HS Evidence About Earth’sPast

Dana Desonie, Ph.D. (DanaD)

Say Thanks to the AuthorsClick http://www.ck12.org/saythanks

(No sign in required)

To access a customizable version of this book, as well as otherinteractive content, visit www.ck12.org

CK-12 Foundation is a non-profit organization with a mission toreduce the cost of textbook materials for the K-12 market bothin the U.S. and worldwide. Using an open-content, web-basedcollaborative model termed the FlexBook®, CK-12 intends topioneer the generation and distribution of high-quality educationalcontent that will serve both as core text as well as provide anadaptive environment for learning, powered through the FlexBookPlatform®.

Copyright © 2013 CK-12 Foundation, www.ck12.org

The names “CK-12” and “CK12” and associated logos and theterms “FlexBook®” and “FlexBook Platform®” (collectively“CK-12 Marks”) are trademarks and service marks of CK-12Foundation and are protected by federal, state, and internationallaws.

Any form of reproduction of this book in any format or medium,in whole or in sections must include the referral attribution linkhttp://www.ck12.org/saythanks (placed in a visible location) inaddition to the following terms.

Except as otherwise noted, all CK-12 Content (including CK-12Curriculum Material) is made available to Users in accordancewith the Creative Commons Attribution-Non-Commercial 3.0Unported (CC BY-NC 3.0) License (http://creativecommons.org/licenses/by-nc/3.0/), as amended and updated by Creative Com-mons from time to time (the “CC License”), which is incorporatedherein by this reference.

Complete terms can be found at http://www.ck12.org/terms.

Printed: September 22, 2013

AUTHORDana Desonie, Ph.D. (DanaD)

CONTRIBUTORKurt Rosenkrantz, (KurtR)

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

CHAPTER 1 HS Evidence About Earth’sPast

CHAPTER OUTLINE

1.1 Fossils

1.2 Relative Ages of Rocks

1.3 Absolute Ages of Rocks

1.4 References

Identifying locations where abundant and interesting fossils are found is a paleontologist’s first step in unravelingEarth history. First, rocks of the right age need to be identified. Desert areas are better for fossil hunting becausethe rocks are better exposed and weathering processes have not degraded the rocks or their fossils. But there are alot of desert areas in the world and it is not possible to search them all on foot. Paleontologists now use satellitesto locate good fossil sites. This Landsat image of Mongolia’s Gobi Desert allowed researchers to locate exposedsedimentary rocks. While the true-color image gave a broad look at the area of interest, the false-color image shownhere elucidates so much more. Different colors highlight vegetation and individual rock types. By studying thedifferent colors scientists can single out the area they think is most likely to produce fossils. In this chapter, you willlearn about some of the ways that scientists study the history of Earth and how they use clues from rocks and fossilsto piece together pictures of how the Earth has changed over billions of years.

1

1.1. Fossils www.ck12.org

1.1 Fossils

Lesson Objectives

• Explain why it is rare for an organism to be preserved as a fossil.• Distinguish between body fossils and trace fossils.• Describe five types of fossilization.• Explain the importance of index fossils, and give several examples.

Vocabulary

• amber• body fossil• cast• fossilization• index fossil• microfossil• mold• permineralization• trace fossil

Introduction

Throughout human history, people have discovered fossils and wondered what they are and what they represent. Inancient times, fossils inspired legends of monsters and other strange creatures. The Chinese writer, Chang Qu, 2,000years ago reported the discovery of “dragon bones,” which were probably dinosaur fossils (Figure 1.1). Look at thetwo photos below and try to trace the origin of the creature on the left.

FIGURE 1.1The griffin, a mythical creature with alion’s body and an eagle’s head and wings(left), was probably based on skeletonsof Protoceratops (right) that were discov-ered by nomads in Central Asia.

2

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.2Ammonites (left) and elephant skull(right).

Ancient Greeks named ammonites after the ram god Ammon since they look like the coiled horns of a ram. Legendsof the Cyclops may be based on fossilized elephant skulls found in Crete and other Mediterranean islands(Figure1.2). Can you see why?

Many of the real creatures whose bones became fossilized were no less marvelous than the mythical creatures theyinspired (Figure 1.3).

FIGURE 1.3(a) The giant pterosaur Quetzalcoatlus had a wingspan of up to 12 meters (39 feet). (b) Argentinosaurus hadan estimated weight of 80,000 kg, equal to the weight of seven elephants! Other fossils, such as the trilobiteKolihapeltis ch (c) impress us with their bizarre forms. These suture marks on an ammonite fossil (d) display adelicate beauty.

How Fossils Form

A fossil is any remains or traces of an ancient organism. Fossils include body fossils, left behind when the softparts have decayed away, and trace fossils, such as burrows, tracks, or fossilized coprolites (feces) (Figure 1.4).Collections of fossils that are found together are known as fossil assemblages.

3

1.1. Fossils www.ck12.org

FIGURE 1.4Coprolite from a meat-eating dinosaur.

The process of a once-living organism becoming a fossil is called fossilization. Fossilization is very rare: Only atiny percentage of the organisms that have ever lived become fossils.

Why do you think only a tiny percentage of living organisms become fossils after death? Think about an antelopethat dies on the African plain (Figure 1.5).

FIGURE 1.5Hyenas eating an antelope. Will the ante-lope in this photo become a fossil?

Most of its body is eaten by hyenas and other scavengers and the remaining flesh is devoured by insects and bacteria.Only bones are left behind. As the years go by, the bones are scattered and fragmented into small pieces, eventuallyturning into dust. The remaining nutrients return to the soil. This antelope will not be preserved as a fossil.

Is it more likely that a marine organism will become a fossil? When clams, oysters, and other shellfish die, the softparts quickly decay, and the shells are scattered. In shallow water, wave action grinds them into sand-sized pieces.The shells are also attacked by worms, sponges, and other animals (Figure 1.6).

How about a soft bodied organism? Will a creature without hard shells or bones become a fossil? There is virtuallyno fossil record of soft bodied organisms such as jellyfish, worms, or slugs. Insects, which are by far the mostcommon land animals, are only rarely found as fossils (Figure 1.7).

Despite these problems, there is a rich fossil record. How does an organism become fossilized?

4

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.6Shell that has been attacked by a boring sponge.

FIGURE 1.7A rare insect fossil.

Usually it’s only the hard parts that are fossilized. The fossil record consists almost entirely of the shells, bones,or other hard parts of animals. Mammal teeth are much more resistant than other bones, so a large portion of themammal fossil record consists of teeth. The shells of marine creatures are common also.

Quick burial is essential because most decay and fragmentation occurs at the surface. Marine animals that die near ariver delta may be rapidly buried by river sediments. A storm at sea may shift sediment on the ocean floor, coveringa body and helping to preserve its skeletal remains (Figure 1.8).

Quick burial is rare on land, so fossils of land animals and plants are less common than marine fossils. Landorganisms can be buried by mudslides, volcanic ash, or covered by sand in a sandstorm (Figure 1.9). Skeletons canbe covered by mud in lakes, swamps, or bogs.

Unusual circumstances may lead to the preservation of a variety of fossils, as at the La Brea Tar Pits in Los Angeles,California (Figure 1.10).

In spite of the difficulties of preservation, billions of fossils have been discovered, examined, and identified bythousands of scientists. The fossil record is our best clue to the history of life on Earth, and an important indicatorof past climates and geological conditions as well.

5

1.1. Fossils www.ck12.org

FIGURE 1.8This fish was quickly buried in sediment tobecome a fossil.

FIGURE 1.9People buried by the extremely hot eruption of ash and gases at Mt.Vesuvius in 79 AD.

FIGURE 1.10Although the animals trapped in the TaBrea Tar Pits probably suffered a slow,miserable death, their bones were pre-served perfectly by the sticky tar.

Exceptional Preservation

Some rock beds contain exceptional fossils or fossil assemblages. Two of the most famous examples of soft organismpreservation are from the 505 million-year-old Burgess Shale in Canada (Figure 1.11). The 145 million-year-old

6

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

Solnhofen Limestone in Germany has fossils of soft body parts that are not normally preserved (Figure 1.11).

FIGURE 1.11(a) The Burgess shale contains soft-bodied fossils. (b) Anomalocaris, mean-ing “abnormal shrimp” is now extinct. Theimage is of a fossil. (c) A brittle starfossil. (d) The famous Archeopteryx fossilfrom the Solnhofen Limestone has dis-tinct feathers and was one of the earliestbirds.

Types of Fossilization

Most fossils are preserved by one of five processes outlined below (Figure 1.12):

FIGURE 1.12Five types of fossils: (a) Insect preservedin amber, (b) petrified wood (permineral-ization), (c) cast and mold of a clam shell,(d) pyritized ammonite, and (e) compres-sion fossil of a fern.

Preserved Remains

Most uncommon is the preservation of soft-tissue original material. Insects have been preserved perfectly in amber,which is ancient tree sap. Mammoths and a Neanderthal hunter were frozen in glaciers, allowing scientists the rareopportunity to examine their skin, hair, and organs. Scientists collect DNA from these remains and compare theDNA sequences to those of modern counterparts.

7

1.1. Fossils www.ck12.org

Permineralization

The most common method of fossilization is permineralization. After a bone, wood fragment, or shell is buriedin sediment, mineral-rich water moves through the sediment. This water deposits minerals into empty spaces andproduces a fossil. Fossil dinosaur bones, petrified wood, and many marine fossils were formed by permineralization.

Molds and Casts

When the original bone or shell dissolves and leaves behind an empty space in the shape of the material, thedepression is called a mold. The space is later filled with other sediments to form a matching cast within themold that is the shape of the original organism or part. Many mollusks (clams, snails, octopi, and squid) are foundas molds and casts because their shells dissolve easily.

Replacement

The original shell or bone dissolves and is replaced by a different mineral. For example, calcite shells may bereplaced by dolomite, quartz, or pyrite. If a fossil that has been replace by quartz is surrounded by a calcite matrix,mildly acidic water may dissolve the calcite and leave behind an exquisitely preserved quartz fossil.

Compression

Some fossils form when their remains are compressed by high pressure, leaving behind a dark imprint. Compressionis most common for fossils of leaves and ferns, but can occur with other organisms.

Clues from Fossils

Fossils are our best form of evidence about Earth history, including the history of life. Along with other geologicalevidence from rocks and structures, fossils even give us clues about past climates, the motions of plates, and othermajor geological events.

History of Life on Earth

That life on Earth has changed over time is well illustrated by the fossil record. Fossils in relatively young rocksresemble animals and plants that are living today. In general, fossils in older rocks are less similar to modernorganisms. The history of life will be discussed in the Earth’s History chapter.

Environment of Deposition

By knowing something about the type of organism the fossil was, geologists can determine whether the region wasterrestrial (on land) or marine (underwater) or even if the water was shallow or deep. The rock may give cluesto whether the rate of sedimentation was slow or rapid. The amount of wear and fragmentation of a fossil allowsscientists to learn about what happened to the region after the organism died; for example, whether it was exposedto wave action.

8

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

Geologic History

The presence of marine organisms in a rock indicates that the region where the rock was deposited was once marine.Sometimes fossils of marine organisms are found on tall mountains indicating that rocks that formed on the seabedwere uplifted (Figure 1.13).

FIGURE 1.13The summit of Mt. Everest, the world’stallest mountain, is limestone that formedin an ancient sea.

Climate

By knowing something about the climate a type of organism lives in now, geologists can use fossils to decipher theclimate at the time the fossil was deposited. For example, coal beds form in tropical environments but ancient coalbeds are found in Antarctica. Geologists know that at that time the climate on the Antarctic continent was muchwarmer. Recall from the chapter about plate tectonics that Wegener used the presence of coal beds in Antarctica asone of the lines of evidence for continental drift.

Index Fossils

An index fossil can be used to identify a specific period of time. Organisms that make good index fossils aredistinctive, widespread, and lived briefly. Their presence in a rock layer can be used to identify that period of timeover a large area.

KQED: Science on the SPOT: Lupe the Mammoth Comes to Life

The fossil of a juvenile mammoth found near downtown San Jose California reveals an enormous amount aboutthese majestic creatures: what they looked like, how they lived, and what the environment of the Bay Area was likeso long ago. Learn more at: http://science.kqed.org/quest/video/science-on-the-spot-lupe-the-mammoth-comes-to-life/

9

1.1. Fossils www.ck12.org

MEDIAClick image to the left for more content.

Lesson Summary

• Fossils are the remains of ancient life. Body fossils are the remains of the organism itself; trace fossils areburrows, tracks, feces, or other evidence of activity.

• Fossilization is a very rare process. The chances of becoming a fossil are enhanced by quick burial and thepresence of hard parts, such as bones or shells.

• Fossils form in five ways: by preservation of the remains, permineralization, molds and casts, replacement,and compression.

• Types of organisms that make good index fossils are widespread but only existed for a short period of time.Index fossils help scientists to determine the approximate age of a rock layer and to match that layer up withother rock layers.

• Fossils give clues about the history of life on Earth, environments, climate, geologic history, and other eventsof geological importance.

Review Questions

1. What factors make it more likely that an animal will be preserved as a fossil?

2. What are the five main processes of fossilization?

3. A scientist wants to determine the age of a rock. The rock contains an index fossil and an ancient relative of aliving organism. Which is more useful for dating the rock, and why?

4. The island of Spitzbergen is in the Arctic Ocean, near the North Pole. Fossils of tropical fruits have been foundin coal deposits in Spitzbergen. What does this indicate?

Further Reading / Supplemental Links

This site is all about fossils: http://www.fossils-facts-and-finds.com/index.html

American Museum of Natural History site devoted to the links between mythic creatures and the organisms thatinspired them: http://www.amnh.org/exhibitions/mythiccreatures

Fossil myths and legends: http://www.tonmo.com/science/fossils/mythdoc/mythdoc.php

More about the Burgess Shale: http://www.geo.ucalgary.ca/ macrae/Burgess_Shale

More about the Solenhofen Limestone: http://www.ucmp.berkeley.edu/mesozoic/jurassic/solnhofen.html

The story of Otzi the Iceman: http://en.wikipedia.org/wiki/%C3%96tzi_the_Iceman

10

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

Points to Consider

• What are some other examples of mythical creatures that may be based on fossils?• Why is it so rare for an animal to be preserved as a fossil?• Some organisms are more easily preserved than others. Why is this a problem for scientists who are studying

ancient ecosystems?• Why are examples of amazing fossil preservation so valuable for scientists?• Many fossils of marine organisms have been found in the middle of continents, far from any ocean. What

conclusion can you draw from this?

11

1.2. Relative Ages of Rocks www.ck12.org

1.2 Relative Ages of Rocks

Lesson Objectives

• Explain Steno’s laws of superposition and original horizontality.• Based on a geological cross-section, identify the oldest and youngest formations.• Explain what an unconformity represents.• Know how to use fossils to correlate rock layers.

Vocabulary

• biozone• cross-cutting relationships• geologic time scale• key bed• lateral continuity• microfossil• original horizontality• relative age• superposition• unconformity• uniformitarianism

Introduction

Something that we hope you have learned from these lessons and from your own life experience is that the lawsof nature never change. They are the same today as they were billions of years ago. Water freezes at 0o C at 1atmosphere pressure; this is always true.

Knowing that natural laws never change helps scientists understand Earth’s past because it allows them to interpretclues about how things happened long ago. Geologists always use present-day processes to interpret the past. If youfind a fossil of a fish in a dry terrestrial environment did the fish flop around on land? Did the rock form in water andthen move? Since fish do not flop around on land today, the explanation that adheres to the philosophy that naturallaws do not change is that the rock moved.

Fossils were Living Organisms

In 1666, a young doctor named Nicholas Steno dissected the head of an enormous great white shark that had beencaught by fisherman near Florence, Italy. Steno was struck by the resemblance of the shark’s teeth to fossils found

12

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

in inland mountains and hills (Figure 1.14).

FIGURE 1.14Fossil Shark Tooth (left) and ModernShark Tooth (right).

Most people at the time did not believe that fossils were once part of living creatures. Authors in that day thoughtthat the fossils of marine animals found in tall mountains, miles from any ocean could be explained in one of twoways:

• The shells were washed up during the Biblical flood. (This explanation could not account for the fact thatfossils were not only found on mountains, but also within mountains, in rocks that had been quarried fromdeep below Earth’s surface.)

• The fossils formed within the rocks as a result of mysterious forces.

But for Steno, the close resemblance between fossils and modern organisms was impossible to ignore. Instead ofinvoking supernatural forces, Steno concluded that fossils were once parts of living creatures. He then sought toexplain how fossil seashells could be found in rocks and mountains far from any ocean. This led him to the ideasthat are discussed below.

Superposition of Rock Layers

Steno proposed that if a rock contained the fossils of marine animals, the rock formed from sediments that weredeposited on the seafloor. These rocks were then uplifted to become mountains. Based on these assumptions, Stenomade a remarkable series of conjectures that are now known as Steno’s Laws. These laws are illustrated below in(Figure 1.15).

Other scientists observed rock layers and formulated other principles. Geologist William Smith (1769-1839) identi-fied the principle of faunal succession, which recognizes that:

• Some fossil types are never found with certain other fossil types (e.g. human ancestors are never found withdinosaurs) meaning that fossils in a rock layer represent what lived during the period the rock was deposited.

• Older features are replaced by more modern features in fossil organisms as species change through time; e.g.feathered dinosaurs precede birds in the fossil record.

• Fossil species with features that change distinctly and quickly can be used to determine the age of rock layersquite precisely.

Scottish geologist, James Hutton (1726-1797) recognized the principle of cross-cutting relationships. This helpsgeologists to determine the older and younger of two rock units (Figure 1.16).

13

1.2. Relative Ages of Rocks www.ck12.org

FIGURE 1.15(a) Original Horizontality: Sediments are deposited in fairly flat, horizontal layers. If a sedimentary rock is foundtilted, the layer was tilted after it was formed. (b) Lateral continuity: Sediments are deposited in continuous sheetsthat span the body of water that they are deposited in. When a valley cuts through sedimentary layers, it isassumed that the rocks on either side of the valley were originally continuous. (c) Superposition: Sedimentaryrocks are deposited one on top of another. The youngest layers are found at the top of the sequence, and theoldest layers are found at the bottom.

The Grand Canyon provides an excellent illustration of the principles above. The many horizontal layers of sedi-mentary rock illustrate the principle of original horizontality (Figure 1.17).

• The youngest rock layers are at the top and the oldest are at the bottom, which is described by the law ofsuperposition.

• Distinctive rock layers, such as the Coconino Sandstone, are matched across the broad expanse of the canyon.These rock layers were once connected, as stated by the rule of lateral continuity.

• The Colorado River cuts through all the layers of rock to form the canyon. Based on the principle of cross-cutting relationships, the river must be younger than all of the rock layers that it cuts through.

Determining the Relative Ages of Rocks

Steno’s and Smith’s principles are essential for determining the relative ages of rocks and rock layers. In the processof relative dating, scientists do not determine the exact age of a fossil or rock but look at a sequence of rocks to tryto decipher the times that an event occurred relative to the other events represented in that sequence. The relative

14

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.16If an igneous dike (B) cuts a series ofmetamorphic rocks (A), which is older andwhich is younger? In this image, A musthave existed first for B to cut across it.

FIGURE 1.17At the Grand Canyon, the CoconinoSandstone appears across canyons. TheCoconino is the distinctive white layer; itis a vast expanse of ancient sand dunes.

age of a rock then is its age in comparison with other rocks. If you know the relative ages of two rock layers, (1) Doyou know which is older and which is younger? (2) Do you know how old the layers are in years?

An interactive website on relative ages and geologic time is found here: http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/gtpage1.html

In some cases, it is very tricky to determine the sequence of events that leads to a certain formation. Can you figure

15

1.2. Relative Ages of Rocks www.ck12.org

out what happened in what order in (Figure 1.18)? Write it down and then check the following paragraphs.

FIGURE 1.18A geologic cross section: Sedimentaryrocks (A-C), igneous intrusion (D), fault(E).

The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cutsthrough. The fault cuts through all three sedimentary rock layers (A, B, and C) and also the intrusion (D). So thefault must be the youngest feature. The intrusion (D) cuts through the three sedimentary rock layers, so it must beyounger than those layers. By the law of superposition, C is the oldest sedimentary rock, B is younger and A is stillyounger.

The full sequence of events is:

1. Layer C formed.

2. Layer B formed.

3. Layer A formed.

4. After layers A-B-C were present, intrusion D cut across all three.

5. Fault E formed, shifting rocks A through C and intrusion D.

6. Weathering and erosion created a layer of soil on top of layer A.

Earth’s Age

During Steno’s time, most Europeans believed that the Earth was around 6,000 years old, a figure that was basedon the amount of time estimated for the events described in the Bible. One of the first scientists to question thisassumption and to understand geologic time was James Hutton. Hutton traveled around Great Britain in the late1700s, studying sedimentary rocks and their fossils (Figure 1.19).

Often described as the founder of modern geology, Hutton formulated uniformitarianism: The present is the keyto the past. According to uniformitarianism, the same processes that operate on Earth today operated in the past aswell. Why is an acceptance of this principle absolutely essential for us to be able to decipher Earth history?

Hutton questioned the age of the Earth when he looked at rock sequences like the one below. On his travels, he

16

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.19A drawing by James Hutton. "Theory ofthe Earth,” 1795.

discovered places where sedimentary rock beds lie on an eroded surface. At this gap in rock layers, or unconformity,some rocks were eroded away. For example, consider the famous unconformity at Siccar Point, on the coast ofScotland (Figure 1.20).

FIGURE 1.20Hutton’s Unconformity on the Coast ofScotland. Can you find the unconformity?What are the geological events that youcan find in this image? (Hint: There arenine.)

1. A series of sedimentary beds was deposited on an ocean floor.

2. The sediments hardened into sedimentary rock.

3. The sedimentary rocks are uplifted and tilted, exposing them above sea level.

4. The tilted beds were eroded to form an irregular surface.

5. A sea covered the eroded sedimentary rock layers.

6. New sedimentary layers were deposited.

7. The new layers hardened into sedimentary rock.

17

1.2. Relative Ages of Rocks www.ck12.org

8. The whole rock sequence was tilted.

9. Uplift occurred, exposing the new sedimentary rocks above the ocean surface.

Since he thought that the same processes at work on Earth today worked at the same rate in the past, he had toaccount for all of these events and the unknown amount of missing time represented by the unconformity, Huttonrealized that this rock sequence alone represented a great deal of time. He concluded that Earth’s age should not bemeasured in thousands of years, but in millions of years.

Matching Up Rock Layers

Superposition and cross-cutting are helpful when rocks are touching one another and lateral continuity helps matchup rock layers that are nearby, but how do geologists correlate rock layers that are separated by greater distances?There are three kinds of clues:

1. Distinctive rock formations may be recognizable across large regions (Figure 1.21).

FIGURE 1.21The famous White Cliffs of Dover in south-west England can be matched to similarwhite cliffs in Denmark and Germany.

2. Two separated rock units with the same index fossil are of very similar age. What traits do you think an indexfossil should have? To become an index fossil the organism must have (1) been widespread so that it is useful foridentifying rock layers over large areas and (2) existed for a relatively brief period of time so that the approximateage of the rock layer is immediately known.

Many fossils may qualify as index fossils (Figure 1.22). Ammonites, trilobites, and graptolites are often used asindex fossils.

Microfossils, which are fossils of microscopic organisms, are also useful index fossils. Fossils of animals that driftedin the upper layers of the ocean are particularly useful as index fossils, since they may be distributed over very largeareas.

A biostratigraphic unit, or biozone, is a geological rock layer that is defined by a single index fossil or a fossilassemblage. A biozone can also be used to identify rock layers across distances.

3. A key bed can be used like an index fossil since a key bed is a distinctive layer of rock that can be recognizedacross a large area. A volcanic ash unit could be a good key bed. One famous key bed is the clay layer at theboundary between the Cretaceous Period and the Tertiary Period, the time that the dinosaurs went extinct (Figure1.23). This thin clay contains a high concentration of iridium, an element that is rare on Earth but common inasteroids. In 1980, the father-son team of Luis and Walter Alvarez proposed that a huge asteroid struck Earth 66million years ago and caused the mass extinction.

18

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.22Mucrospirifer mucronatus is an index fos-sil that indicates that a rock was laid downfrom 416 to 359 million years ago.

FIGURE 1.23The white clay is a key bed that marks theCretaceous-Tertiary Boundary.

The Geologic Time Scale

To be able to discuss Earth history, scientists needed some way to refer to the time periods in which events happenedand organisms lived. With the information they collected from fossil evidence and using Steno’s principles, theycreated a listing of rock layers from oldest to youngest. Then they divided Earth’s history into blocks of time witheach block separated by important events, such as the disappearance of a species of fossil from the rock record.Since many of the scientists who first assigned names to times in Earth’s history were from Europe, they named theblocks of time from towns or other local places where the rock layers that represented that time were found.

From these blocks of time the scientists created the geologic time scale (Figure 1.24). In the geologic time scalethe youngest ages are on the top and the oldest on the bottom. Why do you think that the more recent time periodsare divided more finely? Do you think the divisions in the scale below are proportional to the amount of time eachtime period represented in Earth history?

In what eon, era, period and epoch do we now live? We live in the Holocene (sometimes called Recent) epoch,Quaternary period, Cenozoic era, and Phanerozoic eon.

19

1.2. Relative Ages of Rocks www.ck12.org

FIGURE 1.24The geologic time scale is based on relative ages. No actual ages wereplaced on the original time scale.

Lesson Summary

• Nicholas Steno formulated the principles in the 17th century that allow scientists to determine the relative agesof rocks. Steno stated that sedimentary rocks are formed in continuous, horizontal layers, with younger layerson top of older layers.

• William Smith and James Hutton later discovered the principles of cross-cutting relationships and faunalsuccession.

• Hutton also realized the vast amounts of time that would be needed to create an unconformity and concludedthat Earth was much older than people at the time thought.

• The guiding philosophy of Hutton and geologists who came after him is: The present is the key to the past.• To correlate rock layers that are separated by a large distance look for sedimentary rock formations that are

extensive and recognizable, index fossils, and key beds.• Changes of fossils over time led to the development of the geologic time scale, which illustrates the relative

order in which events on Earth have happened.

Review Questions

1. A 15th century farmer finds a rock that looks exactly like a clamshell. What did he likely conclude about how thefossil got there?

2. Which of Steno’s Laws is illustrated by each of the images in Figure 1.25?

3. What is the sequence of rock units in Figure 1.26, from oldest to youngest?

4. What kind of geological formation is shown in the outcrop in Figure 1.27, and what sequence of events does itrepresent?

5. The three outcrops in Figure 1.28 are very far apart. Based on what you see, which fossil is an index fossil, and

20

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.25

FIGURE 1.26Sequence of Rock Units

why?

6. Why didn’t the early geologic time scale include the number of years ago that events happened?

7. Dinosaurs went extinct about 66 million years ago. Which period of geologic time was the last in which dinosaurslived?

8. Suppose that while you’re hiking in the mountains of Utah, you find a fossil of an animal that lived on theocean floor. You learn that the fossil is from the Mississippian period. What was the environment like during theMississippian in Utah?

9. Why are sedimentary rocks more useful than metamorphic or igneous rocks in establishing the relative ages ofrock?

21

1.2. Relative Ages of Rocks www.ck12.org

FIGURE 1.27Outcrop

FIGURE 1.28Three outcrops

10. Which is likely to be more frequently found in rocks: fossils of very old sea creatures or very old land creatures?

Further Reading / Supplemental Links

A US Geological Survey paper on Rocks, Fossils and Time: http://pubs.usgs.gov/gip/fossils/contents.html

Try to guess the mystery fossils in these pictures and see if you’re right. There are more in the archives: http://www.ucmp.berkeley.edu/exhibits/mysteryfossil/mysteryfossil.php

An interactive “Virtual Museum of Fossils” http://fossils.valdosta.edu/.

22

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

The fossil record in North America http://www.paleoportal.org/.

An excerpt of the book “The Seashell on the Mountaintop” is found here http://alan-cutler.com/excerpt.html

Determining the ages of rocks and fossils: http://www.ucmp.berkeley.edu/fosrec/McKinney.html

Points to Consider

• How did preconceived ideas in Steno’s time make people blind to the reality of what fossils represent?• How did Steno explain the presence of marine fossils in high mountains?• Why was Hutton’s recognition of unconformities so significant?• Can the relative ages of two rock layers that are very far apart be determined?• Can the same principles used to study Earth’s history also be used to study the history of other planets?

23

1.3. Absolute Ages of Rocks www.ck12.org

1.3 Absolute Ages of Rocks

Lesson Objectives

• Define the differences between absolute age and relative age.• Describe four methods of absolute dating.• Explain what radioactivity is and give examples of radioactive decay.• Explain how the decay of radioactive materials helps to establish the age of an object.• Estimate the age of an object, given the half-life and the amounts of radioactive and daughter materials.• Give four examples of radioactive materials that are used to date objects, and explain how each is used.• Describe how scientists know earth is billions of years old.

Vocabulary

• absolute age• daughter product• half-life• ice core• parent isotope• radioactive isotope• radioactivity• radiometric dating• tree ring

Introduction

What was missing from the early geologic time scale? While the order of events was given, the dates at which theevents happened were not. With the discovery of radioactivity in the late 1800s, scientists were able to measure theabsolute age, or the exact age of some rocks in years. Absolute dating allows scientists to assign numbers to thebreaks in the geologic time scale. Radiometric dating and other forms of absolute age dating allowed scientists toget an absolute age from a rock or fossil.

Tree Ring Dating

In locations where summers are warm and winters are cool, trees have a distinctive growth pattern. Tree trunksdisplay alternating bands of light-colored, low density summer growth and dark, high density winter growth. Eachlight-dark band represents one year. By counting tree rings it is possible to find the number of years the tree lived(Figure 1.29).

24

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.29Cross-section showing growth rings.

The width of these growth rings varies with the conditions present that year. A summer drought may make the treegrow more slowly than normal and so its light band will be relatively small. These tree-ring variations appear in alltrees in a region. The same distinctive pattern can be found in all the trees in an area for the same time period.

Scientists have created continuous records of tree rings going back over the past 2,000 years. Wood fragments fromold buildings and ancient ruins can be age dated by matching up the pattern of tree rings in the wood fragment inquestion and the scale created by scientists. The outermost ring indicates when the tree stopped growing; that is,when it died. The tree-ring record is extremely useful for finding the age of ancient structures.

An example of how tree-ring dating is used to date houses in the United Kingdom is found in this article: http://www.periodproperty.co.uk/ppuk_discovering_article_013.shtml.

Ice Cores and Varves

Other processes create distinct yearly layers that can be used for dating. On a glacier, snow falls in winter butin summer dust accumulates. This leads to a snow-dust annual pattern that goes down into the ice (Figure 1.30).Scientists drill deep into ice sheets, producing ice cores hundreds of meters long. The information scientists gatherallows them to determine how the environment has changed as the glacier has stayed in its position. Analyses of theice tell how concentrations of atmospheric gases changed, which can yield clues about climate. The longest coresallow scientists to create a record of polar climate stretching back hundreds of thousands of years.

FIGURE 1.30Ice core section showing annual layers.

Lake sediments, especially in lakes that are located at the end of glaciers, also have an annual pattern. In the summer,the glacier melts rapidly, producing a thick deposit of sediment. These alternate with thin, clay-rich layers depositedin the winter. The resulting layers, called varves, give scientists clues about past climate conditions (Figure 1.31).

25

1.3. Absolute Ages of Rocks www.ck12.org

A warm summer might result in a very thick sediment layer while a cooler summer might yield a thinner layer.

FIGURE 1.31Ancient varve sediments in a rock outcrop.

Age of Earth

During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. Whatmethods can you think of for doing this? One example is that by measuring how much sediment a stream depositedin a year, a geologist might try to determine how long it took for a stream to deposit an ancient sediment layer.Not surprisingly, these methods resulted in wildly different estimates. A relatively good estimate was produced bythe British geologist Charles Lyell, who thought that 240 million years had passed since the appearance of the firstanimals with shells. Today scientists know that this event occurred about 530 million years ago.

In 1892, William Thomson (later known as Lord Kelvin) calculated that the Earth was 100 million years old (Figure1.32). He did this systematically assuming that the planet started off as a molten ball and calculating the timeit would take for it to cool to its current temperature. This estimate was a blow to geologists and supporters ofCharles Darwin’s theory of evolution, which required an older Earth to provide time for geological and evolutionaryprocesses to take place.

Thomson’s calculations were soon shown to be flawed when radioactivity was discovered in 1896. Radioactivity isthe tendency of certain atoms to decay into lighter atoms, a process that emits energy. Radioactive decay of elementsinside Earth’s interior provides a steady source of heat, which meant that Thomson had grossly underestimatedEarth’s age.

Radioactive Decay

Radioactivity also provides a way to find the absolute age of a rock. To begin, go back to the Earth’s Mineralschapter and review the material about atoms.

Some isotopes are radioactive; radioactive isotopes are unstable and spontaneously change by gaining or losingparticles. Two types of radioactive decay are relevant to dating Earth materials (Table 1.1):

TABLE 1.1: Types of Radioactive Decay

Particle Composition Effect on NucleusAlpha 2 protons, 2 neutrons The nucleus contains two fewer pro-

tons and two fewer neutrons.Beta 1 electron One neutron decays to form a pro-

ton and an electron. The electron isemitted.

26

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.32Lord Kelvin.

The radioactive decay of a parent isotope (the original element) leads to the formation of stable daughter product,also known as daughter isotope. As time passes, the number of parent isotopes decreases and the number of daughterisotopes increases (Figure 1.33).

An animation of radioactive decay: http://lectureonline.cl.msu.edu/ mmp/applist/decay/decay.htm.

Radioactive materials decay at known rates, measured as a unit called half-life. The half-life of a radioactivesubstance is the amount of time it takes for half of the parent atoms to decay. This is how the material decaysover time (see Table 1.2).

TABLE 1.2: Radioactive Decay

No. of half lives passed Percent parent remaining Percent daughter produced0 100 01 50 502 25 753 12.5 87.54 6.25 93.755 3.125 96.8756 1.563 98.4377 0.781 99.2198 0.391 99.609

27

1.3. Absolute Ages of Rocks www.ck12.org

FIGURE 1.33A parent emits an alpha particle to createa daughter.

Pretend you find a rock with 3.125% parent atoms and 96.875% daughter atoms. How many half lives have passed?If the half-life of the parent isotope is 1 year, then how old is the rock? The decay of radioactive materials can beshown with a graph (Figure 1.34).

FIGURE 1.34Decay of an imaginary radioactive sub-stance with a half-life of one year.

An animation of half-life: http://einstein.byu.edu/ masong/htmstuff/Radioactive2.html.

Notice how it doesn’t take too many half lives before there is very little parent remaining and most of the isotopesare daughter isotopes. This limits how many half lives can pass before a radioactive element is no longer useful for

28

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

dating materials. Fortunately, different isotopes have very different half lives.

Radiometric decay is exponential. Learn how exponential growth and decay can be described mathematically in thisvideo (IE 1e): http://www.youtube.com/watch?v=UbwMW7Q6F3E (4:46).

MEDIAClick image to the left for more content.

The Scientific Method Made Easy explains scientific method succinctly and well (IE 1a, 1b, 1c, 1d, 1f,1g, 1j, 1k):http://www.youtube.com/watch?v=zcavPAFiG14&feature=related (9:55).

Radiometric Dating of Rocks

Different isotopes are used to date materials of different ages. Using more than one isotope helps scientists to checkthe accuracy of the ages that they calculate.

Radiocarbon Dating

Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range isespecially useful for determining ages of human fossils and habitation sites (Figure 1.35).

The atmosphere contains three isotopes of carbon: carbon-12, carbon-13 and carbon-14. Only carbon-14 is radioac-tive; it has a half-life of 5,730 years. The amount of carbon-14 in the atmosphere is tiny and has been relativelystable through time.

Plants remove all three isotopes of carbon from the atmosphere during photosynthesis. Animals consume this carbonwhen they eat plants or other animals that have eaten plants. After the organism’s death, the carbon-14 decays tostable nitrogen-14 by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount ofcarbon-14 that has decayed can be estimated by measuring the proportion of radioactive carbon-14 to stable carbon-12. As time passes, the amount of carbon-14 decreases relative to the amount of carbon-12.

A video of carbon-14 decay is seen here: http://www.youtube.com/watch?v=81dWTeregEA; a longer explanationis here: http://www.youtube.com/watch?v=udkQwW6aLik&feature=related.

Potassium-Argon Dating

Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Argon is a gas so it can escape from moltenmagma, meaning that any argon that is found in an igneous crystal probably formed as a result of the decay ofpotassium-40. Measuring the ratio of potassium-40 to argon-40 yields a good estimate of the age of that crystal.

Potassium is common in many minerals, such as feldspar, mica, and amphibole. With its half-life, the technique isused to date rocks from 100,000 years to over a billion years old. The technique has been useful for dating fairlyyoung geological materials and deposits containing the bones of human ancestors.

29

1.3. Absolute Ages of Rocks www.ck12.org

FIGURE 1.35Carbon isotopes from the black materialin these cave paintings places their cre-ating at about 26,000 to 27,000 years BP(before present).

Uranium-Lead Dating

Two uranium isotopes are used for radiometric dating.

• Uranium-238 decays to lead-206 with a half-life of 4.47 billion years.• Uranium-235 decays to form lead-207 with a half-life of 704 million years.

Uranium-lead dating is usually performed on zircon crystals (Figure 1.36). When zircon forms in an igneous rock,the crystals readily accept atoms of uranium but reject atoms of lead. If any lead is found in a zircon crystal, it canbe assumed that it was produced from the decay of uranium.

Uranium-lead dating is useful for dating igneous rocks from 1 million years to around 4.6 billion years old. Zirconcrystals from Australia are 4.4 billion years old, among the oldest rocks on the planet.

Limitations of Radiometric Dating

Radiometric dating, or the process of using the concentrations of radioactive substances and daughter products toestimate the age of a material, is a very useful tool for dating geological materials but it does have limits:

1. The material being dated must have measurable amounts of the parent and/or the daughter isotopes. Ideally,different radiometric techniques are used to date the same sample; if the calculated ages agree, they are thought tobe accurate.

2. Radiometric dating is not very useful for determining the age of sedimentary rocks. To estimate the age of asedimentary rock, geologists find nearby igneous rocks that can be dated and use relative dating to constrain the ageof the sedimentary rock.

30

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

FIGURE 1.36Zircon crystal.

Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossilages have become increasingly accurate.

All of this evidence comes together to pinpoint the age of Earth at 4.6 billion years. A video discussing the evidencefor this is found here: http://www.youtube.com/watch?v=w5369-OobM4.

The age of Earth is also discussed in this video: http://www.youtube.com/watch?v=lplcRdNDcps&feature=channel.

Lesson Summary

• Earth is very old, and the study of Earth’s past requires us to think about times that were millions or evenbillions of years ago.

• Techniques such as superposition and index fossils can tell you the relative age of objects, which objects areolder and which are younger.

• Geologists use a variety of techniques to establish absolute age, including radiometric dating, tree rings, icecores, and annual sedimentary deposits called varves.

31

1.3. Absolute Ages of Rocks www.ck12.org

• The concentrations of several radioactive isotopes (e.g. carbon-14, potassium-40, uranium-235 and -238) andtheir daughter products are used to accurately determine the age of rocks and organic remains.

Review Questions

1. Name four techniques that are used to determine the absolute age of an object or event.

2. A radioactive substance has a half-life of 5 million years. What is the age of a rock in which 25% of the originalradioactive atoms remain?

3. A scientist is studying a piece of cloth from an ancient burial site. She determines that 40% of the originalcarbon-14 atoms remain in the cloth. Based on the carbon-decay graph (Figure 1.37), what is the approximate ageof the cloth?

FIGURE 1.37Carbon-decay graph.

4. Which radioactive isotope or isotopes would you use to date each of the following objects? Explain each of yourchoices.

1.) A 4-billion-year-old piece of granite.2.) A 1-million-year-old bed of volcanic ash that contains the footprints of human ancestors.3.) The fur of a woolly mammoth that was recently recovered, frozen in a glacier.4.) A fossilized trilobite from a bed of sandstone that is about 500 million years old.

5. Why is it important to assume that the rate of radioactive decay has remained constant over time?

Further Reading / Supplemental Links

Using tree rings and ice cores to track El Nino events: http://www.pbs.org/wgbh/nova/elnino/reach/living.html.

32

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

Points to Consider

• Why are techniques for dating, such as using tree rings, ice cores, and varves only useful for events thatoccurred in the last few thousand years?

• Why is it important for geological and biological processes that the earth is very old?• Why is it important to use more than one method to find the age of a rock or other object?

Opening image courtesy of Barbara Summey/NASA. http://earthobservatory.nasa.gov/IOTD/view.php?id=602. Pub-lic Domain.

33

1.4. References www.ck12.org

1.4 References

1. (left) Photo by Jastrow; (right) [Flickr:Kabacchi]. (left) http://commons.wikimedia.org/wiki/File:Satyr_griffin_Arimaspus_Louvre_CA491.jpg; (right) https://secure.flickr.com/photos/kabacchi/4156040469/. (left) Pub-lic Domain; (right) CC-BY 2.0

2. (left) Mark C [Flickr: MDCPhotographic]; (right) [Flickr: kokeshi]. (left) https://secure.flickr.com/photos/22766882@N05/2710871622/; (right) https://secure.flickr.com/photos/kokeshi/466014509/. (left) CC-BY-NC 2.0; (right) CC-BY-NC-SA 2.0

3. (a) ; (b) James Emery; (c) Kevin Walsh [Flickr: kevinzim]; (d) Vassil. (a) http://en.wikipedia.org/wiki/File:Quetzalcoatlus07.jpg; (b) http://commons.wikimedia.org/wiki/File:Argentinosaurus.jpg; (c) https://secure.flickr.com/photos/86624586@N00/4663012974/; (d) http://commons.wikimedia.org/wiki/File:Ammonite_2582.jpg. (a) Public Domain; (b) CC-BY 2.0; (c) CC-BY 2.0; (d) Public Domain

4. Courtesy of US Geological Survey. http://commons.wikimedia.org/wiki/File:Coprolite.jpg. Public Domain5. Image copyright p.schwarz, 2010. http://www.shutterstock.com/. Used under license from Shutterstock.com6. Photograph taken by Mark A. Wilson (Department of Geology, The College of Wooster) [Wikimedia: Wil-

son44691]. https://commons.wikimedia.org/wiki/File:BoredEncrustedShell.JPG. Public Domain7. Image copyright Roger De Marfa, 2010. http://www.shutterstock.com/. Used under license from Shutter-

stock.com8. Image copyright markrhiggins, 2010. http://www.shutterstock.com/. Used under license from Shutterstock.com9. RoyFokker. https://commons.wikimedia.org/wiki/File:Pompeya3.jpg. Public Domain

10. Charles R. Knight. http://en.wikipedia.org/wiki/File:La_Brea_Tar_Pits.jpg. Public Domain11. (a) Mark A Wilson (Wilson44691); (b) Mark A Wilson (Wilson44691); (c) [Flickr: Kathy__]; (d) Phil

Dokas. (a) http://en.wikipedia.org/wiki/File:WalcottQuarry080509.jpg; (b) http://commons.wikimedia.org/wiki/File:Anomalocaris_Mt._Stephen.jpg; (c) http://flickr.com/photos/kathy_/3860149772/;(d)http://www.flickr.com/photos/dokas/3594544864/. (a) Public Domain; (b) Public Domain; (c) CC-BY-NC-SA 2.0; (d) CC-BY-NC-SA 2.0

12. (a) Adrian Pingstone (Apingstone); (b) Jon Sullivan; (c) Courtesy of US Geological Survey; (d) Kevin Walsh[Flickr: kevinzim]; (e) Woudloper. (a) http://commons.wikimedia.org/wiki/File:Amber.insect.800pix.050203.jpg; (b) https://commons.wikimedia.org/wiki/File:PetrifiedWood.jpg; (c) http://commons.wikimedia.org/wiki/File:Cast_and_mold_of_a_clam_shell.jpg; (d) https://secure.flickr.com/photos/86624586@N00/3005767958/; (e) http://commons.wikimedia.org/wiki/File:Pecopteris_arborescens.jpg. (a) Public Domain; (b) PublicDomain; (c) Public Domain; (d) CC-BY 2.0 ; (e) Public Domain

13. Image copyright Galyna Andrushko, 2010. http://www.shutterstock.com. Used under license from Shutter-stock.com

14. (a) MMM; (b) Eli Hodapp. (a) https://commons.wikimedia.org/wiki/File:Haizahn-foss.jpg; (b) http://www.flickr.com/photos/io_burn/1805341269/. (a) Public Domain; (b) CC-BY 2.0

15. (a) Stephen J. Reynolds; (b) Woudloper; (c) Mark A Wilson (Wilson44691). (a) http://reynolds.asu.edu/geologic_scenery/geologic_scenery_images.htm; (b) http://en.wikipedia.org/wiki/File:Principle_of_horizontal_-continuity.svg; (c) http://en.wikipedia.org/wiki/File:IsfjordenSuperposition.jpg. (a) Noncommercial uses OKas long as source is acknowledged; (b) Public Domain; (c) Public Domain

16. Courtesy of US Geological Survey, modified by CK-12 Foundation. http://commons.wikimedia.org/wiki/File:Skagit-gneiss-Cascades.jpg. Public Domain

17. Miles Orchinik. http://miles-home.smugmug.com/Landscapes/Friends-and-family/Grand-Canyon/17721182_-9xFMV5#!i=1352580465&k=5vTvDBm. Used with permission from the author

18. Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-NC-SA 3.019. John Clerk. http://en.wikipedia.org/wiki/File:Hutton_Unconformity_,_Jedburgh.jpg. Public Domain20. Meg Stewart. http://www.flickr.com/photos/megstewart/4784445104. Used with permission from author

34

www.ck12.org Chapter 1. HS Evidence About Earth’s Past

under CC-BY-NC-SA21. Image copyright Syringa, 2010. http://www.shutterstock.com. Used under license from Shutterstock.com22. [Flickr: Ivanlee8]. https://secure.flickr.com/photos/24994567@N06/2700898489/. CC-BY-NC-SA 2.023. Courtesy of the US Geological Survey. http://esp.cr.usgs.gov/info/kt/stop2b.html. Public Domain24. Courtesy of US Geological Survey. http://commons.wikimedia.org/wiki/File:Geologic_time_scale.jpg. Public

Domain25. Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-NC-SA 3.026. Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-NC-SA 3.027. Lamprus. https://commons.wikimedia.org/wiki/File:Angular_Unconformity_Kutna_Hora_detail_2.jpg. Pub-

lic Domain28. Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-NC-SA 3.029. Lawrence Murray (lawmurray). http://www.flickr.com/photos/22699083@N04/2284340556/. CC-BY 2.030. Courtesy of US Geological Survey, provided by Eric Cravens, Assistant Curator, National Ice Core Laboratory.

http://en.wikipedia.org/wiki/File:GISP2D1837_crop.jpg. Public Domain31. Courtesy of US Geological Survey. http://pubs.usgs.gov/of/2004/1216/tz/tz.html. Public Domain32. Unknown. http://commons.wikimedia.org/wiki/File:Lord_Kelvin_photograph.jpg. Public Domain33. CK-12 Foundation - Kurt Rosenkrantz and Sam McCabe. . CC-BY-NC-SA 3.034. Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-NC-SA 3.035. HTO. http://en.wikipedia.org/wiki/File:Chauvet_cave,_paintings.JPG. Public Domain36. Stephanie Clifford [Flickr: sdixclifford]. http://www.flickr.com/photos/30486689@N08/2933076466/. CC-

BY 2.037. Kurt Rosenkrantz/CK-12 Foundation. . CC-BY-NC-SA 3.0

35