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Chapter 12 Geologic Time 12.1 Discovering Earth’s History Rocks Record Earth History o Rocks ______________ geological events and changing life forms of the past. o We have learned that Earth is much ______________ than anyone had previously imagined and that its surface and interior have been changed by the same ______________ processes that continue ______________. A Brief History of Geology o ____________________ means that the forces and processes that we observe today have been at work for a very long time. The ideas behind uniformitarianism originated with the work of Scottish geologist ______________. Relative Dating—Key Principles o ______________ dating tells us the ______________ in which events occurred, not how long ago they occurred. Law of Superposition o The law of ______________ states that in an ______________ sequence of sedimentary rocks, each bed is ______________ than the one ______________ it and younger than the one below it. Principle of Original Horizontality o The principle of original ______________ means that layers of sediment are generally ______________ in a horizontal position. Principle of Cross-Cutting Relationships o The principle of ______________ relationships states that when a fault cuts ______________ rock layers, or when ______________ intrudes other rocks and crystallizes, we can assume that the fault or intrusion is ______________ than the rocks affected. Inclusions o ______________ are rocks contained within other ______________. o Rocks containing inclusions are ______________ than the inclusions they contain. Unconformities o An ______________ represents a long period during which deposition ______________, erosion removed previously formed rocks, and then ______________ resumed. o An ______________ unconformity indicates that during the ______________ in deposition, a period of deformation (folding or tilting) and erosion occurred. o A ______________ is when the erosional surface separates ______________ metamorphic or intrusive igneous rocks from younger sedimentary rocks. o A ______________ is when two sedimentary rock layers are separated by an ______________ surface. Correlation of Rock Layers o ______________ is establishing the equivalence of rocks of ______________ age in different areas. By correlating the rocks from one ______________ to ______________, it is possible create a more complete ______________ of the geological history of a region. 12.2 Fossils: Evidence of Past Life Fossil Formation o ______________ are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks. o The ______________ of fossil that is formed is determined by the ______________ under which an organism died and how it was buried. o ______________ Remains Some remains of organisms—such as teeth, bones, and shells—may ______have been altered, or may have changed hardly at all over time. o ______________ Remains The remains of an organism are likely to be ______________ over time. Fossils often become ______________ or turned to ______________. ______________ and ______________ are another common type of fossil. ______________ is particularly effective in preserving leaves and delicate animals. It occurs when an organism is buried under fine ______________. o ______________ Evidence ______________ fossils are indirect evidence of prehistoric life such as ______________ o Conditions Favoring Preservation Two conditions are important for preservation: ______________ and the possession of ______________ parts. Fossils and Correlation o The principle of fossil ______________ states that fossil organisms ______________ one another in a definite and determinable order. Therefore, any time period can be recognized by its ______________ content. o ______________ fossils are widespread geographically, are ______________ to a short span of geologic time, and occur in ______________ numbers. Fossil Formation o Interpreting Environments o Fossils can also be used to ______________ and describe ______________ environments. 12.3 Dating with Radioactivity Basic ______________ Structures o Orbiting the nucleus are ______________, which are ______________ electrical charges. o ______________ number is the number of ______________ in the atom’s nucleus. o ______________ number is the number of ______________ plus the number of ______________ in an atom’s nucleus. Radioactivity o ______________ is the spontaneous ______________ of certain unstable atomic nuclei. Half-Life o A ______________ is the amount of time necessary for ______________ of the nuclei in a sample to ______________ to a stable isotope. Radiometric Dating o Each ______________ isotope has been decaying at a ______________ rate since the formation of the rocks in which it occurs. o Radiometric dating is the procedure of ______________ the ______________ ages of rocks and minerals that contain radioactive isotopes. o As a radioactive isotope ______________, atoms of the ______________ product are formed and accumulate. o An ______________ radiometric date can be obtained only if the mineral remained in a ______________ system during the entire period since its ______________. Dating with Carbon-14 o Radiocarbon dating is the method for determining age by comparing the amount of ______________ to the amount of ______________ in a sample. o When an organism dies, the amount of carbon-14 it contains gradually ______________ as it decays. By comparing the ______________ of carbon-14 to carbon-12 in a sample, ______________ dates can be determined. Importance of Radiometric Dating o Radiometric dating has supported the ideas of ______________, ______________, and others who inferred that geologic time must be ______________. 12.4 The Geologic Time Scale Structure of the Time Scale o Based on their interpretations of the rock record, geologists have divided Earth’s 4.56-billion-year history into ______________ that represent specific amounts of ______________. Taken together, these time spans make up the ______________. o ______________ represent the greatest expanses of time. Eons are divided into ______________. Each era is subdivided into ______________. Finally, periods are divided into smaller units called ______________. o There are ______________ eras within the ______________ eon: the ______________, which means “ancient life,” the ______________, which means “middle life,” and the ______________, which means “recent life.” o Each ______________ within an era is characterized by somewhat less profound ______________ in life forms as compared with the changes that occur during an ______________. o The periods of the ______________ era are divided into still smaller units called epochs, during which even less profound changes in life forms occur. Precambrian Time o During ______________ time, there were ______________ life forms. These life forms are more ______________ to identify and the rocks have been ______________ often. Difficulties With the Geologic Time Scale o A ______________ rock may contain particles that contain radioactive isotopes, but these particles are ______________ the same age as the rock in which they occur. o The age of a particular mineral in a ______________ rock does not necessarily represent the time when the rock was ______________ formed. Instead, the date may indicate when the rock was ______________________.