Presentation on theme: "II. Why Do We Study Fossils Found in Rocks?"— Presentation transcript:
1 II. Why Do We Study Fossils Found in Rocks? Studying The PastWhat is a Fossil?A. Definition: The evidence or remains of once-living plants or animalsII. Why Do We Study Fossils Found in Rocks?To provide evidence of the past existence of life formsTo provide information about past environmental conditionsTo provide evidence that populations have undergone change over time due to environmental changes (evolution)
2 III. Types of Fossils Original Preservation Description: plant or animal remains that have not undergone change since death.Uncommon because frozen, extremely dry, or oxygen-free environments are required to form these fossilsExamples:Mummified humansFrozen organisms (Ice Man)Mammoths & cats in La Brea Tar PitsFossilized insects in tree sap (amber)
3 B. Altered Hard PartsDescription: all organic material has been removed and the hard parts of the organism have been changedMinerals seep in slowly and replace the original organic tissue, forming a rock-like fossilThe fossil has the same shape as the original object, but is chemically more like a rock!Examples:Petrified woodRecrystallized shells
4 C. Index FossilsDescription: Remains of unique species that can be used to correlate rock layers or to date a particular rock layerMust be easily recognized, abundant, and widely distributed geographicallyMust have lived during a relatively short time periodExamples:The mollusk Ecphora has a distinctive shape
5 D. Molds and CastsDescription: Fossils that do not contain any shell or boneA mold is formed when original shell parts are weathered and eroded, leaving an impression of the shell.This cavity might later become filled with minerals or sediment to create a cast.Examples:Common with shellfish
6 E. Trace FossilsDescription: Indirect evidence of plant and animal lifeProvide information about how an organism lived, moved or obtained foodExamples:worm trailsburrowsfootprints
7 IV. Dating Fossils Relative-Age Dating: Definition: Dating rocks and fossils by placing them in chronological order without exact dates.Geologic Principles (used in this dating process):Original HorizontalitySedimentary rocks are deposited in horizontal layersThe Law of Superpositionin an undisturbed sequence the oldest rocks are at the bottom and each successive layer is youngerPrinciple of Cross-Cutting Relationships:an intrusion or a fault is younger than the rock it cuts across
8 3. Other Means of Determining Relative Age CorrelationUsed to date rock layers that are far apart from each otherGeologists examine rocks for distinctive fossils (index) and features to help identify and date themRelative Dating - Applying the ConceptsWhich is the oldest rock unit in the outcrop?Explain why the rock layers on the west side of the outcrop do not match the east side.3. Which is the younger layer, layer A or layer C?
9 B. Absolute-Age Dating: Definition: Dating rocks and fossils by using techniques to determine their actual age.Methods:Tree Rings and Seasonal Climatic ChangesEach tree ring represents 1 year of growthVarves are bands of sediment that show a yearly cycle from climate changeAlthough accurate, neither method can be used to date very far back in time.
10 Methods cont.:Radioactive Dating: Dating fossils based on the amount of radioactive material remaining in a substance over timeRadioactive substances (unstable atoms) emit protons and neutrons at a constant rateThe original element (parent) is converted to a different element (daughter)Since the rate of decay is constant, you can measure the parent to daughter ratio to determine the age of the rockThe length of time it takes for one-half of the original amount to decay is called the elements half-life.
11 Absolute Dating - Applying the Concepts Ex. Uranium-238 will decay into an isotope of lead, Pb - 206
12 1 half-life 2 half-lives 9 billion years 4.5 billion years 0 years Example: Uranium-238Lead 2061 half-life2 half-lives9 billion years25 % U75 % lead4.5 billion years50 % U50 % lead0 years100 % UGraphDo you want to see a simulation?%U lefttime