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Geology
Introduction to Geology
Geology literally means "study of the Earth."
• Historical geology
examines the origin
and evolution of our
planet through time.
• Physical geology
examines the
materials and
processes of the
Earth.
Geological Time
© NASA
Geologic Time
• Relative Dating: Putting geologic events into
proper order (oldest to youngest), but without
absolute ages. We use a number of principles and
laws to do this:
• Law of Original Horzontality - Sedimentary units and
lava flows are deposited horizontally.
• Law of Superposition - the layer below is older than the
layer above.
• Principle of fossil succession - life forms succeed one
another in a definite and determinable order and therefore a
time period can be determined by its fossils.
• Law of Cross-cutting Relationships - A rock is younger
than any rock across which it cuts.
Geologic Time
• Absolute (Radiometric) Dating:
Using radioactive decay of elements to
determine the absolute age of rocks.
This is done using igneous and
metamorphic rocks.
Radiometric dating: Discovery
http://en.wikipedia.org/wiki/Image:
Henri_Becquerel.jpg
• In 1896, Discovery of radioactivity paved
the way for the precise dating of events
in the geological record
Henri Becquerel
(1852-1908)
Radiometric dating: Decay
en.wikipedia.org/wiki/Image:Alpha_Decay.svg
• Radioactive ‘parent isotopes’ spontaneously emit protons and
neutrons and decay into ‘daughter isotopes’
• E.g., Uranium-238 decays into Lead-206
Radiometric dating: Half life
Half life: 0
Half life: 1
Exponential
Linear
Half life: 2
• The rate of decay from parent to daughter isotope depends on
its half life. The half life is the amount of time needed
for half the parent isotope to decay to daughter isotope.
Radiometric dating: Clocks
Decay series
40K
to 40Ar
147Sm
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1250 Ma
1060 Ma
235U
to 207Pb
704 Ma
238U
to 206Pb
4468 Ma
14C

to 143Nd
Half life
to 14N
5370 years
Different radioactive isotopes have different half lives
Isotopes with long half lives are useful for dating old rocks.
Radiometric dating: Meteorites
http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg
Canyon Diablo meteorite
en.wikipedia.org/wiki/Image:Canyon-diablo-meteorite.jpg
Crab Nebula
• Radiometric age of meteorites
date the formation of the
Solar System and Earth
(4550 million years old)
Geologic Time

The oldest rocks that have been dated on Earth are
merely 4.03 billion years old (gneisses in Canada).
How, then, do we know that the Earth is 4.6 billion
years old? There are a number of pieces of evidence:
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Some grains of sand in Australia have been dated at 4.2-4.4
billion years - this suggests that there were rocks around at
that time even if they are no longer there,
Rocks collected from the Moon have yielded ages as old as
4.6 billion years old,
Nearly every meteorite that has the same composition as
the Earth has an age of 4.6 billion years.
Geologic Time
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The concept of geologic time is new
(staggering) to many nongeologists.
The current estimate is that the Earth is
~4,600,000,000 (4.6 billion) years old.
As humans we have a hard time
understanding the amount of time required
for geologic events.
Radiometric Dating: History
http://en.wikipedia.org/wiki/Geologic_time_scale
© World Health Org.
first life
en.wikipedia.org/wiki/Image:Eopraptor_sketch5.png
dinosaurs
© NASA
origin of Earth
first complex cells
humans
Geologic Time
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We have a good idea of how long a century
is. One thousand centuries is only 100,000
years. That huge amount of time is only
0.002% of the age of the Earth!
An appreciation for the magnitude of
geologic time is important because many
processes are very gradual.
Geologic Time
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Geologic time is divided into different types of units.
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Eon
Era
Period
Epoch
Note that each Eon, Era or Period represents a
different amount of time. For example, the Cambrian
period encompasses ~65 million years whereas the
Silurian period is only ~30 million years old.
The change in periods is related to the changing
character of life on Earth and other changes in
environment.
Geologic Time
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The beginning of the Phanerozoic represents
the explosion of life.
The time before the Phanerozoic is
commonly referred to as the PreCambrian
and represents over 4 billion years of time.
The Phanerozoic eon (abundant life)
represents only the last 13% of Earth time.