Earths History

Earths HistoryChapters 29 & 301Paleontology is the study of the life that existed in Prehistoric times.

Traces of ancient life (fossils) are the basis for the geologic time scale and an important part of the rock record.

2A fossil is the remains, impression, or any other evidence of a plant or animal preserved in rock.

Fossils include shells, bones, petrified trees, footprints, impressions made by leaves, or even burrows made by worms.

3Fossilization usually occurs when a dead plant, animal, or impression (ex. footprint) is buried by sediments that gradually turn to rock.

Fossils may form in several ways, from original remains or from replaced remains, by being preserved in either molds or casts, as trace fossils, or in carbonaceous film.

4Types of fossilization

A. Original remains: Fossils are rarely the original unchanged material of plants or animals, but in rare cases, an organism is preserved in its entirety.

Example: amber

5B. Replaced remains: Some fossils form as remains are slowly replaced, molecule by molecule, by rock forming minerals.

Examples: Bones, teeth, shell, petrified wood

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C. Molds and Casts:

1. A brachiopod has died andfallen into soft sediments. Thehard shell remains after thesoft body parts decay.

2. The sediments become rock.The shell decays, leaving amold of the shell.

3. Minerals fill the mold, creatinga cast of the original shell.

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D. Trace fossils: When indirect evidence of life is preserved. Trace fossils include any impressions left in rock by an animal such as trails, footprints, tracks, burrows, and even bite marks on other fossils.

8E. Carbonaceous films: Sometimes the only fossil trace is a thin carbon film resembling a silhouette.

9Relative dating is the process of placing events in the sequence in which they occurred. Relative dating does not identify the actual dates on which the events occurred; it places them in order.

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Back scattered electron (BSE) imaging reveals multiple cross-cutting relationships which provide evidence that the meteorite experienced at least three distinct impacts prior to its arrival on Earth. This image shows that vein 1 is cut by vein 2, and both 1 and 2 are cut by vein 3. 123Index fossils are the remains of organisms that lived and died within a particular time segment of Earths history and can be used to correlate rock layers.

12Absolute TimeRadiometric dating: A technique that measures the ratio of radioactive decay a sample has gone through to determine an age. Examples:Radiocarbon DatingUranium-Lead DatingRubidium-strontium DatingPotassium-Argon DatingAbsolute Time# of half-lives elapsedFraction remainingPercentage remaining01/110011/25021/42531/812.541/166.2551/323.12561/641.56371/1280.781Half-life: The time it takes for half the atoms in a sample of radioactive element to decay to a stable product.

Question: If you have 20g of a radioactive isotope that has gone through two half lives, how many grams were in the original sample? Answer: 80g

4.6 BYA15The geologic time scale is a summary of major events in Earths past that are preserved in the rock record.

16The geologic time scale is divided into eons, eras, periods, and epochs. The eon represents the longest segment of geological time.

Eons are divided into eras.

Eras are divided into periods.

Periods are divided into epochs. (Shortest amount of time.)

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