Geologic Time
What is Geology?
 Geology is the study of
the Earth’s form and
composition and the
changes it has
undergone.
Why do we care?
1.
Rock record provides evidence of geological evens and life
forms of the past.
2.
Processes observed on Earth in the present also acted in
the past.
3.
Earth is very old and has changed over time.
Is the Earth changing now?
 Yes! Although many features of the Earth may seem to be
unchanging over our lifetime, they are still changing, but on
time scales of hundreds, thousands or even millions of years.
This image shows
Mount Hood in
Oregon USA at the
same time in late
summer in 1985 and
2002.
How old is the Earth?
How old is the Earth?
 The Earth is 4.6 billion
years old.
 The sun is 5 billion years
old.
 The universe is 13.7 billion
years old
How do we know?
 Relative Dating
 Absolute Dating
Relative Dating
 Relative Dating identifies which rock units formed first,
second, third and so on.
 Relative dating determines the sequence in which events
occurred, but not how long ago they occurred.
Relative Dating
 Law of Superposition – each rock layer is older than the one
above it and younger than the one below it.
 Usually applies to sedimentary rock, but can also be applied
to lava flows and layers of ash from volcanic rock.
Relative Dating
 Principle of Original Horizontality – layers of sediments are
generally deposited in horizontal position.
Relative Dating
 Principle of Cross-Cutting – when a fault cuts through rock
layers, or when magma intrudes other rocks and hardens,
then the fault or intrusion is younger than the rocks around
it.
http://www.schools.utah.gov/curr/science/core/8
thgrd/sciber8/geology/html/rockages.htm
Absolute Dating
 Absolute Dating identifies the exact age of a rock.
 Scientists use radioactive decay to determine the age of the
rock.
Radioactive Decay
 During radioactive decay, unstable atomic nuclei
spontaneously break apart, or decay, releasing energy.
Radioactive Decay
 What is an isotope?
 Radioactive isotopes decay until they are stable.
 Ex. Uranium-238 decays over time to form lead-206.
Half Life
 Half life is the amount of time necessary for one half of the
nuclei in a ample to decay to its stable isotope.
Radiometric Dating
 Radiometric dating, also called radioactive decay dating, is a
way of calculating the absolute ages of rock that contain
radioactive isotopes.
Practice Problems
1. The half life of nickel is 100 years. If you had 94 grams of
nickel, how much would be left after 200 years?
1 half life = 100 years = 47 grams
2 half lives = 200 years = 23.5 grams
Practice Problems
2. Sodium-24 has a half life of 15 hours. How much of a 440
gram sample of sodum-24 remains after 60 hours?
1 half life = 15 hours = 220 grams
2 half lives = 30 hours = 110 grams
3 half lives = 45 hours = 55 grams
4 half lives = 60 hours = 27.5 grams
Practice Problems
3. Iron-59 has a half life of 45 days. A murder weapon was
found in a lake and the original weapon was reported to have
contained 1,200 grams of iron. After 6 months (180 days),
the original sample decayed. How much iron-59 was left?
1 half life = 45 days = 600 grams
2 half lives = 90 days = 300 grams
3 half lives = 135 days = 150 grams
4 half lives = 180 days = 75 grams
KEY TERMS
 Eon – the longest division of
time (2)
 Era – a well defined time span
 Period – a unit of geological
time during which a system of
rocks formed
 Epoch – a portion of a geologic
time period that marks an
event
Eon
Era
Period
Epoch
GEOLOGIC TIME SCALE
http://www.enchantedlearning.com/subjects/Geo
logictime.html
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