How do scientists sequence Earth’s past
events to create the geological time scale?
• Using evidence from rocks and fossils, scientists
can determine the order of events that occurred
in the past.
• Relative and Absolute age are used to
determine WHEN the events occurred.
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Relative Age
The age of an object
compared to the ages of
other objects.
(without knowing the
exact ages)
General Rules for
Determining Relative Age
• Law of Horizontality
• Law of Superposition
• Law of Cross-cutting
Relationships: igneous intrusions & faults
• Unconformities
• Index Fossils
Law of Horizontality
Sediment layers are horizontal
when originally deposited. Layers
that are not horizontal have been
deformed by movement of Earth’s
plates.
Because sediments are deposited under water, they will form flat, horizontal layers. If a sedimentary rock is found
tilted, the layer was tilted after it was formed.
Law of Superposition
In an UNDISTURBED horizontal
layer of rock, the oldest rocks
are on the bottom and the rocks
become younger toward the top.
DISTURBED horizontal layers: Law
of Superposition as a taco shell.
Movement of tectonic plates can disturb rock
layers. A whole set of layers can get turned on
its side. Rock layers can get bent, or even folded
over, like a taco shell.
When horizontal layers have been disturbed, the
youngest may no longer be on top. Scientists
determine the original order by comparing the
disturbed rock layers with a similar undisturbed
stack of layers.
Folded rock layers
`
Law of Crosscutting Relationships:
Igneous intrusions
Igneous rock forms when magma or lava
hardens. Beneath the surface, magma
may push into bodies of rock. The magma
cools and hardens
into a mass of igneous rock called
intrusions. An intrusion (magma) is always
younger than the rock layers it cuts
through.
Igneous intrusions
Intrusion
Extrusion
Law of Crosscutting Relationships:
Faults
A fault is a break in the Earth’s crust. Forces
inside Earth cause movement of the rock
on opposite sides of a fault.
A fault is always younger then the rock it
cuts through. To determine the relative age
of a fault, geologists find the relative age
of the youngest layer cut by the fault.
Law of Crosscutting Relationships:
Faults
Unconformities
Gaps in a sequence of rock
layers, resulting from erosion.
Even though rock layers are
lost, this can help scientists
understand how geologic
forces acted on Earth.
Index Fossils
These fossils are used to estimate
the age of rock layers in which they
are found. Index fossils can provide
a relative age.
THREE REQUIREMENTS FOR A
FOSSIL TO BE A GOOD INDEX
FOSSIL
 Found in many areas on the
Earth’s surface.
 Very common
 Existed for a short period of
time.
Example of Index Fossil
The mollusk
Inoceramus labiatus, for
example, is a kind of sea
animal that appeared 144
million years ago and went
extinct 65 million years
ago. So, if you find a rock
that contains a fossil of this
mollusk, the rock must be
between 144 million and
65 million years old
because this mollusk lived
during that time span.
Law of Faunal Succession
• The law states that fossils are found in
rocks in a very definite order.
• This law led others to use fossils to define
increments of time within a relative time
scale.
Fossil Record: A Timeline of Earth’s
History
Review of Relative Age Dating
1. What can you tell from undisturbed rock layers?
2. Are only sedimentary rocks used for relative age
determinations?
3. Explain the relative age relationship of crosscutting formations to the rocks they cut?
4. How can index fossils help scientists determine
the ages of rock layers?
Absolute Age
The exact age of a fossil
or rock formation
Can you ask a rock its age?
How do scientists determine the
exact age of rocks & fossils?
Scientist use radioactive dating to
determine the absolute age (exact
age) of fossils and rock formations.
Radioactive dating relies on
radioactive decay.
What is Radioactive Decay?
•Many elements are stable and do not
change, but some elements are unstable and
change over time.
•Unstable elements are used to find the exact
age of fossils.
•These unstable elements are said to be
radioactive.
What are some examples of
radioactive elements?
• Carbon-14 – changes into
Nitrogen-14
• Potassium-40 – changes into
Argon-40
• Urianium-235 – eventually
changes into Lead-207
What is radioactive decay?
• RADIOACTIVE DECAY is when
an unstable element changes in
some way to become stable.
• The atoms of an unstable element
(parent material) break down to
become atoms of another
element (daughter material).
What is a half-life?
• Half-life is the time it takes for half of the
radioactive atoms (unstable atoms) to
change into stable element.
• Each different radioactive element has its
own personal half-life.
Think of it as a geologic CLOCK
Half-life Example
•The half-life of C-14 is about 5700 years.
(Every 5700 years, HALF of the C-14 changes!)
•You have 60 grams of Carbon-14.
• After 5700 years pass, how much C-14 would you
have? N-14?
•Now you have 30g of C-14 & 30g of N-14
•Another 5700 years go by.
•How much C-14 would you have? N-14?
•15g of C-14 & 45g of N-14
•This would continue until all the unstable element
becomes stable.
When does radioactive decay
begin?
• Radioactive decay starts as
soon as the molten magma
or lava begin to harden.
How is absolute age
determined from
radioactive dating?
• In radioactive dating, scientists first
find out how much radioactive element
is in the sample.
• Then, they compare the radioactive
amount to the stable amount.
• The ratio between stable and unstable
lets scientists know how many halflives the fossil or rock has gone
through.
Radioactive dating can only be
used on what kind of rocks?
• Absolute age can only be determined from
igneous rocks
• Igneous rocks are those rocks made from
magma or lava.
Why can’t we find the absolute age
of sedimentary rocks?
• Sedimentary rocks are
formed from millions of
pieces of particles that
come from rocks of all
different ages.
• Radioactive dating
would find the ages of
the particles not the age
of the whole rock.
• We’d never know how
old the rock really is!
How do scientists date sedimentary
rocks?
• To date sedimentary rocks, scientists find
the age of any intrusions or extrusions that
are near the sedimentary rock.
• So, any layer of sedimentary rock above
an intrusion or extrusion, is YOUNGER!
(RELATIVE AGE!!!)
How old is the Earth?
• The oldest rocks ever found on the Earth
have been dated at about 4.0 billion years
old, but scientists believe the Earth is
really…
• 4.6 billion years old.
How do we know how old the Earth is?
• Scientist believe the
Earth and the moon
formed at the same
time.
• Scientist have dated
moon rock brought
back to Earth by
astronauts.
• Therefore the Earth is
believed to be at least
as old as the moon
rocks.