CLUES TO EARTH’S PAST 8th grade

Distant Past • How do you know what life was like in the distant past?

What does a Triceratops look like? How do we know?

•  The answer is fossils.

•  Paleontologists are scientists who study fossils. We can learn about extinct animals from their fossil remains.

Formation of Fossils •  Fossils are the remains, imprints, or traces of animals or

plants that died long ago.

•  Scientists have used fossils to determine when life first appeared, when plants and animals first lived on land, and when organisms became extinct.

•  Fossils can tell a lot about the past. (When, where and how they lived)

• Most animals and plants decay soon after they die. Some animals (scavengers) may eat and scatter the remains of dead organisms.

•  Fungi and bacteria may cause the remains to rot. In time, no trace is left.

• But some dead animals and plants do become fossils. Sometimes conditions are just right for fossils to form

• One way a dead organism can be protected is for sediment to bury the body quickly.

•  For example, a dead fish might sink to the bottom of a lake. If it is quickly buried by sediment dropped from a stream, it would be protected from scavengers.

• Over time, the fish may become a fossil in a layer of rock. But quick burial alone isn’t always enough to make a fossil.

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• Organisms with hard parts, such as bones, shells, or teeth, have a better chance of becoming fossils.

•  These hard parts are less likely to be eaten by scavengers than soft parts.

• Hard parts also decay more slowly than soft parts do. Most fossils are the hard parts of organisms.

Types of Preservation • Mineral replacement, • Carbon Films, • Coal, • Molds and casts, • Original remains, •  Trace fossils

Mineral Replacment •  The hard parts of living things, such as bones, teeth, and

shells, have tiny spaces in them.

•  When organisms are alive, the spaces can be filled with cells, blood vessels, nerves, or air. When organisms die, the soft parts decay and leave empty spaces.

•  If the hard part is buried, groundwater can seep into these spaces and deposit minerals. The result is a type of fossil..

Permineralized! • Permineralized remains are fossils in which the spaces

are filled with minerals from groundwater.

• Sometimes minerals replace all of the original hard parts of an organism.

• Scientists learn about past forms of life from remains that are permineralized

Carbon Films •  The tissues of living organisms contain carbon. Some

fossils are made only of carbon.

•  Fossils usually form when sediments bury a dead organism. As sediment builds up, heat and pressure force all the gases and liquids out of the organism. Then, just a thin layer, or film, of carbon is left.

•  It looks like a shadow of the organism’s body. A carbon film is a thin film of carbon left from an organism and preserved as a fossil.

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Coal •  Large amounts of dead plants may build up in swamps.

Over millions of years, heat and pressure change the plant material into coal, which contains large amounts of carbon.

• Coal is another kind of fossil, but it doesn’t reveal much about the past. As the coal forms, the structure of the original plant is usually lost.

Molds • When the hard part of a dead organisms fall into a soft

sediment, such as mud, then more sediment buries the object a cast mold will form. In time, pressure and cementation turn the sediment into rock.

• Cementation is when minerals from water are deposited in the spaces between sediment particles. Then water and air flow through open spaces in the rock and dissolve the organism’s hard parts.

•  This leaves a hole, or cavity, in the rock called a mold.

• A mold is a body fossil that forms when an organism decays, or dissolves, and leaves a cavity in rock.

Cast •  Later, water and minerals may enter the mold and form

new rock. This produces a copy, or cast, of the original object.

• A cast is a type of body fossil that forms when minerals fill a mold and harden into rock

Original Remains •  From time to time, conditions are just right for the soft

parts of an organism to be preserved. Insects can be trapped in amber, a tree sap that turns into a solid.

•  The amber surrounds and protects the insect’s original parts.

• Some organisms, such as mammoths, have been found preserved in frozen ground in Siberia. Some original remains have been found in natural tar deposits.

Trace Fossils • Animals that walked on Earth long ago left tracks in soft

mud. Some of those tracks have been preserved. They are trace fossils.

•  Trace fossils may be footprints, trails, burrows, or any marks that tell something about how an animal moved and lived.

•  In some cases, tracks can tell more about how an organism lived than any other type of fossil.

• Some trace fossils are burrows and trails left by worms and other animals.

•  These fossils give scientists clues about an animal’s way of life.

•  For example, examining fossil burrows can reveal how firm the sediment was that the animal lived in. From this information, scientists can learn more about the animal who dug the burrow.

Index fossils • By studying fossils, scientists can learn that species of

organisms have changed over time.

• Some species lived on Earth for a long time without changing.

• Other species changed a lot in a short time. Scientists use these organisms as index fossils.

•  Index fossils are the remains of species that lived for a short time, were numerous, and were found in many places.

• Organisms that became index fossils lived only during a specific time. Because of this, scientists can estimate the ages of rock layers based on the index fossils they contain.

• Another way to estimate when rocks formed is done by comparing time spans in which more than one fossil appears.

Fossils/ Environment • Using fossils, scientists can determine whether an area

was land or covered by ocean. If the area was covered by ocean, it might even be possible to learn how deep the water was.

•  Fossils also can give clues about the past climate of an area. For example, rocks in parts of the eastern United States contain fossils of tropical plants.

• But today the environment of this area isn’t tropical. Because of these fossils, scientists know that the climate was tropical when these plants were living.

• Crinoids are animals with many arms that usually live in warm, shallow waters.

• But fossils of crinoids have been found in deserts in parts of western and central North America. What do scientists learn from these fossils?

• When the fossil crinoids were living, a shallow sea must have covered this area of North America

•  .Fossils give clues about past life on Earth. Fossils give information about plants and animals that are now extinct.

•  They provide information about the rock layers that contain them and about the ages of the rock layers. By studying fossils, scientists learn about the climate and environment that existed when the rocks formed.

Question after outline • Why are original remains seldom found?

• Because the conditions necessary for the preservation of original remains are very rare.

•  For original remains to be preserved, an organism must be surrounded and protected by a substance like amber, ice, or tar.

Section 1 outline •  A. Paleontologists study fossils and reconstruct the appearance of animals. •  B. Fossils—remains, imprints, or traces of prehistoric organisms •  1. Fossils can form if the organism is quickly buried by sediments. •  2. Organisms with hard parts are more likely to become fossils than organisms with soft

parts. •  C. Types of preservation •  1. Fossils in which spaces inside are filled with minerals from groundwater are called •  permineralized remains. •  2. Carbon film results when a thin film or carbon residue forms a silhouette of the original •  organism; carbonized plant material becomes coal. •  3. Mold—cavity in rock left when the hard parts of an organism decay •  4. If sediments wash into a mold, they can form a cast of the original organism. •  5. Occasionally original remains are preserved in a material such as amber, ice, or tar. •  6. Trace fossils—evidence of an organism’s activities •  a. Can be footprints left in mud or sand that became stone •  b. Can be trails or burrows made by worms and other animals •  D. Index fossils—abundant, geographically widespread organisms that existed for relatively

short periods of time •  E. Fossils can reveal information about past land forms and climate.

Superposition •  The principle of superposition states that in layers of rock

that have not been disturbed, the oldest rocks are on the bottom and the rocks become younger and younger toward the top.

Rock Layers • Sediments build up, forming layers of sedimentary rocks.

The first layer to form is on the bottom. A new layer forms on top of the first one. A third layer forms on top of the second layer.

•  The bottom layer is the oldest, because it was formed first. Sometimes, the layers of rock are disturbed.

•  When layers have been turned upside down, other clues are needed to tell which rock layer is oldest.

Relative Age • Relative age is the age of something compared with the

ages of other things. Scientists figure out the relative ages of rocks by studying their places in a sequence.

•  For example, if layers of sedimentary rock have been moved by a fault, or a break in Earth’s surface, the rock layers had to be there before the fault cut through them.

• So, the relative age of the rocks is older than the relative age of the fault.

• Relative age doesn’t tell you how old the rock is in actual years.

•  The rock layer could be 10,000 years old or one million years old.

•  The relative age only tells you that the rock layer is younger than the layers below it and older than the fault cutting through it.

•  If a fossil is found in the top layer that is older than a fossil in the lower layer, it shows that the layers have been turned upside down.

•  This could have been caused by folding during mountain building

Unconformities •  Layers of rock form a record of the past. But the record

may not be whole. Layers or parts of layers might be missing.

•  These gaps in the rock layers are called unconformities. Unconformities develop when erosion removes rock layers by washing or scraping them away.

•  There are three types of unconformities.

Angular unconformity •  The unconformity that results when new layers form on

tilted layers is called an angular unconformity

• A layer of rock can be missing from a stack of sedimentary rock layers. Careful study reveals an old surface of erosion. At one time the rocks were exposed and eroded. Later, younger rocks formed above the erosion surface when sediments were deposited again. Even though all the layers are parallel, the rock record still has a gap.

Disconformity • When a rock layer is missing, this type of unconformity is

called a disconformity.

•  A disconformity also forms when a long period of time passes without any new layers of rock forming.

Nonconformity • A nonconformity occurs when metamorphic or igneous

rocks are uplifted and eroded. • Sedimentary rocks are then deposited on top of the

erosion surface. The surface between the two rock types is a nonconformity.

Matching up Rock Layers • Often, layers of rocks that are far apart can be matched

up, or correlated. One way to correlate exposed rock layer from two places that are far apart is to walk along the layer from one place to the next.

• Walking along a layer can prove it is unbroken. Layers can also be matched using fossil evidence. If the same types of fossils are found in the same rock layer in both places, it shows that the rock layer in each place is the same age and also that it is from the same deposit.

Discussion question after out line • What is the difference between a disconformity and a

nonconformity?

•  Disconformity—horizontal sedimentary rock layers are exposed, eroded, and then covered with younger sedimentary rock.

•  A nonconformity develops when sedimentary rock forms over metamorphic or igneous rock.

Section 2 outline •  A. Principle of superposition—process of reading undisturbed rock layers •  1. oldest rocks in the bottom layer •  2. younger rocks in the top layers •  B. How old something is in comparison with something else is its relative

age. •  1. The age of undisturbed rocks can be determined by examining layer

sequences. •  2. The age of disturbed rocks may have to be determined by fossils or

other clues •  C. Unconformities—gaps in rock layers •  1. Angular unconformity—rock layers are tilted and younger sediment

layers are deposited horizontally on top of the eroded and tilted layers. •  2. A layer of horizontal rock once exposed and eroded before younger

rocks formed over it is called a disconformity. •  3. Nonconformity—sedimentary rock forms over eroded metamorphic or

igneous rock. •  D. The same rock layers can be found in different locations; fossils can

be used to correlate those rock layers.

Absolute Age • Absolute age is the age, in years, of a rock or other

object.

• Scientists who study rocks, or geologists, are able to figure out the absolute age of rocks.

• Geologists use the properties of atoms in rock material to determine absolute age.

Structure of atoms • Each atom has a dense center called the nucleus, which

is surrounded by particles with a negative charge called electrons.

•  Inside the nucleus are protons, which have a positive charge, and neutrons, which have no electric charge.

•  . The number of protons determines the identity of the element.

•  The number of neutrons determines the form of the element, or isotope. For example, every atom with just one proton is a hydrogen atom.

•  Hydrogen atoms can have no neutrons, one neutron, or two neutrons. This means that there are three isotopes of hydrogen. Some isotopes break down into other isotopes, giving off a lot of energy.

Isotopes of carbon

Radioactive decay • Radioactive decay is the process in which the nucleus of

an atom breaks down.

• Beta

• Alpha

Beta Decay •  In some isotopes, a neutron breaks down into a proton

and an electron.

•  This type of radioactive decay is called beta decay, because the electron leaves as a beta particle.

•  The nucleus loses a neutron but gains a proton. • 

Alpa decay • Other isotopes give off two protons and two neutrons in

the form of an alpha particle. This is called alpha decay.

Half Life •  The half-life of an isotope is the time it takes for half of

the atoms in the isotope to decay.

• Based on its decay rate, it takes a certain period of time for one half of the parent isotope to decay to its daughter product.

• Example: the half-life of carbon-14 is 5,730 years. So, it will take 5,730 years for half of the carbon-14 atoms to change into nitrogen-14 atoms. After many half-lives, such a small amount of isotope remains that it is not measurable.

• Ex. Start with 100 g

• Present _____g---à ______g 17,190 years from present

Radiometric Dating • Decay of radioactive isotopes is like a clock keeping track

of time that has passed since rocks have formed.

• As time passes, the amount of parent isotope in a rock decreases and the amount of daughter product increases.

• Scientists can use this information to figure out the absolute age of the rock

• Radiometric dating is the process used to calculate the absolute age of rock by measuring the ratio of parent isotope to daughter product in a mineral and knowing the half-life of the parent.

Carbon dating • Carbon-14 is useful for dating bones, wood, and charcoal

up to 75,000 years old.

•  Living organisms take in carbon from the environment to build their bodies.

• Most of the carbon is carbon-12, but some is carbon-14.

•  The ratio of these two isotopes in the environment is always the same.

•  

• After the organism dies, the carbon-14 slowly decays.

• Scientists can compare the isotope ratio in the sample to the isotope ratio in the environment.

• Once scientists know the amount of carbon-14 in a sample, they can determine the age of bones, wood, or charcoal.

FYI the ratio •  These isotopes are present in the following amounts C12

- 98.89%, C13 - 1.11% and C14 - 0.00000000010%. •  Thus, one carbon 14 atom exists in nature for every

1,000,000,000,000 C12 atoms in living material.

•  http://www.c14dating.com/int.html

Other rocks for radiometric dating • Rocks that can be radiometrically dated are usually

igneous and metamorphic rocks.

• Most sedimentary rocks can’t be dated this way. • Why?

• Many sedimentary rocks are made up of particles that eroded from older rocks. Dating these pieces only gives the age of the original rocks they came from.

Oldest know rocks • Radiometric dating has been used to date the oldest rocks

on Earth. These rocks are about 3.96 billion years old.

• Scientists estimate Earth is about 4.5 billion years old. Rocks older than 3.96 billion years probably were eroded or changed by heat and pressure.

Uniformitarianism • Uniformitarianism states that Earth processes occurring

today are similar to those that occurred in the past.

•  In the 1700’s, Scottish scientist James Hutton estimated the Earth to be much older than what was thought at the current time( thousands of years).

• Hutton observed that the processes that changed the landscape around him were slow.

• He inferred that they were just as slow all through Earth’s history.

• Hutton hypothesized that it took much longer than a few thousand years to form rock layers and erode mountains. “The present is the key to the past”

Fast and Slow •  Today, scientists agree that Earth has been shaped by

two types of change.

•  There are slow, everyday processes that take place over millions of years.

•  There are also sudden, violent events such as the collision of a comet that might have caused the dinosaurs to become extinct

Discussion question after outline •  To determine the age of ancient rock, is it better to use

potassium-argon dating or carbon-14 dating?

• Why?

•  Potassium-argon dating is better because the parent isotope has a longer half-life.

Section 3 outline •  A. Absolute age—age, in years, of a rock or other object; determined by properties

of atoms •  B. Unstable isotopes break down into other isotopes and particles in the process

of radioactive decay. •  1. Beta decay—an isotope’s neutron breaks down into a proton and an electron

with the electron leaving the atom as a beta particle; a new element forms due to proton gain.

•  2. Alpha decay—an isotope gives off two protons and two neutrons as an alpha particle; a new element forms.

•  3. The time it takes for half the atoms in an isotope to decay is the isotope’s half-life.

•  C. Calculating the absolute age of a rock using the ratio of parent isotope to daughter product and the half-life of the parent is called radiometric dating.

•  1. Potassium-argon dating is used to date ancient rocks millions of years old. •  2. Carbon-14 dating is used to date bones, wood, and charcoal up to 75,000 years

old. •  3. Earth is estimated to be about 4.5 billion years old; the oldest known rocks are

about 3.96 billion years old. •  D. Uniformitarianism—Earth processes occurring today are similar to those which

occurred in the past