1. Dating methods and the age of the Earth
Geology 103

2. Two types of dating
Relative dating asks “Is a given event older or younger than another event?”
Numerical (absolute) dating asks “How many years ago did an event take place?”

3. What is an “event”?
A discrete occurrence that can be inferred from the rock or fossil record
Examples: the deposition of a sedimentary layer, or the death of an organism

4. Relative dating
Steno’s principles (1669) are all very common sense but do order strata from oldest to youngest
Stratum = layer
Strata = layers
Stratigraphy = study of the order of events in sedimentary rocks

5. Principle of original horizontality
Any distortion of sedimentary strata is due to deformation after the strata were deposited

6. Principle of lateral continuity
Strata are deposited in a basin and thus will be continuous from end to end
Any breaks are due to erosion or deformation after deposition

7. Principle of superposition
Younger strata overlie older strata
True of all gravity-driven deposits

8. Principle of cross-cutting relationships
The body that cuts through is younger than the body that got cut through
True for dikes, faults, joints and any non-concordant stratum

9. Principle of inclusions
Sometimes called “xenoliths”
The inclusion is older than the rock in which it is included

10. Correlation
Similar strata separated by gaps, were once part of a continuous layer
Led to the idea of “formations”: a mappable unit of rocks sharing a common origin event

11. Faunal succession
William Smith and George Cuvier (around 1800): There is an order to the fossils found in strata
Smith and Cuvier did not think of evolution but the principle led Charles Darwin to think of how the order might arise

12. Evolution of geologic timescale

13. Modern geologic timescale

14. Points about the timescale
Divided up by major changes in the fossils found in the strata (Phanerozoic): mass extinctions mark the boundaries
Major divisions: eon, era, period, epoch
Rocks don’t care about life: that is, one period ≠ one formation
Thus, lithostratigraphy is not biostratigraphy

15. Unconformities An unconformity is a time gap in the rock record
Classified into different types, but all represent significant missing time

16. “The Great Unconformity”

17. Numerical dating Two types: radiometric and non-radiometric
Radiometric dating always involves the decay of a radioactive isotope
Non-radiometric dating always involves a change in the number or condition of a material

18. Basic atomic structure
Atoms are the basic unit of matter with chemical properties
Made of sub-atomic particles: electrons, protons and neutrons
All elements (atoms of one type) have isotopes that differ in mass
Some isotopes are unstable and will decay (radio-isotopes)

19. Radiometric dating
Typically need to know: how much of the radioactive isotope (radio-isotope) was present when the “geo-clock” started (i.e., the event occurred), how much of the radio-isotope is present currently and/or the half-life of the radio-isotope

20. Radioactive decay series
Not all decays are as complicated as this!
Start with the principal radio-isotope and end with the stable isotope

21. The “geo-clock” The shape of the curve is called “exponential decay”,
because half of the radioisotope (“parent isotope”) is con-verted to the daughter isotope each successive half-life.

22. Many different systems

23. Non-radiometric dating
Examples: tree ring dating (dendrochronology), lichenometry, fission track dating, electron spin resonance (ESR) dating
In the photo above, the path of an alpha particle ejected
from a radioisotope nucleus is called a “fission track”. By
enhancing these tiny tracks, counting them allows the
researcher to figure out how old numerically the sample is.

24. Two key questions about numerical dating
1. Is the dating method appropriate for the estimated age and composition of the dated material?
2. What is the event being dated? In other words, when does the geo-clock start?

25. Test case
You discover early homonid (i.e., pre-Homo sapiens) skeletons in a sandstone deposit
How do you obtain the numerical age? What method(s) would you use?

26. What are appropriate materials?
Radiometric dating relies on closed systems – typically this means a material that does not gain or lose components, like radioisotopes.
Examples are: igneous rocks, some metamorphic rocks (note they are all crystalline)
Notable bad material: Sedimentary rocks

27. Use boundary (bounding) numerical ages
If one can find a datable material stratigraphically near a fossil, then the age of the datable material can be used to constrain the age of the fossil.
In the example to the right the ash layer was dated (U-Pb) at 563 ± 3 My, so fossils are at least that old.

28. So how old is the Earth?
And how do we know?

29. Count the days of recorded history
James Ussher, Archbishop of Armagh, uses biblical history to determine the first day of Genesis.
It is a complex calculation.
Calculates Sunday, October 23, 4004 BCE as the start date.

30. Estimate the Earth’s cooling
William Thomsen, Lord Kelvin (University of Glasgow, Scotland), 1897.
Reasoned that the Earth was initially molten and had cooled off over time, enough to make a crust.
Measured the rates of cooling of iron spheres.
Age: 20 to 40 million years.

31. Measure the ocean’s salinity
John Joly (Trinity University, Ireland), 1899.
Reasoned that the oceans had started off without any sodium content, and thus, by measuring the amount of sodium brought to the ocean annually by all the rivers in the world and measuring the ocean’s salinity, one could calculate the age of the ocean, which is close to the age of the Earth.
Age: 80 to 100 million years

32. Biologists (and geologists) had a problem with these ages
As Charles Darwin, originator of the theory of evolution by means of natural selection pointed out, even 100 million years was not long enough to give the mechanism of evolution time to generate the diversity of Earth’s life.

33. Radioactivity provides the key
As mentioned earlier, the phenomenon of radioactivity generates heat – something that Kelvin could not have predicted.
Henri Becquerel (Museum of Natural History, Paris) found radioisotopes emitting invisible rays (1896).
Ernest Rutherford (McGill University, Montreal) suggested uranium isotopes might be useful in dating earth materials (1905).

34. Refining the technique
Inspired by a talk by Rutherford, Bertram Boltwood (Yale, 1907) reasons that by measuring the amount of lead, the stable end product of uranium decay, one could determine the age of a rock sample.
Ages ranged from 250 to 1300 million years.

35. ...to the age of the Earth
Arthur Holmes (Imperial College, London, 1913) realizes that the decay of thorium also contributes to the final lead amount and refines Boltwood’s technique.
By 1927, he publishes the age of the Earth as about 1.6 to 3.0 billion years.
In 1931, he co-authors a report the establishes radiometric dating as the only reliable method to determine the Earth’s age.

36. The current age
Earth materials were not solid for a while after the Earth formed – big problem, as radioisotopes are not “fixed” in a crystal until after crystallization.
Oldest rock: Acasta gneiss from the Slave craton, North America: ± Gyr
Oldest mineral: Zircon from Jack Hills, Western Australia: ± Gyr
Other solar system bodies cooled much more quickly after formation: Canyon Diablo meteorite (shown in photo): 4.53 to 4.58 Gyr
Also useful: Lunar rocks, meteorites from Mars
Age of solar system can be independently
determined by helioseismology, which
uses the distribution of helium in the Sun’s
core to figure out how long the Sun has
been active.