Chapter 12 Geologic Time



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.

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 ______________________.

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