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

Transcription

1 I. What is a Fossil? Studying The Past A. Definition: The evidence or remains of once-living plants or animals Why Do We Study Fossils Found in Rocks? QQ#1 what is a fossil? Why are they important? 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 II. Types of Fossils A. Original Preservation - plant or animal remains that have not undergone change since death. 1. Uncommon because frozen, extremely dry, or oxygenfree environments are required to form these fossils 2. Examples: QQ#2 can you think a. Mummified humans of a fossil like this? b. Frozen organisms (Ice Man) c. Mammoths & cats in La Brea Tar Pits d. Fossilized insects in tree sap (amber)

3 B. Altered Hard Parts - all organic material is replaced with rock 1. Minerals seep in slowly and replace the original organic tissue with silica, calcite, or pyrite, forming a rock-like fossil 2. The fossil has the same shape as the original object, but is chemically more like a rock! 3. Examples: a. Petrified wood b. Recrystallized shells

4 C. Molds and Casts - Common with shellfish 1. A mold is an impression/cavity of the shell of an organism. 2. Cast - cavity might later become filled with minerals or sediment.

5 D. Trace Fossils - evidence of life other than the organism itself. 1. Provide information about how an organism lived, moved or obtained food 2. Examples: a. worm trails b. burrows c. footprints QQ#3 These fossils give us more information than just the way organisms look. What kind of info do we get? Why is that

6 E. Index Fossils used to define and identify geological periods 1. Layers of sediment may look different but contain index fossils of the same species and are thus from the same time period 2. Must be easily recognized, abundant, and widely distributed geographically 3. Must have lived during a relatively short time period 4. Examples: most fossil-bearing rocks formed in the ocean, thus most major index fossils are marine organisms The mollusk Ecphora has a distinctive shape

7 IV. Dating Fossils A. Relative-Age Dating: Dating rocks and fossils by placing them in chronological order without exact dates. 1. The Law of Superposition: in an undisturbed sequence the oldest rocks are at the bottom and each successive layer is younger 2. Principle of Cross-Cutting Relationships: an intrusion or a fault is younger than the rock it cuts across 3. Correlation matching rocks/fossils of one geographic region to another Used to date rock layers that are far apart from each other Geologists examine rocks for distinctive fossils (index) and features to help identify and date them

8 Relative Dating - Applying the Concepts QQ#4: Which is the oldest rock unit in the outcrop? QQ#5: Explain why the rock layers on the west side of the outcrop do not match the east side. QQ#6: Which is the younger layer, layer A or layer C?

9 Absolute- Age Dating used to determine the actual age of a rock, fossil, or another object. 1. Tree Rings - Each ring represents 1 year of growth 2. Varves are bands of sediment that show a yearly cycle from climate change Although accurate, neither method can be used to date very far back in time!

10 Radioactive Dating Dating fossils based on the amount of radioactive material remaining in a substance over time a. Basic Atomic Structure: Atoms consist of a nucleus composed of protons and neutrons, surrounded by orbiting electrons b. Isotopes: An atom with an abnormal amount of neutrons. This makes atoms unstable and they may break down. Radioactive

11 The Process of Radioactive Dating 1. Radioactive substances (unstable atoms) emit protons and neutrons at a constant rate 2. As the numbers of nuclear particles changes during decay, the original element (parent) is converted to a different element (daughter) 3. The rate of emission remains constant regardless of environmental changes. 4. Since the rate of decay is constant, you can measure the parent to daughter ratio to determine the age of the rock Half- Life: The length of time it takes for one-half of the original amount of an element to decay QQ#7 explain half life in your own words

12 Carbon and Radioactive Dating 1. All living organisms contain carbon-12 and carbon Even though carbon-14 slowly decays, it is continuously replaced so that the ratio of carbon-14 to carbon-12 is constant while the organism is living» Food in poop out = constant ratio 3. When the organism dies, carbon-14 is no longer replaced and the constant ratio of carbon 14 to carbon-12 changes 4. An estimate of the amount of time that has lapsed since the death of the organism can be found using radioactive dating to determine the amount of carbon-14 found in dead tissue compared to the amount of carbon 12 (which does not decay).

13 QQ#8 a. Why is carbon dating good for looking at the age of once living things? b. Explain how carbon dating works

14 time Example: Uranium-238 Lead half-life 2 half-lives %U left 0 years 100 % U 4.5 billion years 50 % U 50 % lead Graph 9 billion years 25 % U 75 % lead Do you want to see a simulation?