• Before we can discuss time periods of Earth, we must discuss how we know there are different time periods….distinct times where the Earth had different climates, different land masses, different plants and different animals.

Chapter 19, Clues to Earth’s History

What types of things do we study to make discoveries about the

Earth’s past?

Pictures from: http://www.mt-vernon.k12.oh.us/High_School/Depts/Science/frye/fossils/fossil_pictures.htm

Colonial Coral

Knightia

Fossils• Fossil: A fossil

is the remains or evidence of a living thing.

• A fossil can be the bone of an organism or the print of a shell. A fossil might be a tunnel left by a worm.

• The most common fossils are bones, shells, pollen grains and seeds.

Pictures from:http://www.mtvernon.k12.oh.us/High_School/Depts/Science/frye/fossils/fossil_pictures.htm

Trilobite

Crinoid

Are fossils found completely intact?

• Most fossils are not complete. Why?

• Only the hard parts of dead plants or animals become fossils. The soft tissue will decay or will be eaten before it can fossilize.

• For most fossils to form, they must be buried in sediment soon after the organism dies.

Sediment—What is it?

• Sediment: Particles of rock, shell, and other materials that are broken down over time.

• Things that are buried quickly by sediment are best preserved because they do not decay.

• Plants and animals that live near water were preserved more easily than organisms that are in other places since they are more easily covered by sediment.

• Most fossils are found in sedimentary rock.

Types of Rock

• Sedimentary Rock: rock formed by the compacting or cementing of sediments or other non-igneous processes at the Earth surface.

• Igneous rock: Rock formed by cooled molten lava or magma.

• Metamorphic Rock: Rock that is changed from igneous or sedimentary rock by heat, pressure and/or chemical reactions.

http://www.gpc.edu/~pgore/geology/geo101/igneous.htm

Obsidian—a type of igneous rock

What type of rock is most likely to have fossils in it?

• Sedimentary rock is the main type of rock to have fossils.

• The high temperatures in igneous rock would tend to burn up the remains of living things.

• Likewise, the high temperatures, pressure and chemical in metamorphic rock would eat away at the remains, leaving nothing to fossilize.

Types of fossils

• Petrification: The process by which once-living material is replaced by minerals, turning it to stone. (see page 590)

• Imprints: A fossil formed when a thin object leaves an impression in soft mud that hardens.

Types of fossils continued…• Mold: What forms

when the soft parts of an organism decay and the hard parts are dissolved by chemicals. This fossil shows only the shape of the organism.

• Cast: A fossil that occurs when a mold is filled in with minerals in the sediment. This fossil will have the same shape as the organism.

Under what circumstances can a whole organism be

preserved?• Freezing

• When encased in amber

• When trapped in tar pits.

This is a 95 million year old fossil insect in amber from the Raritan formation of New Jersey (This very bug may have bitten a dinosaur).  This is an example of an organism preserved as UNALTERED ORIGINAL  MATERIAL

http://www.clas.ufl.edu/users/pciesiel/gly3603c/fossils.html

• Trace Fossils

A fossil that tells much about an animal without showing any part of the animal.

http://www.clas.ufl.edu/users/pciesiel/gly3603c/fossils.html

This image shows a cast of a Dinosaur footprint from Dinosaur State Park, in Connecticut. This kind of fossil, evidence of the activity or behavior rather than the actual body, is known as a TRACE FOSSIL (G. Mead).

19.2 History of Fossils

• Law of superposition: Law that states that in undisturbed sedimentary rocks each layer is older than the one above it and younger than the one below it.

• (see p. 595)• This law was first

proposed in 1700 by James Hutton

How do we use the Law of Superposition to study the

Earth’s past?

Is the spiral shell fossil older or younger than the cone-shaped shell fossil?

How do you know?

Index Fossil:Fossil of an organism that existed on Earth for only a short period of time and thus can be used for scientists to determine therelative age of a rock.

*When the English Channel was dug, various fossils were found on both sides. Based on these fossils,scientists determined that the English rock layers were the same age as the French layers.

Which 3 fossils helped them reach this conclusion?

Which one could not be used?

See picture on page 597

12342

England France

Can the layers of rock get out of order?

• Certainly. See page 598.• Unconformity: eroded rock

surface that is much older than the younger rock layers above it. This occurs when the original layers are bent, twisted, or folded.

• Fault: A break or crack along which rocks move. Because faults occur after rock has formed, the rock layers are always older than the faults they contain.

Other Rock disturbances

• Intrusion: Igneous rock formed by magna that cools beneath the Earth’s crust.

Intrusions are younger than the rock it passes through.

• Extrusions: Igneous rocks formed from lava that cools at the Earth’s surface.

Extrusions are younger than the rock layers beneath them.

• Both intrusions and extrusions disturb the natural order “superposition” theory.

Dating of materials

• Originally, scientists tried to date the layers by saying that a steady amount of sedimentary rock was deposited each year.

• If a scientist knew the depth of sedimentary rock, they could simply divide the depth by the total deposited each year to find the age of the rock.

• What is wrong with this theory?

Radioactive Dating

• Any type of radioactive element has a nucleus that will break down, or decay, over time.

• The amount of decay is a constant that depends on the type of element.

• While the amount of sediment deposited each year is not a constant, radioactive decay is a constant.

What is a half-life?

• All radioactive elements have a thing known as a half-life.

• The half-life of element is: The amount of time needed for ½ of the radioactive element to decay into another element.

• Carbon-14 decays into Nitrogen.

• If certain radioactive elements are present in a fossil, radioactive dating can be used to judge the age of the fossil.

• All living things have Carbon in them.

Half-Life Examples:

• Example 1: A fossil is found. If the radioactive element in it has gone through 3 half-lifes, how much of the original substance is left?

First ½ life:

Second ½ life:

Third ½ life:

•Example 2: A 10 gram fossil is found to have 5 grams of Carbon and 5 grams of Nitrogen in it. (So the amount of the original element = the amount of the decay element). How much of the fossil has decayed? •Based on the chart below, how old is the fossil?

• Example 3: A 20-gram fossil is found to have 5 grams of Carbon-14 and 15 grams of Nitrogen. A). What is the ratio of the Nitrogen to

the Carbon?

B). How much (fraction) of the original material is there?

C). How many ½ lives have occurred?

D). What is the age of the fossil?

• Radioactive elements do not all have the same ½ lives. (See page 603).

• Carbon-14 is a radioactive element that is in all living things, but it cannot be used to date all fossils. Why?

• Dinosaurs lived about 200 million years ago. If a dinosaur skeleton is found, why could carbon dating not be used to determine how old the skeleton is?

• The element used to date the oldest things will be the one with the longest ½ life. What is that?

• Radioactive dating of some Earth rocks from South Africa have shown the rocks to be about 4 billion years old. Moon rocks have been found to be between 4 and 4.6 billion years old. However, it is believed that the Earth and moon formed at the same time.

• Thinker: At a dig by a river, the remains of both a dinosaur and a human are found. Some people claim that this is proof the man and dinosaurs lived together. How could you prove or disprove this?

• Before we can discuss time periods of Earth, we must discuss how we know there are different time periods….distinct times where the Earth had different climates, different land masses, different plants and different animals.

Chapter 19, Clues to Earth’s History

What types of things do we study to make discoveries about the

Earth’s past?

Pictures from: http://www.mt-vernon.k12.oh.us/High_School/Depts/Science/frye/fossils/fossil_pictures.htm

Colonial Coral

Knightia

Fossils• Fossil: A fossil

is the ______________

_____of a living thing.• A fossil can be

the _____ of an organism or the _____ of a shell. A fossil might be a tunnel left by a worm.

• The most common fossils are bones, shells, pollen grains and seeds.

Pictures from:http://www.mtvernon.k12.oh.us/High_School/Depts/Science/frye/fossils/fossil_pictures.htm

Trilobite

Crinoid

Are fossils found completely intact?

• Most fossils are not complete. Why?

• Only the _____________ of dead plants or animals become fossils. The soft tissue will ___________ or will be _________ before it can fossilize.

• For most fossils to form, they must be _____________________ soon after the organism dies.

Sediment—What is it?• Sediment: Particles of__________

______________________________

materials that are broken down over time.

• Things that are buried quickly by sediment are best preserved because they do____________.

• Plants and animals that live near water were preserved more easily than organisms that are in other places since they are more easily covered by sediment.

• Most fossils are found in ______________ rock.

Types of Rock

• Sedimentary Rock: rock formed by the ________ or

______________ sediments or other non-igneous processes at the Earth’s surface.

• Igneous rock: Rock formed by _________ _____ or ___________.

• Metamorphic Rock: Rock that is changed from igneous or sedimentary rock by_____________________________________________________

http://www.gpc.edu/~pgore/geology/geo101/igneous.htm

Obsidian—a type of igneous rock

What type of rock is most likely to have fossils in it?

• _________________ rock is the main type of rock to have fossils.

• The high temperatures in igneous rock would tend to _______________ the remains of living things.

• Likewise, the high temperatures, pressure and chemical in metamorphic rock would___________________________, leaving nothing to fossilize.

Types of fossils

• Petrification: The process by which once-living material is replaced by ___________, turning it to _______. (see page 590)

• Imprints: A fossil formed when a _____

___________ leaves an impression in soft ______ that hardens.

Types of fossils continued…

• Mold: What forms when the soft parts of an organism _________ and the hard parts are _________ by chemicals. This fossil shows only the _______ of the organism.

• Cast: A fossil that occurs when a mold is filled in with ____________ in the sediment. This fossil will have the same _______ as the organism.

Under what circumstances can a whole organism be

preserved?• ________

• When encased in ________

• When trapped in ________

This is a 95 million year old fossil insect in amber from the Raritan formation of New Jersey (This very bug may have bitten a dinosaur).  This is an example of an organism preserved as UNALTERED ORIGINAL  MATERIAL

http://www.clas.ufl.edu/users/pciesiel/gly3603c/fossils.html

• Trace Fossils

A fossil that tells much about an animal ___________________________________

http://www.clas.ufl.edu/users/pciesiel/gly3603c/fossils.html

This image shows a cast of a Dinosaur footprint from Dinosaur State Park, in Connecticut. This kind of fossil, evidence of the activity or behavior rather than the actual body, is known as a TRACE FOSSIL (G. Mead).

19.2 History of Fossils

• Law of superposition: Law that states that in undisturbed sedimentary rocks each layer is ______ than the one above it and _______ than the one below it.

• (see p. 595)• This law was first

proposed in 1700 by James Hutton

How do we use the Law of Superposition to study the

Earth’s past?

Is the spiral shell fossil older or younger than the cone-shaped shell fossil?

How do you know?

.

*When the English Channel was dug, various fossils were found on both sides. Based on these fossils, scientists determined that the English rock layers were the same age as the French layers.

Which 3 fossils helped them reach this conclusion?

Which one could not be used?

1

2

3

4

2

See picture on page 597

Index Fossil:

Fossil of an organism that existed on Earth for only a ____________________ and thus can be used for scientists to determine the relative age of a rock

1

2

3

4, 2

Can the layers of rock get out of order?

• Certainly. See page 598.• Unconformity: __________ rock

surface that is much____________ than the younger rock layers above it. This occurs when the original layers are___________________

• Fault: A ____________________ along which rocks move. Because faults occur after rock has formed, the rock layers are always __________ than the faults they contain.

Other Rock disturbances

• Intrusion: Igneous rock formed by magna that cools ______________ the Earth’s crust.

Intrusions are _____________ than the rock it passes through.

• Extrusions: Igneous rocks formed from lava that cools at the Earth’s ___________________.

Extrusions are _____________ than the rock layers beneath them.

• Both intrusions and extrusions disturb the natural order “superposition” theory.

Dating of materials

• Originally, scientists tried to date the layers by saying that a steady amount of sedimentary rock was deposited each year.

• If a scientist knew the depth of sedimentary rock, they could simply divide the depth by the total deposited each year to find the age of the rock.

• What is wrong with this theory?

Radioactive Dating

• Any type of radioactive element has a nucleus that will break down, or decay, over time.

• The amount of decay is a constant that depends on the type of element.

• While the amount of sediment deposited each year is not a constant, radioactive decay is a constant.

What is a half-life?

• All radioactive elements have a thing known as a half-life.

• The half-life of element is: The amount of time needed for ____________________________

to ___________ into another element.

• Carbon-14 decays into ________________.

• If certain radioactive elements are present in a fossil, radioactive dating can be used to judge the age of the fossil.

• All living things have

_______________ in them.

Half-Life Examples:

• Example 1: A fossil is found. If the radioactive element in it has gone through 3 half-lifes, how much of the original substance is left?

First ½ life:

Second ½ life:

Third ½ life:

•Example 2: A 10 gram fossil is found to have 5 grams of Carbon and 5 grams of Nitrogen in it. (So the amount of the original element = the amount of the decay element). How much of the fossil has decayed? •Based on the chart below, how old is the fossil?

• Example 3: A 20-gram fossil is found to have 5 grams of Carbon-14 and 15 grams of Nitrogen. A). What is the ratio of the Nitrogen to

the Carbon?

B). How much (fraction) of the original material is there?

C). How many ½ lives have occurred?

D). What is the age of the fossil?

• Radioactive elements do not all have the same ½ lives. (See page 603).

• Carbon-14 is a radioactive element that is in all living things, but it cannot be used to date all fossils. Why?

• Dinosaurs lived about 200 million years ago. If a dinosaur skeleton is found, why could carbon dating not be used to determine how old the skeleton is?

• The element used to date the oldest things will be the one with the longest ½ life. What is that?

• Radioactive dating of some Earth rocks from South Africa have shown the rocks to be about 4 billion years old. Moon rocks have been found to be between 4 and 4.6 billion years old. However, it is believed that the Earth and moon formed at the same time.

• Thinker: At a dig by a river, the remains of both a dinosaur and a human are found. Some people claim that this is proof the man and dinosaurs lived together. How could you prove or disprove this?