1. Chapter 8: Geologic Time
PowerPoint Presentation
Stan Hatfield . SW Illinois College
Ken Pinzke . SW Illinois College
Charles Henderson . University of Calgary
Tark Hamilton . Camosun College
Copyright (c) 2005 Pearson Education Canada, Inc.

2. Geological Time is of Vast Duration Sediments were buried under a Mountain Range, metamorphosed, half the crust was uplifted & eroded, glaciers carried it, lake storms removed all but the biggest rocks.

3. Geologic Time
Relative age dates – placing rocks and events in their proper sequence of formation. e.g. Tertiary is younger than Cretaceous (rocks, fossils, climate)
Numerical dates – specifying the actual number of years that have passed since an event occurred (known as absolute age dating using isotope clocks)
Geologic time scale – Earth’s history is ~4.5 Ga long and written disproportionately by the rocks formed and processes which operated at different times and places.

4. Ordovician Strata: Chute Vernal, PQ

5. Relative Dating – Key Principles
Law of Superposition
Developed by the Danish physician Nicolaus Steno working in Italy in 1669
In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom
Most sedimentary & volcanic rocks are deposited in sequences of essentially flat lying beds

6. Relative Dating – Key Principles
Principle of Original Horizontality
Layers of sediment are generally deposited in a horizontal position
Rock layers that are flat have not been disturbed
Principle of Cross-Cutting Relationships
Younger features cut across older features
Principle of Inclusions
Younger rocks/features include older ones
All 3 types of rocks can include older rocks and minerals

7. Permian Strata, South Rim
Relative Dating – Key Principles: Horizontality & Law of Superposition
Correlation
Permian Strata, South Rim

8. Lower Paleozoic Strata: Devon, UK
Fold Hinge
Deposition, Deformation
At the Closing of Iapetus

9. Formation of Sedimentary Inclusions

10. Relative Dating – Key Principles
Unconformities
Types of Unconformities
Disconformity – strata on either side of the unconformity are parallel (least missing time)
Angular unconformity – tilted rocks are overlain by flat-lying rocks (10’s of Ma missing)
Nonconformity – metamorphic or igneous rocks in contact with sedimentary strata (most missing time, eroded down to crystalline basement rocks)

11. Siccar Point, Scotland & The development of an angular unconformity.

14. m h i g l k j e f d c What kind of unconformity is “i”? What kind of
Figure out the sequence of geological events,
Oldest on the bottom, youngest on the top. m h i g l k j e f d c
What kind of
unconformity is “i”?
What kind of
Contact is “m”?

15. m h i g l k j e f d c “i” is a disconformity “m” is intrusive,
Dyke A
Dyke B
Intr-l
Fault B k j I
Fault A h g f e d c
Figure out the sequence of geological events,
Oldest on the bottom, youngest on the top. m h i g l k j e f d c
“i” is a disconformity
“m” is intrusive,
& baked margins.

16. Which type of unconformity here at the
South Rim represents the greatest amount
of missing time and why?

17. Correlation of Rock Layers
Matching of rocks of similar ages in different regions is known as correlation
Correlation often relies upon fossils
William Smith (late 1700s and early 1800s) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content
Other than fossils we often rely on widespread instantaneous events like volcanic ash falls
Unconformities bound packages of stratigraphy

18. Correlation across slightly overlapping stratigraphy Arizona to Utah.

19. Correlation of Rock Layers
Correlation often relies upon fossils
Principle of Fossil (Faunal) Succession – fossil organisms succeed one another in a definite and determinable order that documents the evolution of life; therefore any time period can be recognized by its fossil content
Index fossils – represent best fossils for correlation; they are widespread ecologically & geographically and are limited to a short time span (i.e., they evolved rapidly)

20. Time from Concurrent Range Zones

21. Dating with Radioactivity
Reviewing Basic Atomic Structure
Isotope (only a few of these are radioactive)
Variant of the same parent atom, same atomic number
Differs in the number of neutrons (weight)
Results in a different mass number than the common type of atom for this element
Too many additional neutrons makes a nucleus unstable and prone to radioactive decay

22. 3 kinds of radioactive decay

23. Dating with Radioactivity
Parent – an unstable radioactive isotope
Daughter product – the isotopes that result from the decay of a parent
Half-Life – the time required for one-half of the radioactive nuclei in a sample to decay
Half-life differs for every Parent-Daughter pair of isotopes

24. Exponential Decay of Parent Isotopes (& exponential growth of daughters)
The Decay
Equation:
A = A0 e –λt
A0 is the initial
amount of the
parent isotope

25. Dating with Radioactivity
Radiometric Dating
Principle of Radioactive Dating
The percentage of radioactive atoms that decay during one half-life is always the same (50 percent)
However, the actual number of atoms that decay continually decreases
Comparing the ratio of parent to daughter yields the age of the sample

27. Dating with Radioactivity
Radiometric dating
Useful radioactive isotopes for providing radiometric ages
Rubidium-87  Strontium-87 ~Precambrian
Feldspar, micas, amphiboles
Thorium-232  Lead-207 ~Pc
Zircon
Two isotopes of uranium (235 and 238) to Lead (208 & 206), 4.5 Ga & 713 Ma respectively, ~Pc to Mesozoic
Potassium-40  Argon-40 or relative to Argon-39 >2 Ma
Feldspar, micas, volcanic glass

28. Slow U 238U  210Pb (½ life = 4.5 Ga) Used for Ancient Zircons & <200a 210Pb
The whole span is used for dating zircons in igneous & metamorphic rocks usually of Mesozoic or older age and requires the measurement of both parent isotope 238U and 210Pb. With care and high precision Laser ablation Mass Spectrometry this can be done for zircons as young as ~10 Ma. Mainly this is the dating work horse for Precambrian rocks older than 1 Ga.
Disequilibrium dating of Pleistocene Carbonates can be done using 226Ra. Muds younger than 200 years can be dated using 210Pb accumulation from atmospheric 222Rn.

29. Dating with Radioactivity
Rules for Radiometric Dating
Mineral contains both parent & daughter as for Zircon with 238U and 206Pb or feldspar with 87Rb and 87Sr
Formed at time of event to be dated as for a lava flow, dyke or contact metamorphism
The mineral is a closed system and has neither gained nor lost parent & daughter

30. Dating with Radioactivity
Radiometric dating
Sources of error
A closed system is required
If temperature is too high, daughter products may be lost
To avoid potential problems, only fresh, unweathered rock samples should be used

31. Production of 14C in the upper atmosphere (& decay once organisms die)
Cosmic rays
Expel neutrons from
Atmospheric gases (N,O).
This expels a proton
From nitrogen
Forming radioactive 14C.
The extra neutron
in radioactive 14C
decays back to 14N
With a ½ life of
5730 years.

32. Dating with Radioactivity
Dating with carbon-14 (radiocarbon dating)
Half-life of only 5730 years
Used to date very recent events
Carbon-14 is produced in the upper atmosphere
Useful tool for anthropologists, archaeologists, historians, and geologists who study very recent Earth history
Only works for plant or animal tissue, not rocks

33. The Geological Time Scale

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

35. The Geological Time Scale

36. 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

38. Precambrian time Nearly 4 billion years prior to the Cambrian period
Not divided into smaller time units because the events of Precambrian history are not known in great enough detail
Also, first abundant fossil evidence does not appear until the beginning of the Cambrian

39. Difficulties in dating the geologic time scale
Not all rocks can be dated by radiometric methods
Grains comprising detrital sedimentary rocks are older than the rock in which they formed
The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed as porphyroblasts may grow for many Ma
The rock needs minerals with the right parent and daughter isotope pairs and in the right age span to be measureable.

40. Difficulties in dating the geologic time scale
Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages
Dates change as brackets become narrower and methods refined; e.g., base of Triassic is now 252 Ma, base of Permian is now 299 Ma, base of Cambrian is 543 Ma…

41. Igneous events permit
Absolute dating
Of sedimentary rocks.

43. Dating Terrains On other Worlds

44. Crater counts & cross cutting relations mostly Precambrian

46. Dendrochronology (wiggle matching) For wood in 14C realm <70Ka

47. Extinctions: Dinosaur Graveyards

48. Bolide impact & Deccan Trap Volcanism both occur at 65Ma
Bolide impact & Deccan Trap Volcanism both occur at 65Ma. Biodiversity dwindled for 10 Ma prior to this. The K/T massive extinction is debated.