Studying The Past. II. Why Do We Study Fossils Found in Rocks?

Transcription

1 Studying The Past I. What is a Fossil? A. Definition: The evidence or remains of once-living plants or animals II. Why Do We Study Fossils Found in Rocks? A. To provide evidence of the past existence of life forms B. To provide information about past environmental conditions C. To provide evidence that populations have undergone change over time due to environmental changes (evolution)

2 III. Types of Fossils A. Unaltered Remains 1. Description: plant or animal remains that have not undergone change since death. a. Uncommon because frozen, extremely dry, or oxygenfree environments are required to form these fossils b. Examples: Mummified humans Frozen organisms (Ice Man) Mammoths & cats in La Brea Tar Pits Fossilized insects in tree sap (amber)

3 B. Altered Remains 1. Description: all organic material has been removed and the hard parts of the organism have been changed a. Minerals seep in slowly and replace the original organic tissue, forming a rock-like fossil b. The fossil has the same shape as the original object, but is chemically more like a rock! c. Examples: Petrified wood Recrystallized shells

4 B. Altered Remains - Continued 2. Types of Altered Remains - Molds and Casts a. An organism dies and gets trapped/buried in sediment (sand, ash, etc) b. The original organism decomposes or dissolves away leaving a hollow impression of the organism (mold) c. This cavity might later become filled with minerals or sediment to create a cast. d. Examples: Plant fossils Trilobites

5 C. Indirect Evidence 1. Description: Trace fossils of plant and animal life a. Provide information about how an organism lived, moved or obtained food b. Examples: Coprolites (animal waste) Burrows/trails footprints

6 IV. Dating Fossils A. Relative-Age Dating: 1. Definition: Dating rocks and fossils by placing them in chronological order without exact dates. 2. Geologic Principles (used in this dating process): a. Original Horizontality Sedimentary rocks are deposited in horizontal layers b. The Law of Superposition in an undisturbed sequence the oldest rocks are at the bottom and each successive layer is younger c. Principle of Cross-Cutting Relationships: an intrusion or a fault is younger than the rock it cuts across

7 3. Other Means of Determining Relative Age a. Correlation Used to date rock layers that are far apart from each other Geologists examine rocks for distinctive fossils and features to help identify and date them b. Inclusions If a rock contains fragments of another rock, than the rock that is containing the fragments must be younger than the fragments themselves c. Unconformities Gaps in the geologic record May indicate erosion or deformation of the rock layers

8 Relative Dating - Applying the Concepts 1. Which is the oldest rock unit in the outcrop? 2. Which is the youngest rock unit in the outcrop? 3. What does H represent? 4. Why don t the layers on the left side of the diagram match with the layers on the right side of the diagram? 5. What do we call what is happening between layer F and M?

9 B. Absolute-Age Dating: 1. Definition: Dating rocks and fossils by using techniques to determine their actual age. 2. Radioactive Dating: Dating fossils based on the amount of radioactive material remaining in a substance over time When nuclei are unstable they spontaneously break apart (decay) in a process called radioactivity The original unstable element (parent) is converted to a different stable element (daughter) Since the rate of decay is constant, you can measure the parent to daughter ratio to determine the age of the rock The length of time it takes for one-half of the original radioactive amount to decay is called the elements half-life

10 B. Absolute Age Dating Continued 3. Uranium 238 When rock forms from magma, it contains U-238 and there is no lead present U-238 can be used to date rocks formed in the early Earth 4. Carbon 14 When an organism is alive, it s C-14 is continuously replaced When that organism dies, the C-14 decays to nitrogen and it does not get replaced C-14 can be used to date geologic events involving organisms within the past 30,000 years

11 time Example: Uranium-238 Lead half-life 2 half-lives 0 years 100 % U 4.5 billion years 50 % U 50 % lead Graph 9 billion years 25 % U 75 % lead %U left

12 C. A Special Case of Relative/Absolute Dating - Index Fossils 1. Description: Remains of unique species that can be used to correlate rock layers or to date a particular rock layer a. Must be easily recognized, abundant, and widely distributed geographically b. Must have lived during a relatively short time period c. If we can date a rock layer absolutely (exact age), then we can use the index fossil to date similar rock layers absolutely d. If we can only date the rock layer relatively (approximate age), then we can use the index fossil date similar rock layers relatively. e. Examples: Ammonites were common 245 to 65 million years ago, when they went extinct