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The Rock Record
Chapter 8
The earth is approximately 4.6 billion years old.
James Hutton and
Uniformitarianism
•
18th century Scottish physician and
farmer farmer
• Believed that studying the present is the
key to understanding the past.
• Principle of Uniformitarianism – the processes
that shape the Earth today are the same as the
processes that shaped the Earth in the past.
– Processes such as volcanism, erosion, etc.
– Hutton’s theory suggested that the Earth was much
older than 6000 years because these processes
happen very slowly.
Geologic processes slowly change the
Earth surfaces.
Relative Age
• Rock layers (or strata) show the sequence of events
that took place in the past.
• Scientists can determine the order in which the rock
layers formed.
• Once they know the order, they can determine a
Relative Age for each layer.
• Relative Age indicates that one layer is older or
younger than another layer.
– Relative Age does NOT indicate the age of the rock layer in
years.
– Although many rocks form layers, sedimentary rock layers
are usually used to determine Relative Age.
Law of Superposition
• Sedimentary rocks form when new sediments are
deposited on top of old layers of sediments.
• As the sediments accumulate, they are compressed
and harden into sedimentary rock layers called beds.
• The boundary between two beds is called a bedding
plane.
• The Law of Superposition states that an undeformed
(unchanged) sedimentary rock layer is older than the
layers above it and younger than the layers below it.
– Generally, the deepest layer is the oldest and the top layer
is the most recent.
– The deepest layer was laid down first and the top layer
last.
Principle of Original Horizontality
• Sedimentary rocks generally form in horizontal
layers.
• The Principle of Original Horizontality states
that if sedimentary rock layers are left
undisturbed they will remain in horizontal
layers.
• Scientists assume that sedimentary rock layers
that are not horizontal have been tilted or
deformed by crustal movements that happen
after the layers were formed.
Horizontal strata
Deformed strata
(undeformed/unchanged)
(changed)
Unconformity
• A break in the geologic record (missing strata)
• Movements of Earth’s crust can lift up rock
layers that were buried and erode them; then
new layers are deposited on top.
• The eroded rock forms a gap in the geologic
record = Unconformity
• All rocks beneath an Unconformity are older
than the rock above the Unconformity.
• Three types of unconformities
The development of a Unconformity
Nonconformity
– The boundary between stratified sedimentary
rock and unstratified igneous rock
Angular unconformity
– The boundary between a set of tilted sedimentary
rock layers and a set of horizontal layers
Disconformity
– The boundary that forms between older eroded
rock layers and new rock layers deposited on top
Law of Crosscutting relationships
• Rock layers may be disturbed by faults or intrusions
that cut across the rock layers.
• A fault is a break or crack in Earth’s crust along which
rocks shift their position.
• An intrusion is a mass of igneous rock that forms when
magma is injected into rock and then cools and
solidifies (vertical intrusions are called dikes and
horizontal intrusions are called sills).
• The Law of Crosscutting Relationships states that a fault
or igneous intrusion is always younger than all the rocks
layers it cuts through.
• If the fault or intrusion cuts through an Unconformity,
the fault or intrusion is younger than all the rocks it cuts
through above or below the Unconformity.
Determining Absolute Age
• Determines the numeric age of rock layers.
• Absolute dating methods include
– Rates of erosion
• By studying how long it takes for rock to erode,
scientists can estimate the age.
• Can be accurate for geologic features that are between
10,000 and 20,000 years old.
• Not accurate for older features because the rates of
erosion can change over time.
– Rates of deposition
• Studying how long it takes for sediments to deposit
Radiometric Dating
• Rocks generally contain small amounts of radioactive
materials (isotopes).
• Radioactive isotopes are unstable
• Radioactive isotopes break down (decay) at a unique
and consistent rate.
– The Half-Life is the time it takes for ½ the mass of given
amount of radioactive material to decay into its daughter
isotope.
• These decay rates can be used to measure absolute
ages of rocks (radiometric dating).
• Scientists measure the concentrations of the original
radioactive isotope (parent isotope) to the newly
formed more stable isotopes (daughter isotopes).
Radioactive Isotopes & Half-Lives
Parent
Isotope
Daughter
Isotope
Half - life
Effective dating
range
Carbon -14
Radiocarbon dating
Nitrogen -14
5.730 years
> 70,000 years
Potassium -40
Argon -40
1.25 billion years
50,000-4.6 billion
years.
Rubidium - 87
Strontium- 87
48.8 billion years
10 million to 4.6
billion years
Uranium - 235
Lead – 207
704 million years
10 million to 4.6
billion years
Uranium - 238
Lead- 206
4.5 billion years
10 million to 4.6
billion years
Thorium - 232
Lead - 208
14 billion years
> 200 million years
Alpha Decay
Mass
unstable element
Atomic #
Mass

stable element
Atomic #
Example:
136
Cs
55
4

+
He
2
4
+
He
2
Beta Decay
Mass
unstable element
Atomic #

Mass
stable element
Atomic #
Example:
136
Cs
55
0

+
e-1
0
+
e-1
Radioactive decay of Uranium-238
Calculating Half-Life
T½
Time
(years)
Unstable
amt.
(mg)
Stable amt
(mg)
Unstable
amt
(%)
Stable
Amt.
(%)
Unstable
amt.
(fractions)
Stable
Amt.
(fractions)
0
0
4000
0
100
0
1
0
1
2
3
4
5
6
The Fossil Record
• The remains of plants and animals (organisms)
that lived in the past.
• Fossils are found in sedimentary rock
• Paleontologists study fossils
• Fossils can be used for:
– Relative and Absolute Dating
– Understanding past geologic events
– Provides information about the geologic history of
Earth
– Recreating past climates
– Provides clues to environmental changes that have
occurred in Earth’s past.
– Tracking the evolution of organisms
How Fossils can form
Fossilization
• Mummification – organisms dried out
• Amber – hardened tree sap
• Tar seeps- petroleum surface deposits trapped
organisms
• Freezing – frozen soil and ice preserved
organisms
• Petrification – mineral solutions in ground
water replaced organic materials. (silica,
calcite, pyrite)
• Imprints – carbonized impressions of leaves,
stems, flowers, fish made in soft mud or clay.
• Molds and Casts – shells often leave a cavity
which fills with sediment (mold) and a replica
of the original organism (cast).
• Coprolites – fossilized dung
• Gastroliths – digestive stones from dinosaurs
• Trace Fossils – evidence of past animal
movements (footprints, tracks, borings,
burrows).
Index Fossils
• Fossils can be used to determine the Relative Ages
of rock layers that contain the fossils.
• Index Fossils can be used to estimate Absolute Ages
of specific rock layers that contain the fossils.
• Index Fossils are fossils that only occur in rock layers
of a particular age.
–
–
–
–
–
Index fossils must be present over a large area
Index fossils must be unique
Index fossils must have lived for short periods of time
Index fossils must occur in large numbers
Example: ammonites…180-206 million years ago
Grey Fossil Site, Tennessee