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A TRIP THROUGH
GEOLOGIC TIME
FOSSILS
Chapter 4
Section !
FOSSILS
 Fossils are the preserved remains or traces of living things.
 Fossils:
1.
2.
Help scientists infer how Earth’s surface has changed
Are clues to what the environments were like
HOW A FOSSIL FORMS
 Most fossils form when living things die and are buried by
sediments. The sediments slowly harden into rock and
preserve the shape of the organisms.
 Fossils are normally found in sedimentary rock.
 Sedimentary rock is the type of rock that is made of hardened
sediment.
 Made of remains of dead organisms and rock particles
HOW A FOSSIL FORMS
1. An animal dies and sinks into shallow water
2. Sediment covers the animal
3. The sediment becomes rock, preserving parts of the animal.
4. Weathering and erosion eventually expose the fossil at the
surface.
T YPES OF FOSSILS
Fossil Types

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Molds and cast
Petrified fossils
Carbon films
Trace fossils
Preserved remains
MOLDS AND CASTS
 The most common fossils are molds and casts
 A mold is a hallow area in sediment in the shape of an
organism or part of an organism.
 A cast is a solid copy of the shape of an organism.
 A cast is the opposite of its mold.
MOLDS AND CASTS
PETRIFIED FOSSILS
 The term petrified means “turned to stone”
 Petrified fossils are fossils in which minerals replace all or
part of an organism.
 Over time minerals harden in the spaces in the object.
 Some of the original substance remains but the minerals have
hardened and preserved it.
PETRIFIED FOSSILS
CARBON FILM
 Carbon film is an extremely thin coating of carbon on rock.
CARBON FILM
 How they form:
1.
2.
3.
Sediment buries the organism
Some of the organism evaporates and becomes gas
Carbon is left behind leaving behind only the carbon from the
organism
TRACE FOSSILS
 Trace Fossils provide evidence of activities of ancient
organisms.
 Example:
 Footprints
TRACE FOSSILS
 There are many things that trace fossil tells us:






Animal size
Behavior of an organism
Two legs or four
Size and shape of the organism
Where the organism lived
How the organism obtained food
PRESERVED REMAINS
 Some processes preserve and organism with little or no
change.
 When organisms get stuck in tar, amber, or ice its prevents it
decay and preserves the remains
PRESERVED REMAINS
CHANGE OVER TIME
 Scientists who study fossils are called Paleontologists.
 Collect information from fossils
 Classify organisms
 Arrange organisms in order of which they lived
 Group similar organisms together
 All the information that paleontologists have gathered about
past life is called the Fossil Record.
CHANGE OVER TIME
 The Fossil Record provides evidence about the history of life
and past environments on Earth.
 The fossil record also shows that different groups of
organisms have changed over time.
FOSSILS AND PAST ENVIRONMENTS
 Fossils provide evidence about the past climate of a region.
 Example coal is found in Antarctica.
 Since coal is formed from the remains of dead plants and animals,
this means that at one time in Earth’s history, Antarctica had to be
warm enough to support that type of climate.
FOSSILS AND PAST ENVIRONMENTS
 Scientists also use fossils to learn about changes in Earth’s
surface.
 Example:
 If you find a fossil of an aquatic animal in the middle of the desert,
you can infer that at one time, there was a region of water where the
fossil was found.
THE RELATIVE AGE OF
ROCKS
Chapter 4
Section 2
AGE OF ROCKS
 The relative age of a rock is its age compared to the ages of
other rocks
 The absolute age of a rock is the number of years since
rocked formed
THE POSITION OF ROCKS
 The sediment that forms sedimentary rock are formed in flat
layers on top of the other. Over time the sediments harden.
 Geologists use the law of superposition to determine the
relative ages of sedimentary rock layers
THE POSITION OF ROCKS
 According to the law of superposition, in horizontal
sedimentar y rock layers the oldest layer is at the bottom.
Each higher layer is younger than the first.
DETERMINING RELATIVE AGE
 To determine relative age, geologists also study extrusions
and intrusions of igneous rock faults and gaps in the geologic
record.
CLUES FROM IGNEOUS ROCKS
 Lava that hardens on the surface is called extrusion
 An extrusion is always younger than the rocks below it
 Below the surface where magma pushes through bodies of
rocks and cools is called an intrusion
 An intrusion is always younger than the rock layers around and
beneath it
CLUES FROM FAULTS
 A fault is a break in Earth’s crust
 Forces inside the earth cause movement of the rock on
opposite sides of the earth
 The fault is always younger then the earth it cuts through
GAPS IN THE GEOLOGIC RECORD
 The geologic record is not always complete
 The surface where new rock layers meet a much older rock
surface beneath them is called an unconformity
 An unconformity shows where some rock layers have been lost
because of erosion
USING FOSSILS TO DATE ROCKS
 To date rock layers, geologists first give a relative age to a
layer of rock at one location.
 Certain fossils, called index fossils help geologists match rock
layers
 To be useful as an index fossil it must:
 Be widely distributed
 Occurs in many areas
USING FOSSILS TO DATE ROCKS
 Index Fossils are useful because they tell the relative ages of
the rock layers in which they occur
 If you know when an organism lived and find that organism in
rock layers it gives the a good idea on how old the rock layer
is.
RADIOACTIVE DATING
Chapter
Four
Section
Three
RADIOACTIVE DECAY
 All matter, including those in rocks, is made of tiny particles
is called atoms
 When all the atoms in a particular type of matter are the
same, the matter is an element
RADIOACTIVE DECAY
 The majority of elements are stable. But some elements
exists in forms that are unstable.
 Over time these unstable elements break down or decay by
releasing particles and energy in a process called radioactive
decay
 These unable elements are said to be radioactive
RADIOACTIVE DECAY
 During radioactive decay, the atoms of one element
break down to form atoms of another element
 Radioactive elements occur naturally in igneous
rocks. Scientists use the rate at which these
elements decay to calculate the rock’s age
RADIOACTIVE DECAY
 As a radioactive element within the igneous rock decays, it
changes into another element
 The amount of radioactive element goes down, but the
amount of the new element goes up.
RADIOACTIVE DECAY
 The rate of decay of each radioactive element is constant ---it
never changes
 This rate of decay is the element’s half -life
 This rate of decay is the element’s half -life. The half-life of a
radioactive element is the time it takes for half of the
radioactive element to decay
DETERMINING ABSOLUTE AGE
 Geologists use radioactive dating to determine the absolute
age of rocks
Steps to determine absolute age using radioactive dating:
1. Determine the amount of radioactive element
2. Compare the amount with the amount of the stable
element into which the radioactive element decay
POTASSIUM-ARGON DATING
 Scientists date rocks using Potassium -40
 Potassium-40 is useful in dating the most ancient rocks
because of its long half -life
 Half-life of 1.3 billion years
CARBON-14 DATING
 A radioactive form of Carbon is Carbon -14
 All plants and animals contain carbon atoms and small
amounts of carbon-14
 Carbon-14 is very useful in dating materials from plants and
animals that lived up to about 50,000 years ago.
CARBON-14 DATING
 Carbon-14 has a half life of only 5,730 years.
 For this reason, it cant be used to date very ancient fossil or
rocks
 The amount of carbon-14 would be to small
RADIOACTIVE DATING OF ROCK LAYERS
 Radioactive dating works well of igneous rocks, but not for
sedimentary rocks
 Rock particles in sedimentary rocks are from other rocks that are all
different ages.
 Scientists use intrusion and extrusions to date the age of
sedimentary rocks
THE GEOLOGIC TIME
SCALE
Chapter
Four
Section
Four
THE GEOLOGIC TIME SCALE
 The ear th is approximately 4.6 billion years old.
 Because the time span of Earth’s past is so great, geologists
use the geologic time scale to show Earth’s history.
 Geologic time scale is a record of the life forms and geologic
events in the Earth’s history
DIVISIONS OF GEOLOGIC TIME
 Geologists have found that over time, major changes in life
form at certain times.
 These changes mark where one unit of geologic time ends and the
next begins.
 The divisions of the geologic timescale depends on the history of life
on Earth
PRECAMBRIAN
 Geologic time begins with a long span of time called
Precambrian time.
 Precambrian Time:
 Covers 88% of Earth’s history
 Was the time at the beginning of Earth
 Beginning -4.6 billon years ago to 344 million years ago
DIVISIONS OF GEOLOGIC TIME
 Af ter Precambrian Time, the basic units of the geologic time
scale are eras and periods.
 The time after Precambrian are divided into three units of
time called eras
 Paleozoic Era
 Mesozoic Era
 Cenozoic Era
PALEOZOIC ERA
 The Paleozoic
 began about 544 million years ago-ended 245 million years ago.
 Lasted approximately 300 million years
 Paleo-means “ancient or early”
Anomalocaris,
the first great
predatory animal
of the Cambrian
MESOZOIC ERA
 Mesozoic
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Age of the Reptiles
Began 245 million years ago – ended 66.4 million years ago
Lasted 180 million years
Meso- means middle
Edmonita dinosaur –
from the cretaceous
period
CENOZOIC ERA
 Cenozoic
 Began 66 million years ago and continues through today.
 Ceno – means “recent”
 Mammals became common
during this time
DIVISIONS OF GEOLOGIC TIME
 Eras are subdivided into units of time called periods
 The names of many geologic periods come from places around
the world where geologists first described the rocks and
fossils of that period.