Fossils
Fossils are the preserved remains or traces of living things. Fossils provide clues as to how life has changed over time. Most fossils form when living things die and are buried by sediments. The sediments slowly harden into sedimentary rock and preserve the shapes of the organisms. Fossils are usually found in this type of rock.
Most fossils form from animals or plants that once lived in or near quiet water such as swamps, lakes, or shallow seas. When an organism dies, generally only its hard parts leave fossils. Fossils found in rock include molds and casts, petrified or permineralized fossils, carbonized fossils, and tracefossils. Other fossils form when the original remains of organisms are preserved in substances such as tar, amber, or ice.
The most common fossils are molds and casts, which are copies of the shapes of ancient organisms, and contain details as to what they looked like. A mold is a hollow area in rock in the shape of an organism or part of an organism. A mold forms when the hard part of an organism, such as a shell, is buried in sediment. Later, water carrying dissolved minerals may seep into the empty space of a mold. If the water deposits the minerals there, the result is a cast, a solid copy of the shape of an organism.
Petrified or permineralized fossils are fossils in which minerals replace all or part of an organism, thus making them
rock-like.
Another type of fossil is a carbonized fossil which is sometimes called a carbon film, because it is an extremely thin coating of carbon on rock. Everything that lives contains some amount of carbon in them, thus when an organism dies their body will sink into the earth’s layers, decompose, and will leave a thin layer of carbon showing the body’s characteristics. Usually this type of fossil will focus on the evidence of the delicate parts such as plant leaves and insects.
Trace fossils provide evidence of the activities of ancient organisms. Fossil footprints, trails, and burrows are examples of trace fossils. The organism doesn’t have to die to leave his trace . By observing trace fossils, scientists can use inference to determine an animal’s size and behavior, whether it walked on two legs or four legs, or it lived alone.
Some processes preserve the remains of organisms with little or no change. These fossils are called preserved fossils, or originals remains. Organisms can be preserved in tar, amber, or ice.
When certain fossils are used to help determine the age of rocks or other fossils, it is called an index fossil. Index fossils are used to compare, or correlate, rocks exposed in separate locations. Index fossils can be any of the 6 fossils already mentioned- molds, casts, preserved remains, petrified remains, trace, and carbon film.
Index fossils are the remains of organisms that were once abundant in numbers, lived a short time on the Earth, and were widely distribute geographically. In the picture, which fossil is the oldest?
Scientists who study fossils are called paleontologists. Paleontologists collect and classify fossils on similarities and when they lived. Together, all the information that paleontologists have gathered about past life is called the fossil record. The fossil record is used to explain the history of life on Earth by showing how that groups of organisms have changed over time. It reveals that fossils occur in a particular order, which may be inferred that life on Earth has evolved, or changed. Thus, the fossil record is used to provide evidence to support the theory of evolution. Recall that a scientific theory is a well-tested concept that can try to explain a wide range of observations.
The theory of evolution states that there has been a gradual change in living things over long periods of time. The fossil record shows that millions of types of organisms have evolved. Some have become extinct. A type of organism is extinct if it no longer exists and will never again live on Earth.
The diagram labeled Fossil Record shows the changes that occurred over millions of years.
Paleontologist use fossils to try to provide evidence of Earth's climate in the past. Fossils can also be used to learn about past environments and changes in Earth's surface. For example, what did scientists think about the surface of the Earth when seashell molds and casts were found on mountaintops?
In addition to fossils, there are other things that scientist use to try to understand Earth’s history. Ice cores are used to learn about Earth’s past environment even though ice can’t be classified as a fossil because it was never alive. An ice core, which is a tubular sample drilled from places like the polar ice caps,Antarctica,Greenlandor glaciers, shows the layers of ice and snow that have built up over thousands of years. Scientists can learn a lot about ancient climates, including changes in temperature, CO2 concentration, and if there were volcanic eruptions, all based on air, dust, or ash trapped in the ice.
The rings in trees also tell the story of past climate For example, the spaces in the rings indicate wet or dry seasons.
Name______ Fossils
1. The preserved remains or traces of living things are called ______
2. Fossils can give us clues as to ______.
3. Describe the steps as to how most fossils are form.
A. ______
B. ______
4. What type of rock are fossils usually found? ______
Why not igneous or metamorphic rock? (previous knowledge)
______
5. What is sedimentary rock made of?______
6. What do you think? Why do only the hard parts of organisms generally leave fossils?
______
7. Circle the letter of each sentence that is true about molds and casts
a. Molds and casts both copy the shape of ancient organisms.
b. A mold forms when the hard part of an organism is buried in sediment.
c. A cast is a hollow area in sediment in the shape of an organism.
d. Molds and casts do not show details of the organism's structure.
8. What is another name for a petrified fossil? ______.
9. How are the fossils in question 8 formed?______
______
10. What is a carbon film? ______
11. True or False? A carbon film forms when minerals preserve the delicate parts of an organism.
12. Circle the letter of each trace fossil.
a. footprints b. animal trails c. animal shells c. burrows
13. What can a scientist infer by looking at fossil footprints?
______
______
14. True or False? Fossils can form only when the remains of an organism decay.
15. Preserved fossils are also called ______.
16. A. What are three substances in which the remains of organisms have been preserved?
a. ______b. ______c.______
17. Give an example of a preserved fossil. ______
18. A type of fossil that is used to help date layers of rock and/or other fossils is known called
an ______
19. True or False? Index fossils are very different from all the other fossils.
20. Scientists who study fossils are called ______.
21. True or False? Paleontologists classify organisms based on their similarities and when they lived.
22. All the information that paleontologists have gathered about past life is called the ______.
23. The fossil record shows a. ______
and b. ______
24. The fossil record is used to support the theory of evolution. What is evolution?
______.
25. What is a scientific theory? ______
______
26. type of organism that no longer exists and will never again live on Earth is ______.
27. Paleontologists use fossils to try to provide evidence of
a. Earth’s ______in the past
b. Earths ______in the past
c. ______in the Earth’s ______
28. What do you think? What does the presence of seashell molds and casts on mountaintops tell you about the surface of the Earth? ______
29. What is an ice core? ______
30. True or False? An ice core is a fossil.
31. What can ice cores tell scientists about the past environments? ______
______
Absolute Age and the Radioactive Dating of Rocks
Rocks are a form of matter. All the matter you see, including rocks, is made of tiny particles called atoms. When all the atoms of a particular type of matter are the same, the matter is an element.
Most elements are stable. They do not change under normal conditions. But some elements exist in forms that are unstable. Over time, these elements break down, or decay, by releasing particles and energy in a process called radioactive decay. These unstable elements are said to be radioactive. During radioactive decay, the atoms of one element break down to form atoms of another element. Radioactive elements occur naturally in igneous rocks.(rocks formed from magma). For an igneous rock, its "birthday" is when it first hardens to become rock. As a radioactive element within the igneous rock decays, it changes into another element. Therefore, the composition of the rock changes slowly over time. The amount of the radioactive element (parent) goes down. But the amount of the new element (daughter) goes up. The rate of decay of each radioactive element is constant; it never changes. This rate of decay is the element'shalf-life. The half-life of a radioactive element is the time it takes for half of the radioactive atoms (parent) to decay, or change into a stable element(daughter). Scientists use this rate of decay to calculate a rock’s age.
Geologists use radioactive dating to determine the absolute age of rocks.
The absolute age of a rock is its age in numbers of years.In radioactive dating, scientists first determine the amount of a radioactive element in a rock. Then they compare that amount with the amount of the stable element into which the radioactive element decays. So if a rock has 50% parent element X and 50% daughter element Z, and the half life of the X is 100 years, the rock would be 100 years old. If it was 25 % X and 75% Z that means it went through 2 half lives and is 200 years old.
Scientists often date rocks using potassium-40. This form of potassium decays to form the stable element argon-40 and has a half-life of 1.3 billion years. The long half-life of potassium-40 makes it useful in dating the most ancient rocks.
All plants and animals contain some carbon-14, a radioactive form of carbon which changes to Nitrogen 14. Carbon-14 is useful in dating materials from organic substances-plants and animals- that lived less than 50,000 years ago. Because carbon-14 has a half-life of only 5,730 years, it can't be used to date more ancient fossils or rocks. You cannot use radiocarbon dating to determine the age of rocks because they were never alive.Radioactive dating works well for dating igneous rocks but not for sedimentary rocks. Rock particles in sedimentary rocks are from other rocks, all of different ages. Radioactive dating would provide the ages of particles, not the sedimentary rock as a whole. But radioactive dating can be used to determine absolute dates of igneousintrusionsand extrusions near sedimentary rock layers.
Igneous intrusionsare pockets of igneous rock that formed when magma pushed up through cracks and hardened into rock forming dikes, sills, baccoliths, and other formations. Intrusions are always younger than the layers of rock they push through. Igneous extrusionsare rocks that formed from lava on the surface of the earth.Sedimentary rock above an igneous intrusion must be younger than that intrusion.
Diagram (1): Dike B is younger than Sedimentary Rock A. Erosion surface C is younger than Dike B. Sedimentary Rock D is younger than Erosion Surface C.
Diagram (2) Sill B is younger than Sedimentary Rock A. Dike C is younger than sill B.
Diagram (3) Stock B is younger than
Sedimentary Rock A. Dike C is the youngest.
Name ______Absolute Age and Radioactive Dating
1. A type of matter in which all the atoms makingup the matter are the same is called a (n) ______. 2. What does the word stable in paragraph 2 mean? ______
______
3. What are unstable elements called? ______
4. What occurs during radioactive decay? ______
______
5. Circle the letter of the kind of rock for which radioactive dating works well.
a. sedimentary b. igneousc. metamorphicd. layered
6. Carbon -14 changes to Nitrogen 14. Which is the parent element and which is the daughter element? Parent______and Daughter______
7. What is the half life of an element?______
______
8. What is another word for decay? ______Why is it used in the word radioactive decay? ______
9. True or False? Scientists use the rate at which radioactive elements decay in rocks to determine its
relative age.
10. Circle the letter of each sentence that is true about radioactive decay.
a. Over time, the amount of a radioactive element in igneous rock will go up.
b. The rate of decay of each radioactive element is always changing.
c. The rate of radioactive decay is an element's half-life.
d. The half-life of a radioactive element is the time it takes for half of all the atoms in a rock to decay.
11.True or False? Geologists use radioactive dating to find the absolute age of rocks.
12. What is meant by the absolute age of a rock? ______
13. What two things must scientists measure to find the absolute age of a rock?
a.. ______
b. ______
14. True or False? By comparing the amount of the radioactive element with the amount of the stable element, scientists can determine the absolute age of a rock if they know how long it takes for the radioactive element to become stable.
15. Complete the table to compare two different types of radioactive dating.
Elements Used In Radioactive DatingElements / Decays to / Used for Dating / Half-life
Potassium-40 / A. / B. / C.
Carbon-14 / D. / E. / F.
16. Suppose a geologist wants to determine the absolute age of an igneous rock. The geologist thinks the rock might have formed about 400 million years ago. Would you use potassium-40 or carbon-14 to date the igneous rock? Explain your answer. ______
______
17. True or False? Carbon -14 can only be used to date organic materialsless than 50,000 years old.
18. True or False? Radioactive dating can only be used for igneous rocks, not sedimentary rocks.
19. Explain your answer for number 18.______
______
20. What do scientists use to help them date sedimentary layers?
______
21. True or False? Igneous intrusions are always older than the sedimentary layers they cut through.
22. Intrusions are made of ______while extrusions are made of ______.
23. A horizontal intrusion is called a ______. A vertical intrusion is called a ______and.
a large igneous intrusion covering several layers of rock in all directions is called a ______.
(Use the last diagram on the reading sheet to find the answer )