Rock Record and Time

1
Rock Record and Time
2
Geologic Time

• A major difference between geologists and most
  other scientists is their concept of time.
• A "long" time may not be important unless it is
  greater than 1 million years

3
(No Transcript)
4
Two Ways to Date Geologic Events

• 1) relative dating (fossils, structure,
  cross-cutting relationships) how old a rock is
  compared to surrounding rocks
• 2) absolute dating (isotopic, tree rings, etc.)
  actual number of years since the rock was formed

5
(No Transcript)
6
Relative Dating
7
Steno's Laws

• Nicholas Steno (1669)
• Principle of Superposition In a sequence of
  undisturbed layered rocks, the oldest rocks are
  on the bottom.
• Principle of Original Horizontality Layered
  strata are deposited horizontal or nearly
  horizontal or nearly parallel to the Earths
  surface.

These laws apply to both sedimentary and volcanic
rocks.
8
Paleontology

• The study of life in the past based on the fossil
  of plants and animals.
• Fossil evidence of past life
• Fossils that are preserved in sedimentary rocks
  are used to determine 1) relative age 2) the
  environment of deposition

9
Fossils
10
(No Transcript)
11
Unconformity

• A buried surface of erosion

12
(No Transcript)
13
Cross-cutting Relationships

• Geometry of rocks that allows geologists to place
  rock unit in relative chronological order.
• Used for relative dating.

14
Angular unconformity
Fig. 10.8
15
Cross cutting relations
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
Fig. Story 10.11
20
Fig. Story 10.11
21
Fig. Story 10.11
22
Fig. Story 10.11
23
Sequence Stratigraphy

• Stratigraphic analysis in which the major
  geologic units are unconformities
• Used widely with seismic data

24
The Geologic Timescale
Divisions in the worldwide stratigraphic column
based on variations in preserved fossils
25
(No Transcript)
26
(No Transcript)
27
10
1600
28
Absolute Geochronology

• Add numbers to the stratigraphic column based on
  fossils
• Based on the regular radioactive decay of some
  chemical elements

Isotopes
Atoms of elements with the same number of protons
and varying numbers of neutrons Examples 235U,
238U 87Sr, 86Sr 14C, 12C
29
Isotopic Dating

• Radioactive elements (parents) decay to stable,
  non-radioactive elements (daughters)
• The rate at which this decay occurs is constant
  and known
• If we know the rate of decay and the amount
  present of parent and daughter we can calculate
  how long this reaction has been occurring.

30
Types of Decay

• alpha decay loss of a 4He (2n, 2p)
• e.g., 147Sm ? 143Nd
• beta decacy neutron ? proton
• e.g., 87Rb ? 87Sr
• electron capture proton ? neutron
• e.g., 40K ? 40Ar

31
Requirements for Isotopic Dating

• Closed system
• decay rate constant
• Initial concentration of daughter is known

32
Half-life
The half-life of a radioactive isotope is defined
as the time required for half of it to decay.
33
Geologically Useful Decay Schemes
parent daughter half life (years)
235U 207Pb 4.50 x 109 238U 206Pb 0.71 x
109 40K 40Ar 1.25 x 109 87Rb 87Sr 47.0 x
109 14C 14N 5730
34
Fig. 10.13
35
(No Transcript)
36
Direct Measurement of the Rates of Geologic
Processes
The precision now available through the Global
Positioning System (GPS) allows measurements of
processes, such as plate motion, to within 1
mm/year.
37
Fig. 10.15
38
Dating the Order of Deformation

• Use geometry
• Inclusions
• Cross-cutting relationships
• Combine with fossils and radiometric dating

39
Fig. 11.22
40
Example 1 What Type of Fault is This?
young

Cretaceous
Cambrian
old
41
Example 2 What Type of Fault is This?
old

Silurian
young
Tertiary
42
Example 3 What Type of Fold is This?
young
Tertiary
Devonian
Silurian
old
43
Example 4 When did the folding occur?
young

Tertiary
Triassic
Devonian
Silurian
old
44
Example 5 When did the faulting occur?

Cretaceous
Jurassic
Jurassic
Triassic
Triassic
Cambrian
Cambrian