GEOLOGIC TIME

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					                                        GEOLOGIC TIME
                                           Relative Dating
                                     The Geologic Time Scale
Introduction
       •Geology involves deciphering the present attributes, history, and origin of the Earth
       •We do this to better understand the Earth and its processes today
       •Geology is an historical science
Uniformitarianism
       •The fundamental assumption of geology
       •The same processes and principles which govern natural changes on Earth today were
       active in the past
       •The laws of chemistry and physics applied in the ancient past, just as they do today
       •The present is the key to the past
Catastrophism
       •Earth history was a series of catastrophes
       •The model was Noah’s flood
                –Entire Earth was covered in water
                –Then the waters receded
       •Cataclysmic events were thought to produce mountains like the Alps and places like the
       Grand Canyon (not known to Europeans at the time)
       •Why catastrophism?
                –No concept of geologic time and that Earth could be old
                –Biblical description of creation in six days
                –Earth was centered on humans
Hutton and Uniformitarianism
       •James Hutton
                –Scottish doctor, gentleman farmer, and geologist
                –Lived from 1726-1797
                –First suggested the concept in 1795
       •Small changes over long periods of time could produce major geologic features
       •Earth is in a constant state of change
       •Earth must have a long history
Hutton and Siccar Point
       •Famous geologic locality
       •Nearly horizontal sequence of sedimentary rocks resting on a second sequence that had
       been compressed into wave-like folds
       •Boundary between the two sequences was an unconformity
               –A gap in the rock record
               –Took time to develop, given rates of erosion
Siccar Point
Siccar Point History
Siccar Point History
Uniformitarianism (cont.)
       •John Playfair (1748-1819)
               –Friend and countryman of Hutton
               –A better writer than Hutton
               –Pressed the concept
       •Charles Lyell (1797-1875)
               –Englishman in philosophical agreement with Hutton
               –Friend of Charles Darwin, and their thinking converged
               –Documented the necessity of uniformitarianism by his observations of landforms
               and rocks
Uniformitarianism (cont.)
       •A theory, well established, not proved
               –No evidence that it has ever been violated
               –It works—Uniformitarianist explanations seem to fit together as a coherent
               whole
       •Useful—Allows us to find modern analogs for ancient processes
Uniformitarianism (cont.)
       •Limitations
               –Same chemical and physical laws
                –Rates can change
                –Location of activity can change
                –Dominance of process can change
Uniformitarianism (cont.)
       •There is still a role for catastrophes
                –They clearly exist
                –Their effect is disproportionately large
       •Uniformitarianism requires that the Earth must be quite old
                –Eastern cultures (Hindus in India) recognized antiquity of the Earth quite early
                –Europeans were much slower
Geologic Time
       •In the 17th century, Archbishop Ussher (1581-1656) of the Irish Protestant Church
                –Counted generations in the Bible
                –Concluded that the Earth was formed in 4004 BCE
       •Dr. John Lightfoot of Cambridge University
                –Earth formed at 9:00 a.m. on October 23, 4004 BCE
       •An ―old‖ Earth produces a major philosophical change
                –Diminishes the significance of man
                –Tends toward view of humans as one step in an on-going chain of events
Catastrophism
       •Prior to Uniformitarianism there was no way to explain fantastic features such as
       mountains, canyons, and fossils on mountain peaks
                –Earth history was a series of catastrophes
                –Much of Earth history was assigned to Noah’s flood
       •Uniformitarianism removes the need for such fantastic explanations
Geologic Time
       •Relative Time
                –Placing events in their correct chronological sequence
       •We should review our notes before the quiz
       •I’ll study after the TV show
                –No measure of the amount of time elapsed between events
               –This is what Hutton and other early geologists did
Geologic Time
      •Absolute dating
               –Had to wait for the discovery of radioactivity
               –Evaluates the amount of time elapsed
      •I arrived on campus on September 8, 1998, at 9:00 a.m.
      •This volcano erupted on June 10, 1956
      •Reading the rock record allows us to put events in the correct chronological order
      •The original geologic time scale is based on relative dating
Reading the Rock Record
      •Original Horizontality
               –Sediments are deposited as horizontal layers
               –If layers are not horizontal, some event has disturbed them
               –Proposed by Nicholas Steno (1638-1687), a Danish court physician, in 1669
               –Ignore details of cross beds
Reading the Rock Record
      •Superposition
               –Also proposed by Steno
               –Younger rocks are deposited on top of older rocks in an undisturbed sequence
               –This means that ancient erosion surfaces (unconformities) are older than the
               rocks above them, but younger than the rocks below them
Unconformities
      •Angular unconformity—Rocks above and below the unconformity do not have the same
      orientation
Grand Canyon
Unconformities
      •Disconformity—Rocks above and below the unconformity have the same orientation
Nonconformity
      •Between rocks of different types, such as igneous and sedimentary or metamorphic and
      sedimentary
Reading the Rock Record
       •Cross-cutting relationships
               –Features are younger than any feature they cut
               –Igneous rocks are younger than any rock they intrude
               –Faults are younger than any layers they displace
               –Rocks that are folded are older than the deformation
Cross-cutting relationships


Inclusions
       •A special case of cross-cutting relationships
–Inclusions are older than the rock layer that surrounds them
       •This means that if you find inclusions of granite in a sandstone, the granite is older than
       the sandstone
               –The older rock had to be there to get incorporated in the newer rock
               –Applies to both igneous and sedimentary rocks
Inclusions
Lateral Continuity (Correlation)
       •Sedimentary layers form continuous sheets that end by thinning to nothing, grading into
       a different type of sediment, or abutting against a barrier which confines the depositional
       area (such as a shoreline)
       •Allows us to trace rock layers long distances, such as the length of the Appalachian
       chain
Ways to Correlate
       •Physical connection
               –Actually trace the layer across the ground
               –The most reliable
       •Similarity of rock types
               –Assume similar rock types formed at the same time across a broad region
               –Must be used with great care because similar rock types can form at different
               times
               –Strength of correlation is enhanced if you can trace a sequence of rock layers
Ways to Correlate
          •Similar fossils—faunal succession
          •A Real Example—The Grand Canyon, Zion National Park, Bryce Canyon National
          Park, and Cedar Breaks National Monument


Grand Canyon to Cedar Breaks
The Rock Units
Faunal Succession
          •13th Century debate about fossils in rocks
                 –Grew from seeds
                 –Marine organisms deposited during Noah’s flood
                 –Placed in the rocks by the Devil to confuse man
          •Leonardo da Vinci first recognized they were marine organisms
          •Robert Hooke suggested they might be useful in the study of Earth history
Faunal Succession (continued)
          •William ―Strata‖ Smith
                 –Widely traveled canal builder
                 –Keen observer
                 –Found he could identify rocks by seeing fossils and tell where in England they
                 came from
          •Since flora and fauna vary with time, rocks of the same age should have the same group
          or assemblage of fossils
          •Enables correlation of rock units over large distances and gaps
Fossils
          •Index Fossil
                 –Used to mark a geologic age
                 –Short-lived species
                 –Widely distributed
Review Diagram
The Geologic Time Scale
          •World-wide unconformities and major extinctions became major boundaries
          •John Lehmann (1756)
             –Alluvial—unconsolidated and stratified rocks formed after Noah’s flood
             –Secondary—Consolidated and stratified rocks formed during Noah’s flood
             –Primary—Crystalline rocks with no fossils; formed at the beginning of the Earth
Geological Time Scale
      •Charles Lyell’s Time Scale (1833)
             –Tertiary—mollusk and mammal remains
             –Secondary—mollusks, cephalopods, ammonites, reptiles including dinosaurs
             –Primary—Unfossiliferous crystalline rocks and sedimentary rocks with fish and
             invertebrate fossils
      •The modern Geological Time Scale
             –Eon  Era  Period  Epoch
The Modern Time Scale
      •Eon
             –Precambrian
                      •Now subdivided   into Hadean, Archean, Proterozoic
      –Phanerozoic
      •Era
             –Hadean, Archean, and Proterozoic are not subdivided into eras
             –Eras of the Phanerozoic
                      •Paleozoic
                      •Mesozoic
                      •Cenozoic
             –Each of the eras are subdivided into periods and epochs
      The Modern Time Scale
             •Periods
                      –Paleozoic Era--Cambrian, Ordovician, Silurian, Devonian, Mississippian,
                      Pennsylvanian, Permian
                      –Mesozoic Era--Triassic, Jurassic, Cretaceous,
                      –Cenozoic Era--Tertiary, Quaternary
                             •More recently,   Paleogene, Neogene, and Quaternary
             •Epoch
                      –Commonly used only in the Cenozoic
                    –Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, Recent or
                    Holocene
                            •Paleogene is   Paleocene, Eocene, and Oligocene
                            •Neogene is   Micocene and Pliocene
                            •Quaternary is   Pleistocene and Holocene


You Should Know
     •Periods of the Paleozoic in order
     •Periods of the Mesozoic in order
     •Periods and Epochs of the Cenozoic in order
     •The following dates
            –Age of the Earth—4.6 billion years ago
            –Proterozoic-Phanerozoic boundary—545 million years ago
            –Paleozoic-Mesozoic boundary—248 million years ago
            –Mesozoic-Cenozoic boundary—65 million years ago
            –Pliocene-Pleistocene boundary—2 million years ago

				
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