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KEY CONCEPT Specific environmental conditions are necessary in order for fossils to form.

Learn about the formation of fossils and how specific environmental conditions are necessary for their preservation. Discover the different ways fossils can form, such as permineralization, natural casts, trace fossils, amber-preserved fossils, and preserved remains. Explore how radiometric dating and relative dating are used to estimate the age of fossils. Understand the concept of isotopes and the decay of unstable isotopes in radiometric dating. Explore the geologic time scale, which divides Earth's history based on major past events and uses index fossils to determine the relative age of rock layers.

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KEY CONCEPT Specific environmental conditions are necessary in order for fossils to form.

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  1. KEY CONCEPT Specific environmental conditions are necessary in order for fossils to form.

  2. Fossils can form in several ways. • Permineralization occurs when minerals carried by water are deposited around a hard structure.

  3. A natural cast forms when flowing water removes all of the original tissue, leaving an impression.

  4. Trace fossils record the activity of an organism.

  5. Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.

  6. Preserved remains form when an entire organism becomes encased in material such as ice.

  7. Only a tiny percentage of living things became fossils. • Specific conditions are needed for fossilization.

  8. Radiometric dating provides an accurate way to estimate the age of fossils. • Relative dating estimates the time during which an organism lived. • It compares the placementof fossils in layers of rock. • Scientists infer the order inwhich species existed.

  9. protrons neutrons • Isotopes are atoms of an element that differ in their number of neutrons. • Radiometric dating uses decay of unstable isotopes.

  10. Radiometric dating uses decay of unstable isotopes. • Isotopes are atoms of an element that differ in their number of neutrons. • A half-life is the amount of time it takes for half of the isotope to decay.

  11. KEY CONCEPT The geologic time scale divides Earth’s history based on major past events.

  12. Index fossils are another tool to determine the age of rock layers. • Index fossils can provide the relative age of a rock layer. • existed only during specific spans of time • occurred in large geographic areas • Index fossils include fusulinids and trilobites.

  13. 100 250 550 1000 2000 PRECAMBRIAN TIME This time span makes up the vast majority of Earth’s history. It includes the oldest known rocks and fossils, the origin of eukaryotes, and the oldest animal fossils. Cyanobacteria The geologic time scale organizes Earth’s history. • The history of Earth is represented in the geologic time scale.

  14. consist of two or more periods • three eras: Cenozoic, Mesozoic, Paleozoic • Eras last tens to hundreds of millions of years.

  15. most commonly used units of time on time scale • associated with rock systems. • Periods last tens of millions of years. • Epochs last several million years.

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