Geologic Time. Earth s History

Transcription

1 Geologic Time Chapter 12 Earth s History Earth s history is recorded in rocks Rocks record geological events and changing life forms of the past. We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today 1

2 Geologic History In the late 1700s, James Hutton published his Theory of the Earth which included the principle o uniformitarianism Uniformitarianism states that the physical, chemical, and biological laws that operate today have also operated in the geologic past. Relative Dating Identifying which rock units formed first, second, third, and so on. This DOES NOT tell us how long ago these events occurred. 2

3 Law of Superposition In an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it. Oct 1 8:30 AM 3

4 Principle of Original Horizontality Layers of sediment are generally deposited in a horizontal position This means that rocks that you can see a curved pattern must have been moved after deposition. Oct 1 8:30 AM 4

5 Principle of Cross Cutting Relationship When a fault cuts through, or when magma intrudes other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected Oct 1 8:31 AM 5

6 Unconformities Represents a long period during which deposition stopped, erosion removed previously formed rocks, and the deposition resumed. An angular unconformity indicates that during the pause in deposition, a period of deformation and erosion occurred. Two sedimentary rock layers that are separated by an erosional surface are called a disconformity. Correlation of Rock Layers To develop a geologic time scale that can be applied to the entire Earth, rocks of similar age in different regions must be matched up. Because rock correlation varies with location, more than one location is needed to give a more accurate correlation. 6

7 Oct 1 8:32 AM Fossils Fossils are the remains or traces of prehistoric life. Important components of sediment and sedimentary rocks 7

8 Fossil Formation The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried. Unaltered Remains some remains are not changed over time. Bones, teeth and shells are often unaltered. Altered Remains Most are changed over time. Molds and Casts Another common type of fossil. Mold is created when a shell or other structure is buried in sediment and then dissolved by underground water Cast fossils are created if the hollow spaces of a mold are later filled with mineral matter. 8

9 Carbonization Preserves leaves and delicate animal forms Occurs when an organism is buried under fine sediment. Pressure squeezes out the liquid and gaseous components and leaves behind a thin residue. Conditions Favoring Preservation Two conditions are important for preservation: rapid burial the possession of hard parts 9

10 Fossils and Correlation The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order. A time period can be recognized by its fossil content. Index Fossils Widespread geographically Limited to a short span of geologic time Occur in large numbers The presence of index fossils provides information about matching rocks of the same age. 10

11 Radioactivity Unstable nuclei spontaneously break apart, or decay Nuclei is called the parent The decay is called the daughter products. Alpha Beta Gamma Half life Common way of expressing the rate of radioactive decay The amount of time necessary for one half of the nuclei in a sample to decay to its stable isotope 11

12 Radiometric Dating Provides a way to calculate the ages of rocks and minerals that contain certain radioactive isotopes. Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. An accurate radiometric date can be obtained only if the mineral remained in a closed system Dating with Carbon 14 Used to date recent events Carbon 14 is continuously produced in the upper atmosphere. All organisms contain carbon 14 When an organism dies, the amount of carbon 14 gradually decreases as it decays. By comparing the ratio of carbon 14 to carbon 12, a date can be determined. 12

13 Geologic Time Scale Based on their interpretations of the rock record geologists have divided Earth s 4.56 billion year history into units that represent specific amounts of time Structure Eons represent the greatest expanse of time Eons are divided into Eras Eras are divided into periods Periods are divided into epochs 13

14 Phanerozoic Means visible life The eras in the Phanerozioc include the Paleozoic, Mesozoic, and Cenozoic Paleozoic ancient life Mesozoic middle life Cenozoic recent life Difficulties A sedimentary rock may contain particles that contain radioactive isotopes, but these particles are not the same age as the rock in which they occur. The age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock first formed. Instead, the date may indicate when the rock was metamorphosed. 14

15 Mar 2 1:57 PM 15