DO NOW: Discuss with your table: Where in this
picture would you expect to find the OLDEST rocks?
Where would you expect to find the NEWEST rocks?
What kinds of events might change that?
Chapter 3
The Rock and Fossil
Record
Section 1: Earth’s Story and
Those Who Listened
The Rock and Fossil
Record
• Geology- Study of Earth’s history
Paleontology
• The science of studying fossils to learn
about past life on Earth.
How does Earth Change?
Most changes on the
Earth are slow – erosion,
deposition, pressure,
melting.
Some changes are quick –
volcanos, earthquakes,
asteroid impacts,
Chapter 3
The Rock and Fossil
Record
Section 2: Relative Dating:
Which Came First?
Relative Dating
• Finding an estimated age
of objects on Earth by
comparing it with rocks
and fossils.
The Law of Superposition
• Superposition - Fossils/Rocks
closer to Earth’s surface will be
younger than Fossils/Rocks
found closer to Earth’s center.
Disturbing Forces
• The law of superposition does not
always hold true!
• Sometimes there are disturbing
forces that cause rocks to tilt, fold,
or get flipped upside down.
• This can be due to:
– Earthquakes
– Magma intrusions
– Folding or tilting of rock layers
Law of Crosscutting Relationships
Any rock layer that cuts across any other
rock layer is younger than the one it cuts
across.
What can cut across a rock layer?
Intrusions – magma can
Faults – cracks in
seep in between rocks and then
harden to intrusive igneous rock
rocks along which the
rocks move
E is older
than B
F, A, and C
are older
than B
Disturbed Rock Layers
1.) Fault- A break in Earth’s crust,
that force the crusts to slide
opposite of each other.
2.) Intrusion- Molten rock pushes
up between existing rock layers.
Disturbed Rock Layers
3.) Folding- When Earth’s layers
bend and buckle from internal
forces such as tectonic plates
4.) Tilting- Internal forces slant
rock layers instead of folding
them.
Disturbed Rock Layers
• All of these disturbances
are younger than the rock
layers they affect!
• The rock layers had to have
been there already for the
change to take place.
The Geologic Column
• A collection of undisturbed fossil/rock
layers from all over the world with
oldest rocks at the bottom.
• An “IDEAL” picture of what rocks
would be present if there had been no
disturbing forces.
• Geologists use it to compare to other
rock sequences and find out what’s
missing.
Index Fossils
Index fossils are fossils of organisms
that lived for a short period of time all
over the world.
If you find an index fossil in a piece of
rock, you know how old that rock is.
Common Index Fossils
Trilobites are fossils that are
found in the MAUV
LIMESTONE layer of the
Grand Canyon. Trilobites are
known to have only existed
on Earth 543 to 505 million
years ago, so the Mauv
Limestone must be 543-505
million years old!
This layer of Bryce Canyon is
called the Winsor Layer. It
contains fossils of
cephalopods which were only
on Earth from 199 to 145
million years ago. So, the
Winsor Layer is 199-145
million years old.
Missing Pieces of the Record
• Missing rock layers create gaps
in rock layer sequences called
unconformities.
• Unconformity - a break in the
geologic record created when rock
layers are eroded or when
sediment is not deposited for a
long period of time.
3 Types of Unconformities:
1. Disconformity
2. Nonconformity
3. Angular unconformity
3 Types of Unconformities
1.) Disconformity- Sequence of
parallel rock is missing! It is hard
to see but very common.
Disconformity
3 Types of Unconformities
2.) Nonconformity - Sedimentary rock
layers lie on top of an eroded surface of
a non-layered igneous or metamorphic
rock.
• Layers are on top of non-layered rock
Nonconformity
3 Types of Unconformities
3.) Angular Unconformity - exists
between horizontal rock layers and
eroded tilted or folded rock layers.
– The tilted or folded layers were
eroded before horizontal layers
formed above them.
Angular Unconformity
Angular Unconformity
Chapter 3
The Rock and Fossil
Record
Section 3: Absolute Dating:
A Measure of Time
Absolute Dating
• Absolute Dating -A very
accurate way of dating and
measuring the age of rocks
and fossils.
–Geologists do this by using
Isotopes and Radioactive
Decay
Isotopes
• Isotopes are unstable forms of
elements.
• They change, at a predictable pace, into
stable forms of the elements.
• Some isotopes change quickly and some
change slowly.
• When the isotope changes, it’s called
radioactive decay.
Radioactive Decay
• An unstable
atom turns
into a stable
atom.
• Unstable =
parent
• Stable =
daughter
Radioactive Decay
• Because radioactive decay occurs
at a steady pace, scientists can
use the relative amounts of stable
daughter and unstable parent
atoms present in an object to
determine the object’s age.
Radiometric Dating
• Using radioactive decay to
determine how old a rock is
• Scientists determine a ratio of
the unstable isotope is present
compared to how much of the
stable isotope is present.
•In other words….
–An element changes
forms over time
helping scientist
accurately date things
Half Life
The amount of time it takes for
one half of the parent isotope
to turn into daughter isotope
Newly
formed rock
= 100%
parent
After 1 half
life = 50%
parent
EXAMPLE
The element we measured has a
half life of 10,000 years.
This rock
is newly
formed
This rock is
one half life old
because half of
it has changed
to daughter.
The rock is
10,000 years
old.
This rock is
two half lives
old because ¾
of it has
changed to
daughter. The
rock is 20,000
years old.
This rock is
three half lives
old because
7/8 of it has
changed to
daughter. The
rock is 30,000
years old.
This rock is
four half lives
old because
15/16 of it has
changed to
daughter. The
rock is 40,000
years old.
Example
The half life of the element we measured
is 8 years.
0 years
8 years
16 years
24 years
32 years
• If ¼ of your sample is parent material
3/4
then ______
is daughter material.
• If ¼ of the your sample is parent
material, how many half lives has it
been through? 2
Example
The half life of the element we measured
is 2000 years.
0 years
2000 years
4000 years
6000 years
8,000 years
• If 1/16 of your sample is parent
material then how many half lives has
it been through? ___________
4
• How old is it? ______________
4 x 2000 = 8000 years
Example
If the mineral you’re studying has a half life of 12,000
years, identify the fraction of parent and daughter
isotopes and the ages of each of these rock samples.
Examples of Elements used in
Radiometric Dating
• Uranium-238 - decays to lead-206
– 1/2 life is 4.5 billion years
• Potassium 40- decays to Argon and Calcium
– 1/2 life of 100,000 years
• Carbon-14 -decays to carbon-12
– 1/2 life of 5,000 years
Chapter 3
The Rock and Fossil
Record
Section 4: Looking At Fossils
Fossils
• Any naturally preserved evidence of
life.
• Fossils can indicated changes in the
environment and can give us a time
frame for the life span of certain
plants and animals
Fossilization
Formation of fossils
1. Mummification
• In dry areas
organisms can die
and be preserved
because of low
humidity and
most bacteria
can not survive in
these places.
2. Preservation in Amber
• Amber is
hardened tree
sap
• Amber traps
insects and
preserves them
3. Tar Seeps/Tar Pits
• Thick petroleum
oozes to Earth’s
surface and traps
animals
• You can see the
fossils of ice age
animals from 10,000
to 40,000 years old,
such as a saber
tooth Tiger
Found in LaBrea Tar Pits near Los Angeles
4.) Freezing
• Low
temperatures
protect and
preserve
organisms and
keep bacteria
out
5.) Petrification
• Minerals replace an organism’s tissues.
• Petrified wood is actually stone and
fossilized.
Petrified Wood- It is all
made of minerals now
6.) Imprints
• Made in soft
mud or clay and
preserved in
sedimentary
rock.
7.) Casts
• Formed when
sediments fill an
imprint (mold) and
then cement to
form rocks with
the reverse
impression of the
organism
8.) Coprolites
• Fossilized waste materials
9.) Gastroliths
• Fossilized stones from inside an
organism’s digestive system to help
break food into smaller parts.
– The stones become gastroliths
when the organism is dead
Pleiosaur Gastroliths
Chapter 3
The Rock and Fossil
Record
Section 5 – Time Marches
On
Geologic Time Scale
• Divides Earth’s 4.6
billion year history into
time intervals (4 Eons)
Divisions of Time
• Eons Eras  Periods Epochs
Biggest
Smallest
What Determines a New Era?
• The geologic column is divided into
EONS, ERAS, PERIODS, & EPOCHS
based on major changes in:
1. Earth’s surface
2. Climate
3. Type of organisms
The Phanerozoic Eon is all of
the time that Earth has
been here – 4.6B years. It’s
divided into:
1. Paleozoic Era – 251M-542M years ago
2. Mesozoic Era – 65M-251M years ago
3. Cenozoic Era – 65Myears ago to today
542MYA
251MYA
Paleozoic
65MYA
Mesozoic
TODAY
Cenozoic
Paleozoic Era
• 542 to 251 million years ago
• Begins - with dramatic increase in
plant and animal species.
• Ends - with landform called
Pangaea and mass extinction of
90% marine species and 70% land
species.
Pangea
Mesozoic Era
• 251 to 65.5 million years ago
• Known as “Age of Reptiles”
– Ex. Dinosaurs
• Pangaea breaks up
Cenozoic Era
• 65.5 million years ago to
present
• “Age of Mammals”
• Continents move to present
day positions.