1. Geologic Time and Plate Tectonics Read Chapter 8
Questions
How do geologists place the events of geologic history in sequence?
What is the geologist’s definition of plate tectonics and what evidence underlies the theory?
How do we apply plate tectonics?
Tools
Principles of stratigraphy
The geologic timescale
Radiometric dating

2. Terms to know GEOLOGIC TIME SCALE EON ERA PERIOD TRILOBITE PRECAMBRIAN
CYANOBACTERIA
PALEOZOIC ERA
MESOZIC ERA
CENOZOIC ERA

3. Terms to know FOSSIL INDEX FOSSIL TRACE FOSSIL RELATIVE AGE
LAW OF SUPERPOSITION
MASS EXTINCTION
UNIFORMITARIANISM
RADIOACTIVE DECAY
RADIOMETRIC DATING

4. Geology deals with a wide range of times and rates
Geology is a historical science…it asks what did happen and when.
When considering events in the unobservable past, two basic needs are to establish the relative order of events and to fix the absolute age of events.

5. Time and stratigraphy
Stratigraphy is the branch of geology that places events in history and the preserved products of those events (rocks, fossils, structures) in chronological order.
All stratigraphy begins by constructing a local sequence, putting in order those rocks among which the temporal relations can be directly observed by contact in the field.
Relating sequences or ages measured in one place to events in other places requires correlation, the basic tool for building up a global sequence of events and a globally useful timescale.

6. Time and stratigraphy
Local sequences and correlation…within each outcrop the sequence of colors is fixed by direct observation. Matching this sequence with what is observed in a different outcrop is correlation.

7. Original horizontality and superposition

8. Superposition?
Which way is up?

9. it is a comparatively recent development in the history of geology
Time and stratigraphy
Radiometric dating is the only sure way to establish absolute ages, but…
it is a comparatively recent development in the history of geology
many rocks (e.g., essentially all sedimentary rocks) cannot be dated because their formation did not reset isotopic indicators
the errors on radiometric dates are of a different kind from the errors that can be made in relative stratigraphy

10. NUMERICAL DATING OF THE EARTH
Rocks contain radioactive minerals which are constantly disintegrating at a steady rate
Under certain circumstances, these atomic “clocks” can be red to give a “time”

11. Radioactive elements
Not all elements are radioactive. Those that are and are the most useful for geologic dating are:
U-238 Half-life = 4.5 By
K-40 Half-life = 1.25 By
C-14 Half-life = 5.73 ky
Also, Sm-147, Rb 87, Th-232, U-235

12. Half-life for decay from U-238 all the way to Pb-206 is 4. 5 b. y
Half-life for decay from U-238 all the way to Pb-206 is 4.5 b.y. (billion years).
U-238 Decay Series
Decay rates for intermediate daughter products range from <1 sec (polonium) to 1,622 years (radium 226).

13. Fig. 5.4
Note changing ratio of parent/daughter after 2 half-lives. To get an estimate of the geologic age, you need the ratio of the parent isotope to the daughter isotope, e.g. two measurements.

14. Fig. 5.5
Simple arithmetic plot of a universal isotopic decay curve. After 1 half-life 50% of parent isotope remains; after 2 half-lives, 25% remains.
What happens if the vertical axis is changed from linear to logarithmic?

15. Model radioactive decay

16. The principle of superposition
Lithostratigraphy
The placement of a continuous series of stratified rocks in chronological order is based on two axioms
The principle of superposition
In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.
The principle of original horizontality
Layered strata are deposited horizontally or nearly horizontally or nearly parallel to the Earth’s surface.

18. Biostratigraphy
Life evolves over time and leaves recognizable traces in rocks called fossils
actual preserved body parts, casts or impressions of body parts, or traces left by the passage of an organism (e.g., a worm burrow or footprint)
A distinctive species or assemblage with a limited age range and a wide geographic range is an index fossil and can be used for correlation

19. Geochronology and Stratigraphy
Sedimentary rocks can rarely be dated directly by radiometric techniques, with the principle of superposition or cross-cutting relations ages can be bracketed between underlying and overlying datable horizons.
The major divisions of the geological timescale and their relative sequence are defined mostly by biostratigraphic horizons, the absolute numbers attached to the boundaries are determined by bracketing the boundaries between radiometric dates

20. The geologic timescale
Study figure 8.6, page 212

21. The geologic timescale
The base of the Phanerozoic at 543 Mya is the first widespread appearance of easily-fossilized hard parts (shells).
It is NOT the first appearance of multicellular organisms; that was at least 100 Ma earlier
Edicaran fauna
(pre-Cambrian)
Trilobite (Cambrian)

22. The geologic timescale
The eras of the Phanerozoic eon are:
Cenozoic (65 Ma–present)
Mesozoic (251 Ma–65 Ma)
Paleozoic (543 Ma–251 Ma)
These are marked by major, first-order changes in marine and terrestrial fossil assemblages, and the boundaries between them are the largest mass extinctions of species in the fossil record:
Broadly speaking, the large fauna of the Paleozoic is dominated by invertebrates, the Mesozoic by reptiles, the Cenozoic by mammals.

23. The geologic timescale
The periods of the Cenozoic are
the Quaternary
the Tertiary
(now divided into Paleogene and Neogene)
The periods of the Mesozoic are
Cretaceous
Jurassic
Triassic
The periods of the Paleozoic era are
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian