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Earth History Earth History Dana Desonie, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) AUTHOR Dana Desonie, Ph.D. To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. Copyright © 2014 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (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 incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/terms. Printed: October 29, 2014 www.ck12.org Chapter 1. Earth History CHAPTER 1 Earth History CHAPTER OUTLINE 1.1 How Fossilization Creates Fossils 1.2 Types of Fossilization 1.3 Earth History and Clues from Fossils 1.4 Principles of Relative Dating 1.5 Determining Relative Ages 1.6 Correlation Using Relative Ages 1.7 Geologic Time Scale 1.8 Tree Rings, Ice Cores, and Varves 1.9 Radioactive Decay as a Measure of Age 1.10 Radiometric Dating 1.11 Age of Earth 1.12 Formation of the Sun and Planets 1.13 Formation of Earth 1.14 Early Atmosphere and Oceans 1.15 Precambrian Continents 1.16 Paleozoic Plate Tectonics 1.17 Paleozoic and Mesozoic Seas 1.18 Mesozoic Plate Tectonics 1.19 Cenozoic Plate Tectonics 1.20 References 1 www.ck12.org Introduction What has led to these landforms? How do we know? This is North America right now. You can pick out a lot of features that we’ve been studying. You can even pick out evidence of different processes. Continental rifting? See where Baja California is separating from the rest of Mexico. Mountain building? You can see the Sierra Nevada, Cascades, and Rocky Mountains, among others. Weathering is seen in the red dirt in the Four Corners area. The Great Lakes are remnants of the giant glaciers that covered the northern region. Shallow limestone shelves are seen surrounding Florida. How we know what we know and what we know about Earth history is touched on very briefly in this concept. 2 www.ck12.org Chapter 1. Earth History 1.1 How Fossilization Creates Fossils • Describe body and trace fossils. How do you find fossils? In lots of places, you can go on a field trip to find fossils. The fossils are found in rock layers. You can break apart the rock layers and find the fossils. What Are Fossils? Fossils are preserved remains or traces of organisms that lived in the past. Most preserved remains are hard parts, such as teeth, bones, or shells. Hard parts are less likely to be destroyed before they can become fossils. Even so, a very tiny percentage of living things become fossils. These types of fossils are called body fossils ( Figure 1.1, Figure 1.2, and Figure 1.3). FIGURE 1.1 The preserved remains of an ammonite. Preserved traces can include footprints, burrows, or even wastes. Examples of these types of fossils, called trace fossils are picture below ( Figure 1.4 and Figure 1.5). Vocabulary • body fossil: Remains of an ancient organism; examples include shells, bones, teeth, and leaves. • trace fossil: Evidence of the activity of an ancient organism; e.g. tracks, tubes, and bite marks. 3 1.1. How Fossilization Creates Fossils www.ck12.org FIGURE 1.2 The preserved remains of an Archaeopteryx, a type of dinosaur. FIGURE 1.3 A group of preserved trilobites, a variety of ancient marine arthropods. FIGURE 1.4 The preserved trace of a dinosaur footprint, originally in mud. FIGURE 1.5 A preserved trace: burrows made by prehistoric worms. Summary • Fossils are the remains or traces of living organisms. • Body fossils are usually hard parts, such as bones, teeth, and shells. • Trace fossils are the traces of organisms, not parts of them. Trace fossils include footprints, feces, and burrows. Explore More Use the resource below to answer the questions that follow. • How Fossils are Formed at http://www.youtube.com/watch?v=TVwPLWOo9TE (2:39) 4 www.ck12.org Chapter 1. Earth History MEDIA Click image to the left for use the URL below. URL: http://gamma.ck12.org/flx/render/embeddedobject/3329 1. What do paleontologists study and what is that thing? 2. At least how old are most fossils? 3. What are trace fossils? What are a few examples? 4. What can be learned from trace fossils? 5. What are mold fossils? 6. How do resin fossils? 7. What are body fossils? 8. How do body fossils form? 9. If body fossils form beneath layers of sediment, how do they end up at the surface where they can be found? Explore More Answers 1. Paleontologists study fossils, which are remains and traces of ancient life. 2. Fossils must be at least 3,000 years old to be fossils. 3. Trace fossils are evidence that life was there without being a body part; these include droppings and footprints. 4. Trace fossils indicate how an animal lived and cared for their young. 5. Molds are impressions of an animal in rock that are filled to make a fossil. 6. Small creatures, usually insects, get caught in sticky tree sap. That sap hardens and traps the organisms within. 7. Body fossils are body parts of an animal, usually bones or teeth. 8. If an animal dies in or near water, it is buried beneath sediment quickly so that its bones don’t end up being scattered. The soft parts of the animal rot away and the bones and other hard parts are replaced by minerals. 9. The region is uplifted and exposed by erosion. Review 1. Give three examples of body fossils. 2. Give three examples of trace fossils. 3. Why are most fossils of hard-bodied organisms? Review Answers 1. ammonites, dinosaurs, trilobites 2. footprints, burrows, feces 3. Hard parts are less likely to be destroyed. 5 1.2. Types of Fossilization www.ck12.org 1.2 Types of Fossilization • Learn the processes that create fossils. Where could you find the best fossils? The best fossils would probably be ones that you could get DNA from. DNA holds the genes of an organisms. The genes could tell scientists a lot about what the organisms was really like. Where could you find a fossil with DNA? How about a melting glacier? You might be able to find a creature that had been frozen into the ice for a long time. A frozen animal would be very well preserved! How Fossils Form The process by which remains or traces of living things become fossils is called fossilization ( Figure 1.6). Most fossils are preserved in sedimentary rocks. Fossils in Sedimentary Rock Most fossils form when a dead organism is buried in sediment. Layers of sediment slowly build up. The sediment is buried and turns into sedimentary rock. The remains inside the rock also turn to rock. The remains are replaced by minerals. The remains literally turn to stone. This type of fossilization is called permineralization. 6 www.ck12.org Chapter 1. Earth History FIGURE 1.6 Fossilization. This flowchart shows how most fossils form. Other Ways Fossils Form Fossils may form in other ways. With complete preservation, the organism doesn’t change much. As pictured below ( Figure 1.7), tree sap may cover an organism and then turn into amber. The original organism is preserved so that scientists might be able to study its DNA. Organisms can also be completely preserved in tar or ice. Molds and casts are another way organisms can be fossilized. A mold is an imprint of an organism left in rock. The organism’s remains break down completely. Rock that fills in the mold resembles the original remains. The fossil that forms in the mold is called a cast ( Figure 1.8). Molds and casts usually form in sedimentary rock. With compression ( Figure 1.9), an organism’s remains are put under great pressure inside rock layers. This leaves behind a dark stain in the rock. Why Fossilization is Rare It’s very unlikely that any given organism will become a fossil. The remains of many organisms are consumed. Remains also may be broken down by other living things or by the elements. Hard parts, such as bones, are much more likely to become fossils. But even they rarely last long enough to become fossils. Organisms without hard 7 1.2. Types of Fossilization www.ck12.org FIGURE 1.7 Complete Preservation. This spider looks the same as it did the day it died millions of years ago! FIGURE 1.8 Molds and Casts.
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