Chapter Introduction
Lesson 1
Fossils
Lesson 2
Relative-Age
Dating
Lesson 3
Absolute-Age
Dating
Chapter Wrap-Up
What evidence do
scientists use to
determine the ages
of rocks?
What do you think?
Before you begin, decide if you agree or
disagree with each of these statements.
As you view this presentation, see if you
change your mind about any of the
statements.
Do you agree or disagree?
1.
Fossils are pieces of dead organisms.
2. Only bones can become fossils.
3. Older rocks are always located below
younger rocks.
Do you agree or disagree?
4. Relative age means that scientists are
relatively sure of the age.
5. Absolute age means that scientists are
sure of the age.
6. Scientists use radioactive decay to
determine the ages of some rocks.
Fossils
• What are fossils and how do they
form?
• What can fossils reveal about Earth’s
past?
Fossils
• fossil
• mold
• catastrophism
• cast
• uniformitarianism
• trace fossil
• carbon film
• paleontologist
Evidence of the Distant Past
• Fossils are the preserved remains or
evidence of ancient living things.
• Fossils are some of the most obvious
clues for understanding Earth’s past.
fossil
from Latin fossilis, means “dug
up”
Evidence of the Distant Past (cont.)
• Catastrophism is the idea that
conditions and creatures on Earth
change in quick, violent events.
• Some early scientists explained Earth’s
history as a series of disastrous events
occurring over short periods of time.
Evidence of the Distant Past (cont.)
• James Hutton thought that the
processes responsible for changing the
landscape on his farm could also
shape Earth’s surface.
• Hutton’s ideas were included in a
principle called uniformitarianism,
which states that geologic processes
that occur today are similar to those
that have occurred in the past.
Evidence of the Distant Past (cont.)
• According to uniformitarianism, Earth’s
surface is constantly
being reshaped in a
steady, uniform
manner.
• Scientists know that
catastrophic events,
which can be explained
by natural processes,
do sometimes occur.
Formation of Fossils
• Some conditions increase the chances
of fossil formation.
• An organism is more likely to become a
fossil if it has hard parts—such as
shells, teeth, or bones—that do not
decay easily.
An organism is more likely to form a
fossil if it is buried quickly after it dies
because layers of sand or mud slow
decay.
Formation of Fossils (cont.)
• Not all fossils are large enough to see
without a microscope.
• Tiny fossils are called microfossils.
Formation of Fossils (cont.)
What conditions increase the
chances of fossil formation?
Types of Preservation
• Sometimes the actual remains of
organisms are preserved as fossils.
• For this to happen, an organism must
be completely enclosed in some
material over a long period of time to
prevent it from being exposed to air or
bacteria.
Types of Preservation (cont.)
• Sometimes when an organism is
buried, pressure on the organism is so
great that it drives off the gases and
liquids from an organism’s tissues,
leaving only the carbon behind.
• A carbon film is the fossilized carbon
outline of an organism or part of an
organism.
Types of Preservation (cont.)
• Replicas, or copies, of organisms can
form when the minerals in groundwater
fill in the pore spaces or replace the
tissues of dead organisms.
• This tree has had
the tissues in it
replaced by
minerals,
becoming petrified.
Scientifica/Getty Images
Types of Preservation (cont.)
petrified
Science Use turned into stone by
the replacement of tissues with
minerals
Common Use made rigid with
fear
Types of Preservation (cont.)
• A mold is the impression in a rock left
by an ancient organism.
• A mold can form when sediment
hardens around a buried organism,
making an impression in the shape of
the organism.
• A cast is a fossil copy of an organism
made when a mold of the organism is
filled with sediment or mineral deposits.
Types of Preservation (cont.)
• A trace fossil is the preserved
evidence of the activity of an organism.
• Trace fossils include tracks, footprints,
and nests and help scientists learn
about characteristics and behaviors of
animals.
Ancient Environments
• Scientists who study fossils are called
paleontologists.
• Paleontologists use the principle of
uniformitarianism to learn about ancient
organisms and the environments in
which they lived.
Ancient Environments (cont.)
Studying fossils helped scientists
reconstruct this 450-million-year-old North
American seafloor. Most of what would
become the
United States
was covered
by a shallow
sea during that
time.
Ancient Environments (cont.)
What can fossils tell us about
ancient environments?
Ancient Environments (cont.)
• Fossils also show that Earth’s climate
has warmed and cooled many times in
the past.
• Plant fossils are especially good
indicators of climate change.
Ancient Environments (cont.)
• Fossils of some plants reveal that 100
million years ago, Earth was very warm
and covered by tropical forests and
swamps.
• Fossils of some organisms, like ferns
and woolly mammoths, help scientists
learn about ancient organisms and
past environments.
Ancient Environments (cont.)
What was Earth’s climate like
when dinosaurs lived?
• The principle of
uniformitarianism
is the basis for
understanding
Earth’s past.
• Fossils can form in
many different
ways.
• Fossils help
scientists learn
about Earth’s
ancient organisms
and past
environments.
Scientifica/Getty Images
What is a fossilized outline of an
organism or part of an organism
called?
A. carbon film
B. cast
C. mold
D. trace fossil
Which term describes the
preserved evidence of the activity
of an organism?
A. cast
B. trace fossil
C. mold
D. carbon film
Which principle states that
geologic processes that occur
today are similar to those that
have occurred in the past?
A. catastrophism
B. uniformitarianism
C. paleontology
D. geology
Do you agree or disagree?
1. Fossils are pieces of dead organisms.
2. Only bones can become fossils.
Relative-Age Dating
• What does relative age mean?
• How can the positions of rock layers
be used to determine the relative ages
of rocks?
Relative-Age Dating
• relative age
• correlation
• superposition
• index fossil
• inclusion
• unconformity
I. Relative Ages of Rocks
A. Relative age is the age of rocks and
geologic features compared with other
rocks and features nearby.
B. Superposition is the principle that says
in undisturbed rock layers, the oldest
rocks are on the bottom, youngest on
top.
1. Unless some force disturbs the layers
after they were deposited
I. Relative Ages of Rocks (cont.)
How might you define your
relative age?
I. Relative Ages of Rocks (cont.)
C. Principle of original horizontality- most
rock-forming materials are deposited in
horizontal layers.
1. Even if rock layers are tilted from
being disturbed or deformed, all the
layers were originally deposited
horizontally.
I. Relative Ages of Rocks (cont.)
D. The principle of lateral continuitySediments are deposited in large,
continuous sheets in all lateral
directions.
1. The sheets, or layers, continue until
they thin out or meet a barrier.
I. Relative Ages of Rocks (cont.)
lateral
from Latin lateralis, means
“belonging to the side”
E. Geologic principles help scientists
determine the relative order of rock
layers.
I. Relative Ages of Rocks (cont.)
F. Inclusion- A piece of an older rock
that becomes part of a new rock.
1. According to the principle of
inclusions, if one rock contains pieces
of another rock, the rock containing the
pieces is younger than the pieces.
2. The principle of cross-cutting
relationships says that if one geologic
feature cuts across another feature, the
feature that it cuts across is older.
I. Relative Ages of Rocks (cont.)
What geologic principles are
used in relative-age dating?
II. Unconformities
A. Unconformity- A surface where rock
has eroded away, producing a break,
or gap, in the rock record.
1. Younger rocks have been deposited
on a layer of eroded rocks were.
2. An unconformity does not represent
a gap in the rock; it represents a gap in
time.
Draw
these in
your
notes!
II. Unconformities (cont.)
How does an unconformity
represent a gap in time?
III. Correlation
A. Correlation- Matching rocks and fossils
from separate locations.
1. Geologists use correlation to fill in
gaps in the rock record.
2. Sometimes it is possible to connect
rock layers simply by walking along rock
formations and looking for similarities.
B. Geologists use correlation to
established a historical record.
III. Correlation (cont.)
C. Index fossils represent species that
existed on Earth for a short length of
time, were abundant, and inhabited
many locations.
1. Index fossils found at different
locations, can usually indicate that the
layers are of similar age.
Correlation (cont.)
How are index fossils useful
in relative-age dating?
• Geologic
principles help
geologists learn
the relative ages
of rock layers.
• The rock record is incomplete
because some of it has eroded away.
• Geologists fill in
gaps in the rock
record by correlating
rock layers.
Which of these refers to the
dating of rocks and geologic
features by comparing them to
other rock and features nearby?
A. superposition
B. relative age
C. lateral continuity
D. unconformity
Which object represents species
that existed on Earth for a short
length of time, were abundant,
and inhabited many locations?
A. trace fossils
B. index fossils
C. molds
D. casts
Which principle says that in
undisturbed rock layers, the
oldest rocks are on the bottom?
A. inclusion
B. relative age
C. unconformity
D. superposition
Do you agree or disagree?
3. Older rocks are always located below
younger rocks.
4. Relative age means that scientists are
relatively sure of the age.
Absolute-Age Dating
• What does absolute age mean?
• How can radioactive decay be used to
date rocks?
Absolute-Age Dating
• absolute age
• isotope
• radioactive decay
• half-life
IV. Absolute Ages of Rocks
A. Absolute age refers to the numerical
age, in years, of a rock or object.
1. Helps develop accurate historical
records from many geologic formations.
2. Used since the 20th century, when
radioactivity was discovered.
Absolute Ages of Rocks (cont.)
How is absolute age different
from relative age?
V. Elements, Atoms & Radioactivity
A. All atoms of a
given element have
the same number of
protons.
B. Isotopes are
atoms of the same
element that have
different numbers of
neutrons.
C. Radioactive decay is the process by
which an unstable element naturally
changes into another element that is stable
(the release of energy from unstable atoms).
.
Atoms (cont.)
1. The rate of decay is constant for a
given isotope and measured in time
units called half-lives.
2. Half-life- is the time required for half
of the parent isotopes to decay into
daughter isotopes.
a. Half-lifes range from a few
microseconds to billions of years.
VI. Radiometric Ages
A.Because radioactive isotopes decay at
a constant rate, they can be used to
measure the age of the material that
contains them.
B. Radiometric dating is the process in
which scientists measure the ratio of
the amount of parent isotope to
daughter product
Radiometric Ages (cont.)
C. Radiocarbon, or C-14, is often used in
radiometric dating.
1.Forms in the upper atmosphere,
where it mixes with carbon-12, or C-12.
2. The ratio of C-14 to C-12 in its
tissues is identical to the ratio in the
atmosphere when the organism is alive
Radiometric Ages (cont.)
3. When an organism dies, it stops taking
in C-14 and the ratio of C-14 to C-12
changes
a. Scientist use this ratio to determine
how old the fossil is
b. C-14’s half-life is about 5730 years
4. Useful for measuring the age of the
remains that died up to about 50,000
years ago.
D. The ratio of U-235 to Pb-207 in a
mineral
VI. Radiometric Ages (cont.)
E. Uranium-235, or U-235, is often
trapped in the minerals of igneous
rocks
1. The half-life of uranium-235 is 704
million years
2. Useful for dating rocks that are very
old.
VI. Radiometric Ages (cont.)
F. The grains in many sedimentary rocks
come from a variety of weathered
rocks from different locations.
1. Isotopes in these grains not very
useful for dating b/c it records the age
of the grains, not when deposited.
Radiometric Ages (cont.)
Why are radioactive isotopes
not useful for dating
sedimentary rock?
G. For dating rocks that do not include
organic material, geologists used
different kinds of radioactive isotopes.
VI. Radiometric Ages (cont.)
Why is a radioactive isotope
with a long half-life used in
dating very old rocks?
VI. Radiometric Ages (cont.)
H. The oldest known rock is estimated to
be between 4.03 billion and 4.28 billion
years old.
1. This is how the age of the Earth was
estimated.
2. Radiometric dating of rocks from the
Moon and meteorites indicate that
Earth is 4.54 billion years old.
• When the unstable atoms of
radioactive isotopes decay, they form
new, stable isotopes.
• Because radioactive isotopes decay
at constant rates, they can be used
to determine absolute ages.
• Isotopes with long half-lives are the
most useful for dating old rocks.
Radioactive decay is the process by
which an unstable element naturally
changes into which of these?
A. an isotope
B. a neutron
C. a stable element
D. a proton
Which of these refers to the time
required for half of the parent
isotopes to decay into daughter
isotopes?
A. age
B. radiometric date
C. rate of decay
D. half-life
Isotopes are atoms of the same
element that have different
numbers of what?
A. neutrons
B. atoms
C. protons
D. parent isotopes
Do you agree or disagree?
5. Absolute age means that scientists are
sure of the age.
6. Scientists use radioactive decay to
determine the ages of some rocks.
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
Evidence from fossils,
rock layers, and
radioactivity help
scientists understand
Earth’s history and
determine the ages of
Earth’s rocks.
Lesson 1: Fossils
• The principle that Earth processes occurring today
are similar to those that occurred in Earth’s past is
called uniformitarianism.
• Dead organism are more likely to become fossils if
they have hard parts and are buried quickly after
they die.
• Fossils include carbon films, mold, casts, and trace
fossils.
• Paleontologists use clues from
fossils to learn about ancient life
and the environments ancient
organisms lived in.
Scientifica/Getty Images
Lesson 2: Relative-Age Dating
• The relative age of rock layers can be determined
using geologic principles, such as the principle of
superposition and the principle of inclusion.
• Unconformities represent time gaps in the rock
record.
• Because the rock record is
incomplete, geologists use
correlation to match rock layers.
• Index fossils are especially
useful in correlating rock layers
that are geographically far apart.
Lesson 3: Absolute-Age Dating
• Absolute age is the age in years of a rock or object.
• The radioactive decay of unstable isotopes occurs
at a constant rate, measured as half-life.
• To date a rock or object, scientists measure the
ratios of its parent and daughter isotopes.
Which of these refers to the idea
that conditions and creatures on
Earth change in quick, violent
events?
A. preservation
B. catastrophism
C. fossilization
D. uniformitarianism
An organism is more likely to
become a fossil if it has which of
these?
A. hard parts
B. soft tissue
C. small parts
D. fur
Which of these refers to a surface
where rock has eroded away,
producing a gap in the rock record?
A. correlation
B. an index fossil
C. inclusion
D. unconformity
When an index fossil is found in
rock layers at different locations,
what can geologists infer about
the layers?
A. they are much older
B. they are much younger
C. they are of similar age
D. they are exactly the same age
What are atoms of the same
element that have different
numbers of neutrons?
A. isotopes
B. protons
C. half-lives
D. elements
Which term refers to the
preserved remains or evidence of
ancient living things?
A. rocks
B. minerals
C. fossils
D. shells
Which of these is the impression
in a rock left by an ancient
organism?
A. fossil
B. carbon film
C. cast
D. mold
What term describes matching
rocks and fossils from separate
locations?
A. correlation
B. unconformity
C. comparison
D. inclusion
During which process does an
unstable element naturally
change into another element that
is stable?
A. radiometric dating
B. absolute aging
C. radioactive decay
D. radiometric aging
Of which of the following do all
atoms of a given element have
the same number?
A. neutrons
B. isotopes
C. parent isotopes
D. protons