Transcript Fossils The Fossil Record and Geologic Time Scale

13.1 Ecologists Study Relationships
Rock and Fossil Record
8th Grade Science
The Fossil Record and Geologic Time Scale
Unit Objectives
• Explain how rock layers are affected by the
folding, breaking, and uplifting of rock layers
due to plate motion
• Describe the methods used to estimate
geologic time and the age of the Earth (e.g.,
techniques used to date rocks and rock layers,
presence of fossils)
• Use rock and fossil evidence to make
inferences about the age, history, and
changing life forms and environment of the
Earth
The Fossil Record and Geologic Time Scale
Geology
• The study of Earth’s history, origin, and structure.
The Fossil Record and Geologic Time Scale
Uniformitarianism
• “The present is the key to the past”
• Principle that explains how the same geologic processes
that are shaping Earth have been at work for its entire
history.
– Specifically the Rock Cycle
• Developed by James Hutton
The Fossil Record and Geologic Time Scale
Catastrophism
• Principle that explains how catastrophes are what have
made Earth’s surface into what it is today.
– Earthquakes
– Floods
The Fossil Record and Geologic Time Scale
Modern Geology
• Scientist agree that today the view of geology is a mix
between uniformitarianism and catastrophism.
The Fossil Record and Geologic Time Scale
FIVE MAIN TYPES OF
FOSSILS
Petrified
Fossils
Molds and
Casts
Trace
Fossils
Carbon
Films
Preserved
Remains
The Fossil Record and Geologic Time Scale
Interpreting the Fossil Record
• fossils the trace or remains of an organism that lived
long ago, most commonly preserved in sedimentary rock.
• paleontology the scientific study of fossils.
• Fossils are an important source of information for finding
the relative and absolute ages of rocks.
• Fossils also provide clues to past geologic events,
climates, and the evolution of living things over time.
The Fossil Record and Geologic Time Scale
Interpreting the Fossil Record, continued
• Almost all fossils are discovered in sedimentary rock.
• The fossil record provides information about the geologic
history of Earth.
• Scientists can use this information to learn about how
environmental changes have affected living organisms.
The Fossil Record and Geologic Time Scale
• Specific conditions are needed for fossilization.
• Only a tiny percentage of living things became fossils.
The Fossil Record and Geologic Time Scale
Index fossils are another tool to determine the age of rock
layers.
• Index fossils can provide the relative age of a rock layer.
– existed only during specific spans of time
– occurred in large geographic areas
• Index fossils include fusulinids and trilobites.
The Fossil Record and Geologic Time Scale
Index Fossils
Index fossils
• Index fossil a fossil that is used to establish the age of
rock layers because it is distinct, abundant, and
widespread and existed for only a short span of geologic
time.
• Paleontologists can use index fossils to determine the
relative ages of the rock layers in which the fossils are
located.
The Fossil Record and Geologic Time Scale
Index Fossils
The Fossil Record and Geologic Time Scale
Index Fossils
Index fossils
• To be an index fossil, a fossil must be present in rocks
scattered over a large region, and it must have features
that clearly distinguish it from other fossils.
• In addition, organisms from which the fossil formed must
have lived during a short span of geologic time, and the
fossil must occur in fairly large numbers within the rock
layers.
The Fossil Record and Geologic Time Scale
Index Fossils and Absolute Age
• Scientists can use index fossils to estimate absolute
ages of specific rock layers.
• Because organisms that formed index fossils lived during
short spans of geologic time, the rock layer in which an
index fossil was discovered can be dated accurately.
• Scientists can also use index fossils to date rock layers
in separate area.
• Index fossils are used to help locate rock layers that are
likely to contain oil and natural gas deposits.
The Fossil Record and Geologic Time Scale
Radiometric dating provides an accurate way to estimate
the age of fossils.
• Relative dating estimates the time during which an
organism lived.
– It compares the placement
of fossils in layers of rock.
– Scientists infer the order in
which species existed.
The Fossil Record and Geologic Time Scale
Relative Dating
• The role that
scientist play in
determining if an
object or event is
older or younger
than another.
The Fossil Record and Geologic Time Scale
Principle of Superposition
• Principle that states that younger rocks lie above older
rocks.
– Grand Canyon – Oldest layers are the ones on the
bottom, newest on top.
The Fossil Record and Geologic Time Scale
Superposition
The Fossil Record and Geologic Time Scale
The Fossil Record and Geologic Time Scale
• Forces within the Earth have disturbed some rock
sequences.
• These forces can
push other rocks
into the sequence,
tilt or fold rock
layers, and break
sequences into
movable parts.
• They can even tilt them until they are upside down,
making it difficult for geologists to determine the
relative age of the rocks.
The Fossil Record and Geologic Time Scale
Disturbing Forces
• Sometimes certain events can affect the sequence of
layers.
The Fossil Record and Geologic Time Scale
• To help with this problem, geologists have combined data
collected from all the known undisturbed rock sequences from
different parts of the world.
• From this data geologists created the geologic
column.
The Fossil Record and Geologic Time Scale
The Fossil Record and Geologic Time Scale
Geologic Column
• A sequence of undisturbed rock
layers from all over the world that
scientist have put together and
arranged from oldest to youngest.
The Fossil Record and Geologic Time Scale
• Since scientists assume that sediment is deposited
horizontally to form layers, if the rock layers are not
horizontal, something must have happened to disturb
them.
The Fossil Record and Geologic Time Scale
The
Fossil and
Record
and
Geologic
Time
Scale that
• Folding
tilting
are
two types
of events
disturb rock layers. These events are always
younger than the rock layers they effect.
The Fossil Record and Geologic Time Scale
• Faults, intrusions, and the effects of folding and tilting can
make dating rock layers a challenge.
• Sometimes, layers of rock are missing altogether, creating
a gap in the geologic record.
The Fossil Record and Geologic Time Scale
• Missing rock layers create breaks in rock-layer
sequences called unconformities.
• An unconformity is a surface that represents a missing
part of the geologic column.
• Unconformities also represent missing time —time that
was not recorded in layers of rock.
• When geologists
find an
unconformity, they
must question
whether the
“missing layer”
was never present
or whether it was
somehow removed. This figure shows nondeposition.
The Fossil Record and Geologic Time Scale
b. Absolute dating – age is given in years, instead of
relative terms (before/after, early/late).
i. Radiometric dating is the measurement of radioactive
isotopes found in fossils and rocks, to determine age.
The half-life of an isotope is the number of years it
takes for 50% of the original sample to decay.
The Fossil Record and Geologic Time Scale
ISOTOPES
- alternate forms of an element
- some are RADIOACTIVE
The Fossil Record and Geologic Time Scale
The phenomenon of radioactivity
relates to our story about the age
of the Earth in two ways:
(1) As radioactive elements decay in
the Earth, they heat up the
surrounding rocks.
(2) Radiometric Dating
The Fossil
ethods
cont.: Record and Geologic Time Scale
b. Radioactive Dating: Dating fossils based on the amount of
radioactive material remaining in a substance over time
• Radioactive substances (unstable atoms) emit protons and
neutrons at a constant rate
• The original element (parent) is converted to a different
element (daughter)
• Since the rate of decay is constant, you can measure the
parent to daughter ratio to determine the age of the rock
• The length of time it takes for one-half of the
original amount to decay is called the elements half-life.
The Fossil Record and Geologic Time Scale
To estimate the age of a rock:
D = amount of daughter product.
P = amount of parent.
For a particular radioactive element in a rock,
determine the present ratio = D/P.
D
P
The Fossil Record and Geologic Time Scale

D
P
D
P
As time passes, the amount of parent decreases, and the
amount of daughter product increases. This provides a way
of estimating the amount of time since the "clock" got
started (i.e., since the rock solidified).
The Fossil
Record
and
Geologic
Time
Scale
Half-Lives of Radioactive
Isotpes
The Fossil Record and Geologic Time Scale
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their
number of neutrons.
neutrons
protrons
The Fossil Record and Geologic Time Scale
Radioactivity
release of high-energy particles from unstable
atoms,
maybe cancer-causing
Radioactive isotope
- an isotope that undergoes radioactive decay
- used to determine absolute age
Examples:
Uranium 238 and Carbon 14
The Fossil Record and Geologic Time Scale
Radioactive Decay
radioactive isotopes give off radioactive particles until
they become stable isotopes (new elements)
Example:
Uranium - 238
Radioactive Isotope
Radioactive Decay
Lead-206
Stable Isotope
The Fossil Record and Geologic Time Scale
Amount of time it takes 1/2 the atoms in a given sample
to go through radioactive decay is called it’s
Half - Life
Let’s look at the half - life of
Uranium 238
The Fossil Record and Geologic Time Scale
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their
number of neutrons.
– A half-life is the amount of time it takes for half of the
isotope to decay.
The Fossil Record and Geologic Time Scale
Half-Lives of Special Isotopes
Uranium 238
Lead 206
4.5 x 109 years
(4,500,000,000 years)
Carbon 14
Nitrogen 14
5.7 x 103 years
(5,700 years)
The Fossil Record and Geologic Time Scale
Unknown Radioactive Isotope
half - life = 3000 years
sample size = 200 g
Let’s complete the table below.
Half Life
Remaining Unknown
Radioactive Isotope (g)
Number of Years
0
200
0
1
100
3000
2
50
6000
25
9000
4
12.5
12,000
5
6.25
15,000
3
The Fossil
and Geologic
TimeOFScale
NOWRecord
LET’S GRAPH
THE HALF-LIFE
THE UNKNOWN
200
3
2 H-L
1 Half - Life
0 Half - Lives
4
MASS OF UNKOWN (G)
100
0
DECAY TIME (YEARS)
The Fossil Record and Geologic
Time Scale
14
Carbon
half - life = 5,700 years
sample size = 800 g
Complete the following table.
Half Life
Remaining
Radioactive C14 (g)
Number of Years
0
800
0
1
400
5,700
2
200
11,400
100
17,100
4
50
22,800
5
25
28,500
3
The Fossil
and Geologic
TimeOFScale
NOWRecord
LET’S GRAPH
THE HALF-LIFE
CARBON 14
800
2
1 Half - Life
0 Half - Lives
MASS OF C14 (G)
400
3
4
DECAY TIME (YEARS)
The Fossil Record and Geologic Time Scale
Examples of Very Old Rocks Found on Earth:
3.4 billion year old granites - South Africa.
3.7 billion year old granites - Greenland.
Oldest terrestrial rocks > 4.6 billion years.
Central Canada and Central Australia
Oldest terrestrial rocks (4.0 billion years?) are deep in
the interiors of continents.
The Fossil Record and Geologic Time Scale
KEY CONCEPT
The geologic time scale divides Earth’s history based
on major past events.
The Fossil Record and Geologic Time Scale
The geologic time scale organizes Earth’s history.
• The history of Earth is
represented in the geologic time
scale.
100
250
550
1000
2000
PRECAMBRIAN TIME
This time span makes up the
vast majority of Earth’s history.
It includes the oldest known
rocks and fossils, the origin of
eukaryotes, and the oldest
animal fossils.
Cyanobacteria
The Fossil Record and Geologic Time Scale
• Eras last tens to hundreds of millions of years.
– consist of two or more periods
– three eras: Cenozoic, Mesozoic, Paleozoic
The Fossil Record and Geologic Time Scale
• Periods last tens of millions of years.
– most commonly used units of time on time scale
– associated with rock systems.
• Epochs last several
million years.