Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 8:
CLOCKS IN ROCKS
Timing the Geologic Record
© 2011 by W. H. Freeman and Company
Chapter 8:
Clocks in
Rocks:
Timing the
Geologic Record
About the Clocks in Rocks
• Geologists have uncovered deep time by
studying the “clocks in rocks.”
• These clocks can be used to measure
the duration of geologic processes and
cycles of the Earth system.
• Geologists refer to both relative age and
absolute age.
Lecture Outline
1. Reconstructing geologic history from
the stratigraphic record
2. Geologic time scale: relative ages
3. Measuring absolute time with isotopic
clocks
4. Geologic time scale: absolute ages
5. Advances in timing the Earth system
1. Reconstructing Geologic History from
the Stratigraphic Record
● Principles of stratigraphy
● original horizontality
● superposition
● faunal succession
Original Horizontality and Superposition
Younger
Older
Example: Marble Canyon, Arizona
Original Horizontality and Superposition
Younger
Older
Left undisturbed, the youngest layers always remain
above the oldest.
Original Horizontality and Superposition
sedimentation in
lake or sea
Original Horizontality and Superposition
Sediments are deposited in horizontal
layers and slowly change into rock.
Original Horizontality and Superposition
Fossils as Recorders of Time:
The Principle of Faunal Succession
Outcrop A
I
Outcrop B
II
II
III
Outcrop A
I
Outcrop B
II
II
III
Outcrop A
I
Outcrop B
II
II
III
Some of the fossils found in
outcrop A are the same as
fossils found in outcrop B,
some distance away.
Outcrop A
I
Outcrop B
II
II
III
Some of the fossils found in
outcrop A are the same as
fossils found in outcrop B,
some distance away.
Outcrop A
Outcrop B
I
II
II
Layers with the same
fossils are the same
age.
III
Some of the fossils found in
outcrop A are the same as
fossils found in outcrop B,
some distance away.
Outcrop A
Outcrop B
I
II
II
Layers with the same
fossils are the same
age.
III
1. Reconstructing Geologic History from
the Stratigraphic Record
● Unconformities – gaps in the record
● disconformity
● angular unconformity
Disconformity
Disconformity
Disconformity
Disconformity
Disconformity
Angular
unconformity
Angular
unconformity
Angular
unconformity
Angular
unconformity
Angular
unconformity
Angular
unconformity
The ‘great unconformity’ in the
Grand Canyon, Arizona.
1. Reconstructing Geologic History from
the Stratigraphic Record
● Cross-cutting relationships
● faults
● intrusions
Cross-cutting
relationships
Cross-cutting
relationships
Cross-cutting
relationships
Cross-cutting
relationships
Cross-cutting
relationships
Relative age
relationships
Thought questions for this chapter
As you pass by an excavation in the street, you see a
cross section showing paving at the top, soil below the
paving, and bedrock at the base. You also notice that a
vertical water pipe extends through a hole in the street
into a sewer in the soil. What can you say about the
relative ages of the various layers and the water pipe?
Why did 19th-century geologists who constructed the
geologic time scale find sedimentary strata deposited in
the sea more useful than strata deposited on land?
Thought questions for this chapter
In studying an area of tectonic compression, a geologist
discovers a sequence of older, more deformed
sedimentary rocks on top of a younger, less deformed
sequence, separated by an angular unconformity. What
tectonic process might have created the angular
unconformity?
A geologist documents a distinctive chemical signature
caused by Proterozoic organisms. Would you consider
this chemical signature to be a fossil?
2. Geologic Time Scale:
Relative Ages
● Divisions of geologic time
● eras
● periods
● epochs
3. Measuring Absolute Time
with Isotopic Clocks
● Isotopic dating theory
● radioactive atoms
● parent-daughter
● half-life
3. Measuring Absolute Time
with Isotopic Clocks
● Isotopic dating methods
● uranium-lead
● potassium-argon
● rubidium-strontium
● carbon-nitrogen
Example: Rubidium-Strontium
Neutrons
Protons
Electron
Rubidium-87 nucleus
Neutrons
Protons
Electron
A neutron decays,
ejecting an electron…
Rubidium-87 nucleus
Neutrons
Strontium-87 nucleus
Protons
Electron
A neutron decays,
ejecting an electron…
Rubidium-87 nucleus
Neutrons
Strontium-87 nucleus
Protons
Electron
A neutron decays,
ejecting an electron…
…and producing a proton,
which changes the atom.
Thought questions for this chapter
Is carbon-14 a suitable isotope for dating geologic events
in the Pliocene Epoch?
How does determining the ages of igneous rocks help to
date fossils?
4. Geologic Time Scale:
Absolute Ages
● Four Eons of geologic time
● Hadean
● Archean
● Proterozoic
● Phanerozoic
5. Advances in Timing
the Earth System
● Sequence stratigraphy
● Chemical stratigraphy
● Paleomagnetic stratigraphy
● Clocking the climate system
Key terms and concepts
Absolute age
Eon
Epoch
Era
Geologic time scale
Half-life
Isotopic dating
Mass extinction
Period
Principle of faunal succession
Principle of original horizontality
Principle of superposition
Relative age
Stratigraphic succession
Unconformity