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  • 1. Historical Geology Chapter 12 Geologic Time
  • 2. Determining the Age of the Earth
    • _____________ – Greek (570-470B.C.) realized the fossils were the ancient remains of life on Earth. 
    • _____________ – Greek, (450 B.C.) dug into the Nile River bank and counted the layers. He determined the Earth to be may thousands of years old.
    • _____________– 1779, compared the Earth to a ball of cooling iron. He determined the Earth’s age to be 75,000 years.
  • 3.
    • Several scientists tried to use the saltiness of the ocean.
    • They measured how much salt was being added by rivers and concluded the age to be 90 million years.
    • (Salt precipitates out of ocean water when it becomes supersaturated.)
  • 4.
    • _____________(Lord Kelvin) 1897, used heat conduction combined with actual measurements of the rate of heat flow out of the Earth’s surface.
    • He concluded the Earth to be no more than 20-40 million years old.
    • His calculations upset the biological science community of the time.
    • Most thought the Earth to be much older.
  • 5.
    • _____________ wrote that he would die unhappy if Thompson’s calculation were correct.
    • Radioactivity proved that Darwin was correct.
  • 6. Discovering Earth’s History
    • _____________, 1869, led an expedition down the Colorado River through the Grand Canyon.
    • Rocks record geological events and changing life forms of the past.  
    • How is this history revealed?
    • _____________
  • 7.
    • Geological events are almost meaningless unless they are put into a time perspective.  
    • The Earth is much older than anyone had previously thought and that its surface and interior have been changed.
  • 8. A Brief History of Geology
    • The primary goal of geologists is to interpret Earth’s history .
    • In the mid-1600s, _____________ constructed a chronology or time line of both human and Earth history.
    • He determined the Earth to be 5,000 years old.
    • He believed the Earth to be created in 4004 B.C.
  • 9.
    • In the 1700s, _____________, published his Theory of the Earth.
    • He set forth the _____________.
    • Uniformitarianism means that the forces and processes that we observe today have been at work for a very long time.
  • 10.
    • Scientists know that these processes have not always as observable as they were in the past.
    • EX: large meteorites have hit the Earth even though no one saw them happen.
    • This leads to the belief that the Earth is very old.
  • 11. Relative Dating: Key Principles
    • Relative dating tells us the sequence in which events occurred, not how long ago they occurred.
    • Nicolaus Steno…… _____________ : states that in un-deformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.
  • 12.
    • Principle of _____________ - means that layers of sediment are generally deposited in horizontal position.
    • EX: _____________
  • 13.  
  • 14.
    • Principle of _____________– when a fault cuts through or when magma intrudes other rocks and crystallizes, we can assume the fault and the intrusion are both younger than the rocks affected.
  • 15.  
  • 16.
    • Law of _____________ :
    • _____________ – pieces of one rock contained in another.
    • Ex: _____________ rocks are formed in streambeds.
  • 17.  
  • 18.
    • _____________ – represents a long period during which deposition stopped, and erosion removed previously formed rocks, and then deposition resumed.
  • 19.
    • _____________– indicates that during the pause in deposition, a period of deformation (folding and/or tilting) and erosion occurred.
  • 20.  
  • 21.
    • Two sedimentary rock layers that are separated by an erosion surface are called a _____________.
  • 22.
    • _____________ – means the erosion surface separates older metamorphic or igneous intrusions from younger sedimentary rocks
  • 23.
    • _____________ of Rock Layers – matching rocks of a similar age in different regions.
    • _____________ help with correlation.
  • 24. Distance is 12 miles between these two rock units
  • 25. Distance is 17 miles between these two rock units
  • 26. Fossils: Evidence of Past Life
    • Fossils contain traces of prehistoric life.
    • They are important components of sediment and _____________ rocks
  • 27. Fossil Formation
    • The type of fossil that is formed is determined by the condition under which an organism died and how it was buried.
    • 1. _____________– fully preserved mammoth, insects preserved in amber.
  • 28.  
  • 29.
    • 2. _____________– _____________wood and bones.
    • Mineral rich water soaks into the small pores and cavities of the original organism.
    • The minerals later crystallize.
  • 30.  
  • 31.
    • 3. _____________– common in shelled organisms.
    • Similarly carbonation occurs when pressure squeezes out liquids and gaseous components of an organism leaving behind a thin residue of carbon
  • 32. Fossil Types
  • 33. Indirect Evidence
    • 4. _____________– animal tracks, foot prints, burrows or holes, worm tubs, coprolites or petrified animal dung, gastroliths or gizzard stones
  • 34.  
  • 35.
    • Conditions Favoring Preservation :
    • 1. _____________– slows decay
    • 2. _____________–
  • 36. Fossils and Correlation
    • William Smith in the 18 th century determined that fossils weren’t randomly distributed.
    • Each layer contained distinct fossils that may not occur in the layers above and below it.
  • 37.
    • The Principle of _____________– states that fossil organisms succeeded one another in a definite and determined order.
    • Therefore, any time period can be recognized by its fossil content can recognize any time period.
  • 38.
    • Geologists have identified an order of fossils; an Age of Trilobites, an Age of Fishers, an Age of Coal Swamps, an Age of Reptiles, an Age of Mammals.
    • Once fossils were recognized as tie indicators, they became more useful in correlating rocks of similar age.
  • 39.
    • Index fossils – are…..
    • 1. _____________
    • 2. _____________
    • 3. _____________
    • Ex. _____________
  • 40.  
  • 41. Interpreting Environments
    • Fossils can also be used to interpret and describe ancient environments.
    • Ex: Determining where ocean water and shorelines once were, climate, water temperature, corals indicate warm shallow oceans
  • 42. Dating with Radioactivity
    • Earth is about _____________ billion years old.
    • Basic atomic structure
  • 43.
    • An atom’s _____________ is the number of protons and neutrons in an atom’s nucleus.
    • The number of neutrons can vary, and these variants, or _____________, have different mass numbers.
  • 44. Radioactivity
    • Radioactivity is when atomic nuclei are unstable and spontaneously break apart, or _____________.
    • What is an isotope?
    • _____________
    • An unstable or radioactive isotope of an element is called the _____________.
    • The isotopes that result from the decay of the parent are called the _____________ products .
  • 45.
    • When the unstable nuclei begin to break down, radioactive decay begins and continues until a stable or non-radioactive isotope is formed.
    • Example – _____________ decays until lead 206 (Pb-206) is formed.
    • This process has 13 intermediate steps before the stable Pb-206 is reached in the 14 th
  • 46. Half - life
    • _____________ is the common way of expressing the rate of radioactive decay.
    • _____________ is the amount of time necessary for ½ of the nuclei in a sample to decay to its stable isotope.  
    • The half - life of U-238 is _____________ billion years .
  • 47.  
  • 48. Radiometric Dating
    • is the process by which the age of rocks and minerals can be determined by using certain isotopes.
    • The rates of decay have been precisely measured and _____________ under the physical conditions that exist in the Earth’s outer layers.
  • 49.
    • Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.
    • Example – When uranium is incorporated into a mineral that crystallizes from magma, lead isn’t present from previous decay. The radiometric “clock” starts at this point.
  • 50.
    • As the uranium decays, atoms of the daughter product are formed, and measurable amounts of lead eventually accumulate.
    • The five radioactive isotopes in this table exist in nature and have been useful in determining ages of ancient rocks.
  • 51.  
  • 52.
    • An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation.
    • Example: potassium – argon method stems from the fact that argon is a gas and may lead from the sample making the measurement inaccurate.
  • 53.
    • Cross checking or using more than one radiometric method is used to insure accuracy.
  • 54. Dating with Carbon – 14
    • To date recent events, _____________ is used in a method called radiocarbon dating.
    • _____________ is continuously produced in the upper atmosphere and is quickly incorporated in carbon dioxide. As a result, all organisms contain a small amount of carbon – 14.
  • 55.
    • While the organism lives, carbon – 14 is continually replaced.
    • The ratio of carbon – 14 to carbon – 12 remains constant. 
    • When an organism dies, the amount of carbon – 14 gradually decreases as it decays.
    • By comparing the ratio of carbon – 14 to carbon – 12 in a sample, radiocarbon dates can be determined.
  • 56.
    • Because the half-life of carbon – 14 is only _____________ years, it can be used to date recent geologic events up to about _____________ years ago.
  • 57. Importance of Radiometric Dating
    • Radiometric dating has supported the ideas of James Hutton and Charles Darwin.
    • These dating methods have proved that there has been enough time for the processes we observe to have accomplished tremendous tasks.
  • 58. Other Dating Methods Used
    • 1. _____________ from _____________ trees (3,000 years)
    • 2. _____________ – which are sediment layers found in glacial lakes (15,000 years)
  • 59. Bristlecone Pine
  • 60. Varve
  • 61. The Geologic Time Scale
    • The geologic time scale divides Earth’s 4.56 billion year history into specific units of time.
  • 62. Structure of the Time Scale
    • Time is divided into units, which include:
    • _____________ – longest unit of time
    • _____________
    • _____________
    • _____________ – shortest unit of time.
  • 63.
    • Phanerozoic eon began 450 million years ago. It means “visible life.”
    • There are three eras with in the Phanerozoic.
    • 1 – _____________ – meaning ancient life
    • 2 – _____________ – middle life
    • 3 – _____________ – recent life
  • 64.
    • Any unit of time is separated by a major change in life forms .
    • Each era is subdivided into periods, which are separated by somewhat less significant changes in life forms.
    • The Cenozoic is further divided into epochs.