CHAPTER 13:
GEOLOGIC
TIME
Geology is the “Science of Time”
Oldest known rocks on Earth: gneiss of the Nuvvuagittuq
greenstone belt, Québec, formed ~4.28 bya
What are some of the geological events that had to take place
before this rock in Sego Canyon Utah was available for painting?
Imagine the amount of time required first for these rocks to form, then for
uplift to take place and finally for this deep canyon to be eroded.
Age of the Earth
• In 1788, before the Royal Society of Edinburgh, James Hutton theorized that
the Earth was so old that ...
"The result ... of this physical enquiry is that we find no vestige of a beginning, no
prospect of an end."
This statement was very insightful, but it is no longer strictly true.
Why not?
How do geologists measure time?
• Relative dating is a system of reasoning that is used to determine the
chronological sequence or order of a series of geologic events
• Radiometric (or absolute) dating uses the natural phenomenon of decay of
radioactive isotopes within minerals to calculate their chronological age
Radiometric techniques are not the only way to acquire
absolute geological dates. Can you think of any others?
•
Why is it important to study geologic time?
Some important events in the Earth’s history
Early Estimate
• Early estimates of Earth’s age were
based on religious dogma
• In the mid 1600s, Archbishop James
Ussher use the bible to “determine”
the exact date of the creation of the
Earth as ...
“… nightfall preceding 23 October
4004 BC”
The Age of the Earth
•
Scientific analysis of Earth’s age,
based on thickness and assumed
depositional rates of sedimentary
rocks, and of calculations of the time
it has taken for the Earth to cool
down, led to estimates of hundreds
of millions of years.
• Beginning in the late 1700s,
philosophers and naturalists, and
geologists using Relative Dating
techniques, proposed theories of
"Deep Time".
• In the 1900s, geologists using
Radiometric Dating estimated the
age of the Earth to be ...
• ~4.55 - ~4.6 by
William Smith’s 1815 geological map of England and Wales
Geologic Events Used to Determine Age
•
•
•
•
•
•
Five types of geologic events can be used to determine relative ages
Intrusion
Erosion
Deposition
Faulting
Rock deformation
• Intrusion and erosion in this case
Which of these rock types
is the oldest?
Deposition, faulting & erosion
Erosion & deposition
Deposition, faulting and erosion
Deposition, rock deformation and erosion
Seven stratigraphic principles
•
Seven stratigraphic principles are used in relative dating
Principle of superposition
Principle of
original horizontality
What is the basis of the principle of original horizontality?
Principle of
original lateral continuity
Principle of
cross-cutting relationships
Principle of inclusions
Principle of unconformities
Which rock type is youngest in each of these two situations?
The principle of faunal succession (evolution)
How could you apply the principle of faunal succession to date rocks?
Relative dating determines the order of
geologic events
(Follows tilting
and erosion)
Rise and fall of relative sea level
produces certain rock sequences
North America
during the
Cretaceous
The interior seaways of the Cretaceous are not just a result of a high
sea level. What other geological process is responsible?
Radioactive Decay
• Radiometric dating is based on constant and known rates of radioactive decay
to estimate the age of geologic samples
The concepts of radioactive decay and halflife
What are the relative proportions of the parent and daughter
elements at 2 half-lives and at 4?
Samples
• Geologists must select an appropriate radioisotope when dating a sample…
appropriate to the type of material being studied and to its expected age.
Which isotope pairs might be useful to date something that is
expected to be around 1 million years old?
Accurate dating requires understanding
numerous sources of uncertainty
• What is the relationship between
the sample and the event it
represents?
• What is the approximate age of the
sample?
• What radioisotope is most
appropriate?
• What are the potential sources of
uncertainty in the analytical
process?
Above is a gneiss intruded by a granitic dike. If you dated the
gneiss, what would that tell you about the age of it’s parent rock,
and what about the age of the dike?
Uranium-238 Decay Chain
238U
is an appropriate radioisotope to date Earth's age. Why?
Radon in buildings is the second leading
cause of lung cancer
Potassium-Argon
• Potassium is abundant in the crust in minerals like feldspar, mica and
amphibole
• 40K decays to 40Ca and 40Ar
• Used to date igneous rocks
• Original atmospheric argon is a factor
• Potassium-Argon is an extremely important isotopic clock for older igneous
and metamorphic rocks.
Carbon dating is ideal for relatively young
carbon-rich material
Earth’s age is measured using several
independent observations
• "Absolute" ages of primordial crust, Moon, and meteorites
• Abundance of lead isotopes
• Consistency between these observations
Oldest radiometrically dated mineral ~4.4 bya
The sample shown here is sandstone from the Narryer Gneiss of Western
Australia. The dates are from tiny grains of the mineral zircon (not visible
here). Are they representative of the age of the sandstone? If not, why not?
Oldest radiometrically dated rocks
?
This garnet-bearing gneiss of the Nuvvuagittuq greenstone belt,
on the Québec shore of Hudson Bay, has been dated at ~4.28
bya. Why are such old rocks so rare on earth?
Most meteorites formed ~4.4 - ~4.7 bya
The oldest moon rocks formed ~4.3 - ~4.6 bya. There is relatively little
rock on the moon that is younger than a few bya. Why not?
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