1. Geologic Time Chapter 8

2. Determining geological ages
Relative age dates – placing rocks and geologic events in their proper sequence
Numerical dates – define the actual age of a particular geologic event (termed absolute age dating)

3. First principle of relative dating
Law of superposition
Developed by Nicolaus Steno in 1669
In an undeformed sequence of sedimentary or volcanic rocks, oldest rocks at base; youngest at top

4. Superposition illustrated by strata in the Grand Canyon

5. 2nd, 3rd principles of relative dating
Principle of original horizontality
Layers of sediment are originally deposited horizontally (flat strata have not been disturbed by folding, faulting)
Principle of cross-cutting relationships
Younger features cut across older ones

6. Cross Cutting Relationships in strata

8. Grand Canyon younger strata cutting across older ones
Cambrian Tapeats sandstone over Precambrian Unkar Group

9. Unconformities (loss of rock record)
An unconformity is a break in the rock record produced by erosion and/or nondeposition
Types of unconformities
Angular unconformity – tilted rocks overlain by flat-lying rocks
Disconformity – strata on either side of the unconformity are parallel (but time is lost)
Nonconformity – sedimentary rocks deposited above metamorphic or igneous rocks (basement)

10. Formation of an angular unconformity

11. An angular unconformity at Siccar Point, England

12. Development of a Nonconformity
The basement-cover contact near Boulder (Pennsylvanian sandstone over Precambrian granite) is a nonconformity (visible on Flagstaff Road near bouldering area)

13. Nonconformity in the Grand Canyon - Strata deposited over Schist

14. Correlation of rock layers
Matching strata of similar ages in different regions is called correlation

15. Correlation of strata in
southwestern United States
Sections are
incomplete

16. Correlation of rock layers with fossils
Correlation relies upon fossils
Principle of fossil succession – fossil organisms succeed one another in a recognizable order - thus any time period is defined by the type of fossils in it

17. Determining the ages of rocks using fossils

18. Geologic time scale
The geologic time scale – a “calendar” of Earth history
Subdivides geologic history into units
Originally created using relative dates
Structure of the geologic time scale
Eon – the greatest expanse of time

19. Geologic Timescale
Divisions based on fossils

21. Geologic time scale Structure of the geologic time scale
Names of the eons
Phanerozoic (“visible life”) – the most recent eon, began about 540 million years ago
Proterozoic
Archean
Hadean – the oldest eon

22. This, believe it or not, is the rock with the oldest known minerals ever found. From NW Australia, the rock (a conglomerate) is about 3.0 Billion years old. The rock contains detrital grains of zircon (a mineral formed in granite in the crust) that is 4.4 Billion years old. Age of the Earth is 4.54 Billion (sample and age date courtesy of Steve Mojzsis)

23. Geologic time scale Structure of the geologic time scale
Era – subdivision of an eon
Eras of the Phanerozoic eon
Cenozoic (“recent life”)
Mesozoic (“middle life”)
Paleozoic (“ancient life”)
Eras are subdivided into periods
Periods are subdivided into epochs

24. Geologic time scale Precambrian time
Nearly 4 billion years prior to the Cambrian period
Not divided into small time units because the events of Precambrian history are not know in detail
Immense space of time (Earth is ~ 4.5 Ga)

25. Radioactivity (Used to age date rocks)
Spontaneous changes (decay) in structure of atomic nuclei
Types of radioactive decay
Alpha emission
Emission of 2 protons and 2 neutrons (an alpha particle)
Beta emission
An electron (beta particle) is ejected from the nucleus
Electron capture
An electron is captured by the nucleus
The electron combines with a proton to form a neutron

26. Neutron capture (A) and Beta emission (B)

27. Using radioactivity in dating
Parent – an unstable radioactive isotope
Daughter product – isotopes resulting from decay of parent
Half-life – time required for one-half of the parent isotope in a sample to decay

28. A radioactive decay curve

30. Dating with carbon-14 (radiocarbon dating)
Half-life only 5730 years
Used to date very young rocks
Carbon-14 is produced in the upper atmosphere
Useful tool for geologists who study very recent Earth history (for me this is the history of earthquakes).

31. Using radioactivity in dating
Importance of radiometric dating
Allows us to calibrate geologic timescale
Determines geologic history
Confirms idea that geologic time is immense

32. How do we actually “date” a rock?
Collect sample (geologist as pack animal)
Process for minerals by crushing, sieve, separate magnetically and/or with heavy liquids
Measure parent/daughter ratio of mineral separates with a mass spectrometer

33. Dating sediments without fossils

34. In-class exercise: Determining the age of a young sedimentary rock with carbon-14
Take out a piece of paper and put your name on it to obtain credit
Assume a half-life of 5730 years.
Determine the age of the sample after three half-lives
Determine the amount of parent material in the rock after three half lives
Determine the age of a Precambrian igneous rock using Uranium 238.
Assume a half life of 2.25 Billion years
Assume the rock is 4.5 Billion years old
Determine the relative amount of parent and daughter isotopes (or ratio relative to one another.

35. End of Chapter 8