Geologic Time
Geologic Time helps us visualize the Age of the Earth. By Studying the Earth’s
history, we can better understand how life has changed on this planet. Modern
evidence supports the theory that life began in the oceans and through many
changes has evolved into the forms we see today.
Early Attempts to Age the Earth:
4.6 Billion Years Old
• Estimates based on the bible: Bishop Usher (1581-1656) calculated the
“Biblical creation" of Earth by studying geneology. He concluded the Earth
began in 4004 B.C.
• Estimates based on Salinity: John Joly calculated the amount of salt in the
oceans and carefully determined the amounts of salt in rivers. He estimated
the earth to be 90 million years old.
• Estimates based on the rate of deposition: Individuals have tried to determine
how long it would take to form the largest strata.
• Estimates based on heat lose from Earth: Lord Kelvin estimated this by going
into the deepest caves and taking their temperature. He calculated the earth’s
age at about 100 million. He did not take into account radioactivity.
• Estimates based on age of the sun: In the late 1800’s the only known source
of energy was coal. Tried to figure how long a “sun of coal” would burn. The
estimate was about 1,000,000 years.
• Along came James Hutton (Scottish genius), he concluded by direct
observations of rock strata that a vast amount of time was
necessary for formations to have formed.
If you could compress the history of the Earth (4.6 billion years) into one year, it
would look like this: Jan  Dec.
• Oldest known rocks: Jan Mid- March
• First Living things in Ocean: May
• First Land Plants and Animals: Late November
• Coal swamps: 4 days in early December
• Dinosaurs dominant: Mid-December. They reign for 4 days
• Most Ice sheets recede: 1 minute, 15 seconds before midnight
• Rule of Rome 11:59:45-11:59:50 (5 seconds)
• Columbus discovers America: 3 seconds before midnight
• Hutton (1788): 1 second before midnight
Evidence of Geologic Time in the Rock Record:
• To understand the evidence it is necessary to review some geology- such as
classification. Rocks are classified into three categories: Igneous,
Metamorphic, or Sedimentary. Igneous rocks form from cooling of molten
(melted) rock- thus they do not preserve fossil evidence.
• Metamorphic rocks are formed when rocks undergo change due to extreme
heat and/or pressure- fossils are usually destroyed.
• Sedimentary rocks are formed and deposited at the surface by wind and
water. Most fossils are formed in Sedimentary rocks. Because the Earth is
dynamic (always changing) – rock cycle.
• Sedimentary rocks can be buried so deep that heat and pressure can change
them to Metamorphic rocks or they can be melted and become Igneous rocks.
Thus, our fossil record is only a tiny window into what has lived on this Earthmuch of the evidence has been destroyed by natural processes.
• Each rock type tells a story of its formation. Sedimentary rocks tells us many
things and can be classified into different types based on their environment of
deposition.
• Clastic sedimentary rocks are composed of fragments or pre-existing rocks:
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examples are sandstone and shale.
A chemical sedimentary rock is composed of material precipitated directly from
solution: examples include gypsum and halite or salt. A third type of
sedimentary rock is organic. These rocks are composed mostly of the remains
of plants and animals: examples are coal and limestone.
Since this planet is ¾ water, much of our fossil evidence is marine.
Deposition of sedimentary rocks in the ocean depends on size of particles and
the energy of the environment. The beach is a high energy environment.
The most common rock type deposited here is sandstone. Further from shore,
the particles carried by water get smaller.
Thus the Continental Shelf one might expect to find deposits of silt/mud. In the
open ocean, one would expect to find the formation of limestone because the
sediments from land have already been deposited near shore.
• Draw a marine profile and the expected sedimentary rock deposits.
• Biostratigraphy is the study of layered rocks using fossils. By correlating rock
strata and fossils, evidence of organic extinctions and faunal succession can
be seen.
• The dating of rock strata can be done in one of two ways: Relative dating and
Absolute dating.
• Relative dating provides information on which rock strata is older but does not
tell how old. Determining relative age is like going to the mall and estimating
the age of the shoppers you see. You can tell who is old and who is young
even though you do not know their exact age.
• Absolute dating is used to estimate the actual age of the rock using
radiometric dating.
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Relative Age Dating (certain principals apply)
Original Horizontality- layers tend to be laid in a horizontal fashion. For
example: if one finds shark teeth in rock strata – it is assumed that the rock
and the teeth were deposited at the same time.
Superposition- younger rock strata is laid down on top existing strata. Thus
younger rocks are found on top with older rocks on the bottom (includes:
sedimentary and igneous)
• Cross-cutting Relationships: Intrusive igneous and faults must be younger than
the strata they cross.
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Law of Uniformitarianism- (Hutton) " the processes that we see today are the
same processes that occurred long ago."
• Unconformities- first recognized by Hutton. Unconformities are apparent time
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gaps in the rock record. They can be caused by: erosion- rock layers eroded
away and appear to be missing. Another explanation is that representative
rock layers of that time were never deposited.
Angular Unconformity- occurs when the deposited rock strata is uplifted at an
angle and is eroded flat. Horizontal layers are then deposited on top.
Absolute Age Dating:
This is called Radiometric dating- it involves using radioactive isotopes of
certain elements found naturally occurring in rocks and fossils. An isotope is
an atom with the same atomic number but a different number of neutrons
(unstable). An example is Carbon 12- normal vs. Carbon 14- radioactive
isotope
Each different isotope decays at a certain known rate. This is called the “Half
Life” because it represents the amount of time needed to decay ½ of the
radioactive isotopes to decay.
Examples of isotopes used in Geologic Dating: