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Geologic Time

Discovering Earth’s HistoryRocks Record Earth HistoryRocks record geological events and changing life forms of the past.

Jana Fajutagana

on 20 September 2012

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Transcript of Geologic Time

Geologic Time  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.  Rocks record geological events and changing life forms of the past. Discovering Earth’s History Rocks Record Earth History  Uniformitarianism means that the forces and processes that we observe today have been at work for a very long time. A Brief History of Geology • The law of superposition states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it. 1. Law of Superposition  Relative dating tells us the sequence in which events occurred, not how long ago they occurred. Relative Dating—Key Principles Ordering the Grand Canyon’s History • The principle of original horizontality means that layers of sediment are generally deposited in a horizontal position.  2. Principle of Original Horizontality Disturbed Rock Layers • Rocks containing inclusions are younger than the inclusions they contain. • Inclusions are rocks contained within other rocks.  4. Inclusions • The principle of cross-cutting relationships states that when a fault cuts through rock layers, or when magma intrudes other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected. 3. Principle of Cross-Cutting Relationships Applying Cross-Cutting Relationships Formation of Inclusions • An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred. • An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.  5. Unconformities A Record of Uplift, Erosion,
and Deposition  Correlation is establishing the equivalence of rocks of similar age in different areas. Correlation of Rock Layers Correlation of Strata at Three Locations  Unaltered Remains  The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried. • Some remains of organisms—such as teeth, bones, and shells—may not have been altered, or may have changed hardly at all over time.  Fossils are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks. Fossils: Evidence of Past Life Fossil Formation • Carbonization is particularly effective in preserving leaves and delicate animals. It occurs when an organism is buried under fine sediment. • Molds and casts are another common type of fossil. • Fossils often become petrified or turned to stone. • The remains of an organism are likely to be changed over time.  Altered Remains • Two conditions are important for preservation: rapid burial and the possession of hard parts.  Conditions Favoring Preservation • Trace fossils are indirect evidence of prehistoric life.  Indirect Evidence Types of Fossilization  Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers.  The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content. Fossils and Correlation • Fossils can also be used to interpret and describe ancient environments.  Interpreting Environments Overlapping Ranges of Fossils  Mass number is the number of protons plus the number of neutrons in an atom’s nucleus.  Atomic number is the number of protons in the atom’s nucleus.  Orbiting the nucleus are electrons, which are negative electrical charges. Dating with Radioactivity Basic Atomic Structures is the spontaneous decay of certain unstable atomic nuclei. Common Types of Radioactive Decay  A half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope. Half-Life The Half-Life Decay Curve  Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.  Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. Radiometric Dating  An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation.  As a radioactive isotope decays, atoms of the daughter product are formed and accumulate. Radioactive Isotopes Frequently
Used in Radiometric Dating  When an organism dies, the amount of carbon-14 it contains gradually decreases as it decays. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.  Radiocarbon dating is the method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample. Dating with Carbon-14  Radiometric dating has supported the ideas of James Hutton, Charles Darwin, and others who inferred that geologic time must be immense. Importance of Radiometric Dating  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. Taken together, these time spans make up the geologic time scale.
The Geologic Time Scale Structure of the Time Scale  There are three eras within the Phanerozoic eon: the Paleozoic, which means “ancient life,” the Mesozoic, which means “middle life,” and the Cenozoic, which means “recent life.”  Eons represent the greatest expanses of time. Eons are divided into eras. Each era is subdivided into periods. Finally, periods are divided into smaller units called epochs.  The periods of the Cenozoic era are divided into still smaller units called epochs, during which even less profound changes in life forms occur.  Each period within an era is characterized by somewhat less profound changes in life forms as compared with the changes that occur during an era.  During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often. Precambrian Time The Geologic Time Scale  The age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock was first formed. Instead, the date may indicate when the rock was metamorphosed.  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. Difficulties With the Geologic Time Scale Using Radiometric Methods to
Help Date Sedimentary Rocks Formation of an Angular Conformity • A disconformity is when two sedimentary rock layers are separated by an erosional surface. • A nonconformity is when the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.  7. Unconformities
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