Geologic Time

1
Geologic Time

• Geologic Time Scale
• Principles of Relative Dating
• Absolute Dating

2
The Geologic Time Scale

• Defined as eons, eras, periods, and epochs
• Each eon, era, etc usually relates to
  significant changes in the fossil record

3
(No Transcript)
4
Some Basic Geologic Principles

• Law of Superposition
• Principle of Original Horizontality
• Principle of Cross-Cutting Relationships

5
Law of Superposition

• In a series of sedimentary rocks younger rocks
  are laid on top of older rocks
• e.g. Grand Canyon, as you hike up and out, rocks
  are progressively younger

6
Law of Original Horizontality

• Sedimentary rocks are laid originally in a
  horizontal (lying flat) manner
• Rock beds that are no longer horizontal have been
  tilted

7
Cross-cutting relationships

• What came first?

8
Unconformities

• Interruptions of the geologic record

B
A
9
N
M
L
K
J
I
H
G
F
E
O
D
B
C
A
10
Correlating Rock Layers

• Rock formations are sometimes unique enough to
  identify in multiple places
• This allows for a larger series of rock units to
  be related

11
(No Transcript)
12
Fossils

• We know the progression of life as it evolved on
  earth through time
• Certain fossils occur only during limited time
  periods

13
540-240 Million years old
Trilobites (bottom dwelling critters) -
Paleozoic Dinosaurs - Mesozoic Mammals - Cenozoic
240-65 Million years old
65 Million years old to present
14
Planetary Surfaces

• If we only have pictures, how do we date
  planetary surfaces??
• Superposition and cross-cutting relationships
• Crater density

15
C
4
D
2
A
1
B
3
16
Absolute Dating

• Radiometric Dating
• Based on radioactive decay of atoms (isotopes)
• Most common and reliable method of dating
• Also paleomagnetism, tree rings, etc

17
Radioactive Decay

• Some elements nuclei have a tendency to
  spontaneously break apart
• This occurs with certain isotopes
• e.g. U238, Rb87, K40, C14

18
(No Transcript)
19
Radioactive Decay

• Radioactive decay converts mass into energy
  (emc2)
• This is why the Earths interior is hot
• Radioactive decay happens at a very constant and
  predictable rate

20
Where Does Helium Come From??

• Helium is used for balloons and blimps
• So where can we find helium??
• Alpha particles (2 neutrons and 2 protons) are
  helium atoms released in nuclear fission
• 238U ? 234Th 4H
• Geologists find helium much in the same way they
  find oil and natural gas

21
Half Life

• The half life of a radioactive element is the
  amount of time it takes for half of the original
  amount of material to decay

22
C14
N14
It takes 5730 years for half of the original
carbon-14 to decay to nitrogen-14
23
Radiometric Dating

• Measure ratio of parent to daughter elements
  (isotopes) to determine crystallization age
• Absolute age is dependent on both the ratio of
  parent to daughter elements and half life

24
(No Transcript)
25
How Old is It?

• Half life of 1 billion years Parent isotope is
  K40, daughter is Ar40

Ratio of K40 to Ar40 Age
½ to ½ ¼ to ¾ 1 to 0 1/16 to 15/16
1 Billion years
2 Billion years
0 years
4 Billion years
26
Elements used for Geologic Dating
27
Caveats

• Works mostly on igneous rocks
• Records the date of crystallization
  (solidification), not deposition
• Metamorphic and sedimentary rocks are much more
  difficult to date

28
How Old is It?
Limestone layer is between 200,000 and 400,000
years old.