Geologic Time
Geologic Time
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By examining layers of sedimentary rock, geologists developed a time scale for dividing up earth history.
Earlier in the 20th century, radiometric-dating techniques allowed scientists to ______________
_________________________________________________________ in the geologic time scale.
In this segment, we will learn how geologists:
o determine the ______________________________________________
o determine the __________________________________________________________, and
o how ___________________________________________________________________
__________________________________________________.
How do geologists determine how old rocks are?
o ________________________ -- determine whether the rock is older or younger than other rocks
o _________________________ -- use radiometric dating techniques to determine how long ago the rock
formed in the exact number of years
 *Not all rocks can be dated absolutely, so ______________________________
____________________________.
o Absolute Dating:
 Radiometric Dating Techniques
 _________________________________________________________________
 Isotope -- form of __________________________________________________
__________________________
 Radioisotope -- isotope that __________________________________________
_____________________________
 Rate of Radioactive Decay
 Radioisotopes decay at a _________________________ rate.
 Rate of decay is measured by half-life
 Half-life -- _________________________________________________
______________________________________.
 Decay products
 Radioisotopes may decay to form a different isotope or a stable isotope.
 May be a series of radioactive decays before a stable isotope is formed.
 __________________________________________________________
________________________________________________________________________
_______
 Radiometric Age Dating
 Radioisotopes are trapped in minerals when they crystallize.
 Radioisotopes decay through time, and ____________ isotopes are formed.
 Determining the ratio of ____________________________________
_________________________________________________________
___________________
o Absolute Dating
 Thus, by using the appropriate radioactive isotope (knowing its half-life time), and measuring
the _________________________________________________
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_________________________________________________________________
______________________.
 Radiometric dating uses decay of _____________________________________
______________________.
 Isotopes are atoms of an element that differ in their number of neutrons.
 A ______________________________________________________________
_____________________ of the isotope to decay
Principle of Uniformitarianism
o James Hutton, late 1700s – (considered to be "Father of Geology")
o Hutton realized that most sedimentary layers were deposited from gradual, day-to-day processes. He
realized that it _______________________________________________ rocks. This was far different
from what others believed prior to this time.
o Present is the key to the past"
 whatever ______________________________________________________
______________________ (plate tectonics, volcanism, mountain building, earthquakes,
sedimentation) ______________________________________
___________________________and probably at the same (or very comparable) rates
o ______________________________________________________________
 The comparing of rock units to decipher their age relative to one another
 Principle of Superposition
 ______________________________________________________
___________________________________. (Oldest on bottom, youngest on top)
 May not apply to rocks that have been folded (can get turned upside-down).
o Principles associated with Relative Dating
 Principle of Original Horizontality
 ____________________________________________________________
_________________________________________________________.
 If layers are folded, episode of deformation must have occurred ____________ rocks formed.
Age of folding is younger than youngest deformed rock unit.
 Principle of Crosscutting Relationships
 Any feature (e.g. fault or intrusion) that cuts across rocks is _________________ than the
youngest rock that is cut.
o Principle of Faunal Succession
 Organisms have evolved and gone extinct through time
 Fossil content of rock changes in a systematic way, reflecting evolutionary changes
 Fossil content can be used to help determine age of rock and correlate rocks.
 Paraphrased as "Organisms within rock units change with time".
o Correlation is determining that _________________________________________ (may mean rocks are
the same age)
o Unconformities
 are ______________________________________________________________
__________________________________________. In other words, time has been left out of
the ______________________ geologic rock record.
 There are three (3) principal types of unconformities:
o Angular Unconformity
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______________________________________________________________
__________________________________. Easiest of the three types to recognize because the
units are at an angle truncated with the units above them.
o Nonconformity
 Rocks in a horizontal fashion were _____________________________________
_______________________________ material at which time subsequent deposition of
sedimentary layers commenced.
o ____________________________
 Rocks in a nearly horizontal fashion were _______________________________
________________________________________ by subsequent sedimentary deposition. Only a
small discontinuous layer can be observed (rubble zone or soil profile).
Geologic Time Scale
o Developed in 1800s from ________________________ dating of rocks
o More recently, radiometric techniques have __________________________________
______________________________________________________________________
o Many of the names relate back to localities in England (Ex: Devonian from Devonshire)
Divisions of Geologic Time Scale:
o Eons Eras:
 Paleozoic -- Mesozoic -- Cenozoic
 Oldest -----------------> Youngest
o Periods of the Phanerozoic: Paleozoic Era Permian (youngest)
 Pennsylvanian together with Mississippian are called "Carboniferous" in Great Britain
o Epochs of Tertiary and Quaternary
PaleoceneEoceneOligoceneMiocenePliocenePleistocene
Fossils can form in several ways.
o Permineralization occurs when minerals carried by water are deposited around a hard structure.
o A natural cast forms when flowing water removes all of the original tissue, leaving an impression.
o Trace fossils record the _________________________________________________.
o Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is
buried
o Preserved remains form when an ___________________________________________
_____________________________________ such as ice.
Specific conditions are needed for fossilization
o Only a _________________________________________________________________
__________________________.
Index fossils
o can ___________________________________________________.
o _______________________________________________
o occurred in large geographic areas
o Index fossils include fusulinids and trilobites.