
Paleozoic Era – early life dominated by invertebrate animals
Mesozoic Era – middle life
Cenozoic Era – recent life

22. How was the timescale created?
Mapping in 1800s using the principles of
Superposition
Original Horizontality
Original Lateral Continuity
Cross-cutting relationships
Also Fossil Correlation

23. Absolute Dating
Radiometric dating is the most common method of obtaining absolute ages
calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks
Other methods?
Tree ring counting
Varves
Ice cores

24. Geologic Time Scale
The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scale
The geologic time scale is a dual scale
a relative scale
and an absolute scale

25. Changes in the Concept of Geologic Time
The concept and measurement of geologic time has changed through human history
James Ussher ( ) in Ireland
calculated the age of Earth based on recorded history and genealogies in Genesis
he announced that Earth was created on October 22, 4004 B.C.
a century later it was considered heresy to say Earth was more than about 6000 years old

26. Changes in the Concept of Geologic Time
During the 1700s and 1800s Earth’s age was estimated scientifically
Georges Louis de Buffon ( ) calculated how long Earth took to cool gradually from a molten beginning
used melted iron balls of various diameters
he estimated Earth was 75,000 years old

27. Changes in the Concept of Geologic Time
Others used rates of deposition of various sediments and thickness of sedimentary rock in the crust
gave estimates of <1 million
to more than 2 billion years
Or the amount of salt carried by rivers to the ocean and the salinity of seawater
John Joly in 1899 obtained a minimum age of 90 million years

28. Crisis in Geology Lord Kelvin (1824-1907)
knew about high temperatures inside of deep mines and reasoned that Earth is losing heat from its interior
Assuming Earth was once molten, he used
the melting temperature of rocks
the size of Earth
and the rate of heat loss
to calculate the age of Earth as between 400 and 20 million years

29. Crisis in Geology
This age was too young for the geologic processes envisioned by other geologists at that time
leading to a crisis in geology
Kelvin did not know about radioactivity as a heat source within the Earth

30. Absolute-Dating Methods
The discovery of radioactivity destroyed Kelvin’s argument for the age of Earth
Radioactivity is the spontaneous decay of an atom’s nucleus to a more stable form
The heat from radioactivity helps explain why the Earth is still warm inside
Radioactivity provides geologists with a powerful tool to measure absolute ages of rocks and past geologic events

31. Absolute-Dating Methods
Understanding absolute dating requires knowledge of atoms and isotopes: we have it!
Atomic mass number
= number of protons + number of neutrons
Isotopes: different numbers of neutrons
Different isotopes have different atomic mass numbers but behave the same chemically
Most isotopes are stable
but some are unstable
Geologists use decay rates of unstable isotopes to determine absolute ages of rocks

32. Radioactive Decay
Radioactive decay is the process whereby an unstable atomic nucleus spontaneously changes into an atomic nucleus of a different element
Three types of radioactive decay:
alpha decay, two protons and two neutrons (alpha particle) are emitted from the nucleus

33. Half-Lives
Half-life of a radioactive isotope is the time it takes for one half of the atoms of the original unstable parent isotope to decay to atoms of a new more stable daughter isotope
The half-life of a specific radioactive isotope is constant and can be precisely measured

34. Half-Lives
The length of half-lives for different isotopes of different elements can vary from
less than 1/billionth of a second
to 49 billion years
Radioactive decay
is geometric not linear
a curved graph

35. Geometric Radioactive Decay
In radioactive decay, during each equal time unit, one half-life, the proportion of parent atoms decreases by 1/2

36. Determining Age
By measuring the parent/daughter ratio and knowing the half-life of the parent which has been determined in the laboratory geologists can calculate the age of a sample containing the radioactive element
The parent/daughter ratio is usually determined by a mass spectrometer
an instrument that measures the proportions of atoms with different masses

37. Determining Age For example:
If a rock has a parent/daughter ratio of 1:3  a parent proportion of 25%
and the half-live is 57 million years, how old is the rock?
25% means it is 2 half-lives old.
the rock is 57 x 2 =114 million years old.

38. What Materials Can Be Dated?
Most radiometric dates are obtained from igneous rocks
As magma cools and crystallizes, radioactive parent atoms separate from previously formed daughter atoms
they fit differently into the crystal structure of certain minerals
Geologists can use the crystals containing the parents atoms to date the time of crystallization

39. Long-Lived Radioactive Isotope Pairs Used in Dating
The isotopes used in radiometric dating need to be sufficiently long-lived so the amount of parent material left is measurable
Such isotopes include:
Parents Daughters Half-Life (years)
Uranium Lead billion
Uranium Lead million
Thorium Lead billion
Rubidium Strontium billion
Potassium Argon billion

40. How do we know the Earth can’t be older than about 6-7 b.y.?
Moderate half-life isotopes (1 b.y.)
If Earth was > 6-7 b.y. old, there wouldn’t be many parents left

41. Radiocarbon Dating Method
Carbon is found in all life
It has 3 isotopes
carbon 12 and 13 are stable but carbon 14 is not
carbon 14 has a half-life of 5730 years
carbon 14 dating uses the carbon 14/carbon 12 ratio of material that was once living
The short half-life of carbon 14 makes it suitable for dating material < 70,000 years old
It is not useful for most rocks, but is useful for archaeology and young geologic materials

42. Carbon 14 Carbon 14 is constantly forming in the upper atmosphere
when a high-energy neutron, a type of cosmic ray , strikes a nitrogen 14 atom it may be absorbed by the nucleus and eject a proton changing it to carbon 14
The 14C formation rate
is fairly constant
and has been calibrated against tree rings

43. Carbon 14
The carbon 14 becomes part of the natural carbon cycle and becomes incorporated into organisms
While the organism lives it continues to take in carbon 14
when it dies the carbon 14 begins to decay without being replenished
Thus, carbon 14 dating measures the time of death

44. Tree-Ring Dating Method
The age of a tree can be determined by counting the annual growth rings in lower part of the stem (trunk)
The width of the rings are related to climate and can be correlated from tree to tree
a procedure called cross-dating
The tree-ring time scale now extends back 14,000 years!

45. Tree-Ring Dating Method
In cross-dating, tree-ring patterns are used from different trees, with overlapping life spans

46. Summary Uniformitarianism holds that
the laws of nature have been constant through time
and that the same processes operating today have operated in the past
although not necessarily at the same rates

47. Summary The principles of superposition
original horizontality,
lateral continuity
and cross-cutting relationships
are basic for determining
relative geologic ages and for interpreting Earth history
Radioactivity was discovered during the late 19th century
and lead to radiometric dating
which allowed geologists to determine absolute ages for geologic events

48. Summary
Half-life is the length of time it takes for one-half of the radioactive parent isotope to decay to a stable daughter isotope of a different element
The most accurate radiometric dates are obtained from long-lived radioactive isotope/daughter pairs
in igneous rocks

49. Summary
The most reliable radiometric ages are obtained using two different pairs in the same rock
Carbon 14 dating can be used only for organic matter such as
wood, bones, and shells
and is effective back to about 70,000 years