Section 6

Earth’s History
The Relative Age of Rocks
What does the fossil record show?
What does the fossil record show?
 Evidence of Gradual Change
 Organisms have changed significantly over time.
 In rocks more than 1 billion years old, only fossils of single-celled
organisms are found.
At 550 million years old, fossils of simple, multicellular animals.
At 500 million years ago, ancient fish without jawbones surface.
At 400 million years ago, fish with jaws are found.
At 350 million years ago, reptiles
At 300 million years ago, mammals
At 230 million years ago, and birds.
At 150 million years ago, as the rocks become more and more
recent, the fossils look like the animals we observe today.
Fossils . Mold
 PALAEOZOIC: late Ordovician
 Common name: graptolites
Petrified Fossil
Late middle Cambrian
Common name: trilobite
Carbon Film
 Extremely thin coating of carbon on rock.
 When sediment buries an organism, some gases
escape from the sediment , leaving carbon
Index Fossils
 Trilobite. Cambrian Period
 A hollow area in sediment in the shape of an organism is
a …………………
 A solid copy of the shape of an organism is a ……….
 What type of fossils provide evidence of the activities of
ancient organisms……………………..
 Which type of substance is NOT able to preserve entire
a) ice
b) water
Relative Age of Rocks
 The age of the rock is compared to the ages of
other rocks.
 Ex:
 Sandstone in an area is older than limestone.
Absolute Age
 The absolute age of an igneous rock can
be best determine by the sizes of crystals
found in the upper and lower parts of the
Absolute Age of Rocks
 It is number of years that passed since the rock
 Geologists often use both absolute and relative ages
to determine the age of a rock.
 Geologists use the law of superposition to determine
the relative ages of sedimentary rock layers.
 For example: The sandstone is 300 million years old.
Two different Trash Pits
What can we say and learn from these
 Relative age of trash layers – Because of the shape of the
pits the oldest layers of trash occur below younger layers . The
inhabitants of the area likely deposited the trash by throwing it
in from the top, eventually filling the pits.
 Thus the relative age of the trash layers is, in order from
youngest to oldest.:
 5.25" Disk Layer -Youngest
 Al Cans Layer
 Tin Cans Layer
 Ceramic Cups Layer
 Stone Tools Layer - Oldest
Rock Layers
 Fossils are most often found in layers of
sedimentary rock.
 Geologists use the law of superposition to
determine the relative ages of sedimentary rock
 According to the law of superposition, in
undisturbed horizontal sedimentary rock layers
the oldest layers is at the bottom.
 Each higher layer is younger than the layers below
Law of Superposition
 Magma that reaches the surface is called lava.
 Lava that hardens on the surface and forms
igneous rocks is called an extrusion.
 Magma may push in bodies of rock below the
surface. There the magma cools and hardens into
a mass of igneous rock called an intrusion.
 A mass of igneous rock below the Earth’s surface
is called intrusion.
 A fault is a break in Earth’s crust. Forces inside
Earth cause movement on the rock on the
opposite sides of a fault.
 A fault is always younger than the rock it cuts
 To determine the age of a fault, geologists find
the relative age of the youngest cut by the fault.
 San Andres Fault
Index Fossil
 They help geologists match rock layers .
 Index fossils tell the relative ages of the rock
layers in which they occur.
 To be an index fossil, the fossil must be widely
distributed an represent an organism that existed
for a geologically short period of time.
Index Fossils
Index Fossils
 What is it?
 Any animal or plant preserved in the rock record of the Earth that is
characteristic of a particular span of geologic time or environment.
A useful index fossil must be distinctive or easily recognizable, abundant,
and have a wide geographic distribution and a short range through time.
Index fossils are the basis for defining boundaries in the geologic time
scale and for the correlation of strata.
In marine strata, index fossils that are commonly used include the
single-celled Protista with hard body parts and larger forms such as
In terrestrial sediments of the Cenozoic Era, which began about 65.5
million years ago, mammals are widely used to date deposits. All of these
animal forms have hard body parts, such as shells, bones, and teeth, and
evolved rapidly.
Index Fossils
How can rock layers change?
 Most of Earth’s geologic record has been lost to
 Gaps in the geologic record and folding can
change the position in which rock layers appear.
 Because of erosion most of the geologic record of
sedimentary rock layers has been lost.
Gaps in the Geologic Record
 When rock layers erode away, an older layer
surface may be exposed. Then deposition begins
again building new rock layers.
 It is a gap in the geologic record. It shows where
rock layers have been lost due to erosion.
 Sometimes forces inside Earth fold rock layers so
much that the layers are turned over completely.
 In this case, the youngest rock layers must be on
the bottom.
 Two ways of rock layers can change are folding
and unconformity.
 Folded rock layers are not longer flat and ,
sometimes older layers end up on top.
Geologic Folding
Law of superposition
 According to the law of superposition ,in
horizontal sedimentary rock layers the oldest
layer is at the bottom and the youngest layer is at
the top.
Law of superposition
Law of superposition
Radioactive Decay
 Most elements usually do not change, but some
elements can break down, or decay, over time.
These elements release particles and energy in a
process called radioactive decay.
 These elements are RADIOACTIVE.
 During Radioactive decay, the atoms of one
element break down to form atoms of another
Radioactive Decay
Half- Life
 Half- life of a radioactive element is the time it
takes for half of the radioactive atoms to decay.
Radioactive Dating
 Radioactive elements occur naturally in igneous
 Scientists use the rate at which these elements decay
to calculate the rock’s age.
 As a radioactive element within the igneous rock
decays, it changes into another element. So the
composition of the rock changes over time.
 The amount of the radioactive element DECREASES
and the amount of the new element INCREASES.
Radioactive Dating
Determinig Absolute Ages
 In radioactive dating, scientists first determine
the amount of a radioactive element in a rock.
Then they compare that amount with the amount
of the stable element into which the radioactive
element decays.
Potassium-Argon Dating
 Potassium-40 is an element scientists use to date
 Potassium-40 decays to stable Argon-40 and has a
half-life of 1.3 billion years.
 Potassium-40 is useful in dating the most ancient
rocks because of its long half-life.
Potassium-Argon Dating
Carbon -14 Dating
 Carbon-14 is a radioactive form of carbon.
 All plants and animals have Carbon and some
 After an organism dies, the Carbon-14 in the
organisms’ body decays.
 It changes to Nitrogen-14.
Carbon -14 Dating
 To determine the age of a sample, scientists
measure the amount of Carbon-14 that is left in
the organism’s remains.
 Carbon-14 has been used to date fossils such as
frozen mammoths and the skeletons of
prehistoric humans.
 Carbon-14 half-life is only 5,730 years.
Carbon -14 Dating
Radioactive Dating
 To determine Earth’s age scientists use
radioactive dating and evidence from rocks on
Earth’s and the moon.
 Using fossils, evidence from rock layers, and
radioactive dating, scientists can infer about
events in Earth’s history.
Geologic Time Scale
 It is a record of the geologic events and the evolution
of life forms as shown in the fossil record.
 Scientists have placed Earth’s rocks in order by
relative age.
 Then, they divided geologic time into parts, based on
time in Earth’s history when there were major
changes in life forms.
Geologic Time Scale
 Precambrian time is the earliest part of the Geologic
This part is the longest of the geologic time ended 542
million years ago.
Scientists divided the time between Precambrian time
and the present time in three eras:
Paleozoic ERA
Mesozoic ERA
Cenozoic ERA
 Velociraptor
Geologic Time Scale
 The Paleozoic ERA is made up of six periods.
 The Mesozoic ERA is made up of three periods.
 The Cenozoic ERA is made up of three periods.
 The Quaternary Period is the last and continues to
the present time.
 Tyrannosaurus Rex
 Scientists use the principle of uniformitarianism
to explain Earth’s past.
 Allosaurus
 This principle states that geologic processes that
operate today are the same ones that operated in
the past.
 Natural processes such as weathering, erosion,
and plate tectonics have shaped Earth’s surface .
 The distribution of land and water on Earth’s
surface has changed over time.
 Theropod ( beast-footed )