Earth History - 6th Grade
Objectives and Indicators for this Unit
2.8.B. Earth History
1. Explain how sedimentary rock is formed periodically, embedding plant and animal remains and leaving a record of the sequence in which the plants and animals appeared and disappeared.
a. Explain how sedimentary rock buried deep enough may be reformed by pressure and heat and these re-formed rock layers
may be forced up again (uplift) to become land surface and even mountains.
b. Cite evidence to confirm that thousands of layers of sedimentary rock reveal the long history of the changing surface of the Earth
c. Explain why some fossils found in the top layers of sedimentary rock are older then those found beneath in lower layers.
§ Folding Breaking Uplift Faulting Tilting
2. Recognize and explain that fossils found in layers of sedimentary rock provide evidence of changing life forms.
a. Recognize how different types of fossils are formed, such as petrified remains, imprints, molds and casts.
b. Recognize and explain that the fossil record of plants and animals describes changes in life forms over time.
Vocabulary Terms that have to be looked up and written in your journal
Deposition
Cementation
Compaction
Fossil
Geologist
Extinct
Law of Superposition
Radioactive decay
Period
Era
Sediments
Paleontologist
Cast
Imprints
PRE-TEST - Please copy these questions in your journal and answer them there.
· How can live things be preserved inside of a rock?
· How are different fossil types formed?
· What types of evidence can be gathered about life on Earth from fossils?
· How can the relative and absolute ages of rock be determined? (and thus the fossils in it)
· How can sedimentary rock be reformed into new landforms? (thus transporting fossils to new areas)
· How do fossils show that life on Earth has evolved over time?
How is geologic time measured?
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At least one thousand
At least one hundred
At least five hundredRead this article about the layers of the Earth
HOW MANY LICKS DOES IT TAKE TO GET TO THE CENTER?
Have you ever wondered how many licks it takes to get to the core of a giant jaw breaker? 1, 2,3…slurp. The earth is in some ways like a giant jaw breaker. It is composed of several layers: the crust, the mantel, and the core.
CRUST
What comes to mind when you think of the word crust? Perhaps it is the time old saying," Eat your crust!" The earth's crust is a little different then the crust on a piece of bread. It is not soft and chewy, but it hard and composed of different minerals. The thin, outermost layer of the earth is called the crust. It makes up only one percent of the earth's mass. The continental crust is thicker than the oceanic crust. It can range from 25 km thick at the edges to 70 km thick near the center. The oceanic crust on the other hand is only about 7 km thick and considerably more dense. The crust and the uppermost part of the mantle make up the lithosphere, a solid region that is broken into plates. It is about 65 to 100 km thick.
MANTLE
The mantle is the layer below the crust. It makes up almost two thirds of the earth's mass and is about 2900 km thick. The mantel is divided into two regions, the upper and lower sections. Directly below the upper section is the asthenosphere. Heat and pressure cause a small amount of melting to occur in the asthenosphere. While still solid, the asthenosphere is able to flow. The ability of a solid to flow is calledplasticity. See "What's the matter?" for an activity to demonstrate plasticity. Since the asthenosphere is more liquid than the rest of the mantle, the broken lithosphere plates are able to "float" on it.
When the material in the asthenosphere is heated, it becomes less dense and rises. While the cooler material is more dense tends to sink. Circulating currents carry the warmer material up and the cooler material down. These circular currents in the asthenosphere are called convection currents. The circulating convection currents cause the plates to move.
Assignment= Explain in your journal what "convection currents " are.
CORE
Below the mantle is the core, the center of the earth. It makes up nearly one third the mass of the earth. The core is also divided into two regions, the inner core and the outer core. From seismic or earthquake waves, scientists believe the outer core is a liquid and the inner core is a solid. The outer core is made of iron and is very dense. Scientists hypothesize that the circulation of the outer core causes the magnetic field around the earth. It is believed to be circulating in the counter-clockwise direction giving us the north pole in its present location. It switches about every million years. A record of this "switching" is recorded in the rocks both on land and in the ocean crust. See "Go west young man! But which way is north? "The inner core is made of solid iron and nickel. Many scientists believe it is kept in the solid state because of the extreme pressure from the other layers.
Assignment=Please copy this graphic organizer into your journal and complete it from the reading above.
Assignment= Explain in your journal what " It is believed to be circulating in the counter-clockwise direction" means in regards to the spinning of the outer core.
Read this article about Fossils
Fossils Tell A Story By Patti Hutchison |  |
1 Imagine you are hiking in the woods. As you walk up a steep hill, you find a fossil. It is a mold of many tiny seashells. What would sea shells be doing in the middle of the woods?2 Most fossils are found in sedimentary rocks. These rocks form on the surface of the earth. They record the processes that have happened on the surface, including life. Scientists are able to arrange fossils according to age. This is called the fossil record. By studying the fossil record, scientists have found that the earth and its life forms have gone through many changes in the past.
3 Fossils have taught us how and when rock layers have formed. They have also helped scientists learn about life forms that have come and gone. Fossils have even taught us about the climate of the earth long ago.
Assignment=Please copy this graphic organizer in your journal and complete it from the reading above.
4 The Geologic Time Scale is divided into huge blocks of time called eras. Eras are defined by major changes in the fossils found in the sedimentary rock layers that were formed during those time spans.
5 Some species of organisms can be found on both sides of a time period. Usually, however, fossils found in one rock layer are different than fossils found in the layers above and below. This shows that over time, many organisms have become extinct. In fact, scientists believe that for every organism alive today, at least one hundred others have become extinct.
6 A common example of extinct organisms is the dinosaurs. Hundreds of different kinds of dinosaurs lived on earth millions of years ago. But we don't see any today. Many other animals such as wooly mammoths and saber-toothed tigers have also become extinct.
7 By studying the fossil record, scientists have found that many organisms that are alive today have changed over time. For example, fossil evidence has shown that the ancestors of horses were once only the size of dogs.
Assignment=Please copy this graphic organizer in your journal and complete it from the reading above.
8 Fossils can even tell scientists how past life forms looked and behaved. Footprints help them to know how heavy and animal was. The shapes of teeth give clues about what the animal ate.
9 So, what does the fossil of the seashells found in the woods tell scientists? It tells them that the earth once looked very different than it does now. This fossil indicates that those woods were once covered by an ocean.
10 The fossil record can also show us how the earth's climate has changed over time. For example, fossils of coral have been found in arctic regions. Coral is an organism that lives in warm water. This tells us that these frigid areas were once much warmer than they are today.
11 Each time period has left its own impressions in the rock layers. The fossil record has shown how earth and its life forms have affected each other throughout time.
12 By studying the fossil record, scientists have been able to tell the story of the history of our earth.
Assignment=Please copy this graphic organizer in your journal and complete it from the reading above.Copyright © 2011 edHelper
Assignment=Expain in your journal what the fossils can tell us about the change in animals on the Earth.
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Assignment=Please copy this cloze activity in your journal and complete it there.
Fossils Tell A Story By Patti Hutchison | |
Directions: Fill in each blank with the word that best completes the reading comprehension.
(1) _______________________ you are hiking in the woods. As you walk up a steep hill, you find a fossil. It is a mold of many tiny seashells. What would sea shells be doing in the middle of the woods?Most fossils are found in (2) _______________________ rocks. These rocks(3) _______________________ on the surface of the earth. They record the processes that have happened on the surface, including life. Scientists are able to arrange fossils according to age. This is called the fossil record. By studying the fossil (4) _______________________ , scientists have found that the earth and its life forms have gone(5) _______________________ many changes in the past.
Fossils have taught us how and when rock layers have (6) _______________________ . They have also helped scientists learn about life forms that have come and gone. Fossils have even taught us about the climate of the earth long ago.
The Geologic Time Scale is divided into (7) _______________________ blocks of time called eras. Eras are defined by major changes in the fossils found in the sedimentary rock (8) _______________________ that were formed during those time spans.
Some species of organisms can be found on both sides of a time period. Usually, (9) _______________________ ,(10) _______________________ found in one rock (11) _______________________ are different than fossils found in the layers(12) _______________________ and (13) _______________________ . This shows that over time, many organisms have become extinct. In fact, scientists believe that for every organism alive today, at least one (14) _______________________ others have(15) _______________________ extinct.
A common example of extinct organisms is the dinosaurs. (16) _______________________ of different kinds of dinosaurs lived on earth millions of years ago. But we don't see any today. Many other animals such as wooly mammoths and saber-toothed tigers have also become extinct.
By studying the fossil record, scientists have found that many organisms that are alive today have changed over time. For example, fossil evidence has shown that the ancestors of horses were once only the size of dogs.
Fossils can even tell scientists how past life (17) _______________________ looked and behaved. Footprints help them to know how heavy and animal was. The shapes of teeth give clues about what the animal ate.
So, what does the fossil of the seashells found in the woods tell scientists? It tells them that the (18) _______________________ once looked very different than it does now. This fossil indicates that those woods were once covered by an ocean.
The (19) _______________________ record can also show us how the earth's climate has changed over time. For example, fossils of coral have been found in arctic regions. Coral is an organism that lives in warm water. This tells us that these frigid areas were once much warmer than they are today.
Each time period has left its own impressions in the rock layers. The fossil record has shown how earth and its life forms have affected each other throughout time.
By studying the fossil record, scientists have been able to tell the story of the (20) _______________________ of our earth.
Copyright © 2011 edHelper
How do geologists know how old things are?
Geologists routinely work in millions and billions of years, so normal calendars and methods of representing spans of time don't always work very well. Fortunately, being as clever as they are, geologists have managed to come up with other ways.
Ever since the first human looked at a piece of the earth and wondered about it, we have been assigning a relative order to the events which have taken place. This sandstone was deposited first, then this limestone, and then it was folded, and so on. These attempts to decipher the complexity of the earth and arrange it into definable units of time has evolved into the Relative Geologic Time Scale. This works great, and with some guidance from the Laws of Geology , it really helps geologists make sense out of what we see on the surface of the earth.
The problem still remains, though: we may know that the sandstone is older than the limestone, but by how much? In order to answer this, we need to know the ages of the sandstone and limestone, and be able to subtract. Subtraction is easy - the trick is knowing the ages. Unfortunately, there is no way to know the age of something just by looking, and it wasn't until Henry Becquerel discovered radioactivity in 1896 that a way was found to figure out how old rocks are. The first radiometric age dates were calculated in 1907 by a study of how long it takes for uranium to decay into lead. The science has evolved, and geologist routinely calculate "absolute" ages for all kinds of rocks and minerals. But what if our assumptions concerning the decay of unstable elements are wrong? As with everything else in geology, use the data as needed, but don't carve them in stone!
Assignment=Please copy this graphic organizer in your journal and complete it from the reading above.
ASSIGNMENT= Explain in your journal what "absolute dating of rocks" means.
Adding Time
If you wanted to squeeze the 3.5 billion years of the history of life on Earth into a single minute, you would have to wait about 50 seconds for multicellular life to evolve, another four seconds for vertebrates to invade the land, and another four seconds for flowers to evolve—and only in the last 0.002 seconds would “modern” humans arise.
Biologists often represent time on phylogenies by drawing the branch lengths in proportion to the amount of time that has passed since that lineage arose. If the tree of life were drawn in this way, it would have a very long trunk indeed before it reached the first plant and animal branches.
The following phylogeny represents vertebrate evolution—just a small clade on the tree of life. The lengths of the branches have been adjusted to show when lineages split and went extinct.
How We Know What Happened WhenLife began 3.8 billion years ago, and insects diversified 290 million years ago, but the human and chimpanzee lineages diverged only five million years ago. How have scientists figured out the dates of long past evolutionary events? Here are some of the methods and evidence that scientists use to put dates on events: |  |
 1. Radiometric dating relies on half-life decay of radioactive elements to allow scientists to date rocks and materials directly. | |
 2. Stratigraphy provides a sequence of events from which relative dates can be extrapolated. | |
|  3. Molecular clocks allow scientists to use the amount of genetic divergence between organisms to extrapolate backwards to estimate dates. |
Important Events in the History of Life
A timeline can provide additional information about life's history not visible on an evolutionary tree. These include major geologic events, climate changes, radiations of organisms into new habitats, changes in ecosystems, changes in continental positions, and widespread extinctions. Explore the timeline below to review some of the important events in life's history. | |||||||||
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Important Events in the History of Life (text-only version)
View the Graphical Timeline
A timeline can provide additional information about life's history not visible on an evolutionary tree. These include major geologic events, climate changes, radiation of organisms into new habitats, changes in ecosystems, changes in continental positions, and major extinctions. Explore the timeline below to view some of the major events in life's history.
Years ago | Event |
130,000 | Anatomically modern humans evolve. Seventy thousand years later, their descendents create cave paintings — early expressions of consciousness. |
4 million | In Africa, an early hominid, affectionately named “Lucy” by scientists, lives. The ice ages begin, and many large mammals go extinct. |
65 million | A massive asteroid hits the Yucatan Peninsula, and ammonites and non-avian dinosaurs go extinct. Birds and mammals are among the survivors. |
130million | As the continents drift toward their present positions, the earliest flowers evolve, and dinosaurs dominate the landscape. In the sea, bony fish diversify. |
225million | Dinosaurs and mammals evolve. Pangea has begun to break apart. |
248million | Over 90% of marine life and 70% of terrestrial life go extinct during the Earth’s largest mass extinction. Ammonites are among the survivors. |
250million | The supercontinent called Pangea forms. Conifer-like forests, reptiles, and synapsids (the ancestors of mammals) are common. |
360million | Four-limbed vertebrates move onto the land as seed plants and large forests appear. The Earth’s oceans support vast reef systems. |
420million | Land plants evolve, drastically changing Earth’s landscape and creating new habitats. |
450million | Arthropods move onto the land. Their descendants evolve into scorpions, spiders, mites, and millipedes. |
500million | Fish-like vertebrates evolve. Invertebrates, such as trilobites, crinoids, brachiopids, and cephalopods, are common in the oceans. |
555million | Multi-cellular marine organisms are common. The diverse assortment of life includes bizarre-looking animals likeWiwaxia. |
3.5 billion | Unicellular life evolves. Photosynthetic bacteria begin to release oxygen into the atmosphere. |
3.8 billion | Replicating molecules (the precursors of DNA) form. |
4.6 billion | The Earth forms and is bombarded by meteorites and comets. |
Looking for the oldest fossils
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These dendrites might look like fossils, but they are not. | |
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The problem of determining what is and is not a fossil can be especially difficult when it comes to ancient microfossils. Because these fossils are of relatively simple organisms, such as bacteria and single-celled algae, without much in the way of identifying features (like leaves or horns), it can be a challenge to demystify them — to figure out what sort of living thing they represent, if indeed, they represent any living thing at all. For example, the microscopic fossil shown on the left below comes from 2 billion year old rock. It is only 20 microns long — that's less than the width of a human hair! This fossil looks similar to a modern unicellular red algae, Porphyridium (shown on the right below), but from appearances alone, it can be hard to tell what organism the fossil really represents.
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On the left is Eosphaera, a 2 billion-year-old microfossil, and on the right isPorphyridium, a modern unicellular red algae that bears some resemblance to the microfossil. | ||
Compounding the problem, modern microorganisms can sometimes invade minute pores in ancient rocks, making the identification of real fossils tricky. Even worse, geologic chemical reactions can sometimes produce tiny structures resembling simple bacteria and algae — such as the one shown below, which was cooked up by geologists in a lab. If these same reactions occurred on ancient Earth, they might have left behind traces that would easily be mistaken for fossils. With all these red herrings around, how can a paleontologist figure out what is and is not a fossil?
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On the left is a microbe-like cellular filament found in 3.465 billion year old rock, and on the right is a silica-carbonate filament synthesized from inorganic processes in a laboratory. | ||
Luckily, modern technology and scientific knowledge have come to the rescue:
- Improved microscopic and imaging techniques sometimes allow scientists to zoom in on these fossils to identify hallmarks of life, such as the cell wall.
- Advanced chemical analysis tools can compare the chemical makeup of the fossil itself to the surrounding rock to note any indication that the structure was once alive. These techniques for example, can help identify very tiny samples of kerogen, the organic material into which living things decay.
- Elements come in forms with different weights, called isotopes. Carbon-12 and carbon-13 (the heavier of the two) are both common on Earth, but living things prefer to use carbon-12. Sensitive techniques can determine whether a rock or putative fossil contains more carbon-12 than expected, suggesting that the material may once have been alive.
In order to figure out if a microfossil is really a fossil, paleontologists use the tools above, along with other observations, to evaluate the following criteria:
- Does the alleged fossil look "life-like?" In other words, does it have morphological structures consistent with living things?
- Based on geologic information, did the "fossil" form in an environment that could have sustained life and then preserved a fossil?
- Does the "fossil" have a biogeochemical make-up that suggests it was once alive? For example, is it high in carbon-12?
Earth History Web Quest
Directions: Copy in your journal fill in the blank with the correct terms.
1. Earth contains ____________ main layers.
2. The three main layers are the ______________, the______________, and the _______________.
3. The _______________ is the hottest of the three layers of the earth.
4. The theory that the continents were all one large continent is called ________________ or _______________ ___________________.
5. The theory of continental drift was proposed by _____________ _______________.
6. When two of earth plates slide past one another, a ___________ boundary is formed.
7. When two of earth plates move apart from one another, the create a ___________________ boundary.
8. A _________________ is a huge ocean wave caused by a sudden shift of the ocean floor.
POST TEST
Please copy these questions in your journal and answer them there.
· How can live things be preserved inside of a rock?
· How are different fossil types formed?
· What types of evidence can be gathered about life on Earth from fossils?
· How can the relative and absolute ages of rock be determined? (and thus the fossils in it)
· How can sedimentary rock be reformed into new landforms? (thus transporting fossils to new areas)
· How do fossils show that life on Earth has evolved over time?
How is geologic time measured?
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