Transcript LESSON 10 – Earth`s History: Estimating Geologic Time

LESSON 10 – Earth’s History:
Estimating Geologic Time
Modified from
http://epswww.unm.edu/facstaff/mroy/EPS101/Chapter_07_files/Chapter_07.ppt#376,28,7.9
How do you reconstruct geologic history with rocks?
http://faculty.sdmiramar.edu/gbochicchio/Chapter%2018.ppt#309,23,Figure%2018.6
http://teachersites.schoolworld.com/webpages/bkwscience/files/Earth%20History.ppt#265,1,
Earth History
Determining geological ages
• Geologists use rocks to tell time in two
ways
– In the field, they can look at a landscape and
decipher the order of events that produced it
– To actually know “when” an event occurred, or
to know when a rock formed, requires
laboratory analysis
This field sketch shows observations of a landscape. How do we ascertain the order
in which the rocks were placed there? By first determining the rocks’ relative ages.
How do you determine the order
of events?
• Relative age – the ordering of objects or
features from oldest to youngest. Things
that happened first, then next, and last
• Absolute age – establishing the date of
an event (in years before the present
time).
How are geologic events placed in
relative order?
• By the application of a set of geological “rules”
• First is the Principle of Superposition – This
states that when rocks were placed, newer ones
were placed or formed on top of the older ones
New Rocks
So here at the Grand Canyon –
the old rocks are on the bottom
of the pile, and the new ones
are on top.
Old Rocks
How are geologic events placed in
relative order?
• Principle of Original Horizontality –
sedimentary layers are (more or less) horizontal
when they form.
(These Flat layers
that are no longer
horizontal)
• Principle of Original Horizontality – sedimentary
layers are (more or less) horizontal when they
form.
Newer rock layers
(These Flat layers
that are no longer
horizontal)
Older rock layers
Figure 18.3
How are geologic events placed in
relative order?
• Principle of Cross-Cutting Relationships –
geologic features that cut across rock must be
younger than the rock they cut through.
Older Rock
Newer Rock
Older Rock
How are geologic events placed in
relative order?
• Principle of
Inclusions – This
states that objects
enclosed in rock
must be older than
the time of rock
formation
Review: Principles of Relative Dating
• Law of Superposition
– In an undeformed sequence of
surface-deposited rocks, the oldest
rocks are on the bottom.
• Includes sedimentary rocks, lava flows, ash
deposits and pyroclastic strata.
• Does not include intrusive rocks, which intrude
from below.
Review: Principles of Relative Dating
• Principle of original horizontality
– Layers of sediment are generally deposited in a
horizontal position
– Rock layers that are flat have not been disturbed
• Principle of cross-cutting relationships
– Younger features cut across older features (faults,
intrusions etc)
• Principle of Inclusion
– Objects enclosed in rock must be older than the time of
rock formation
Fossil Record
• Evidence of organisms preserved in the
sedimentary rock record.
• Typically hard shelled organisms or
bones of other organisms are the only
evidence preserved as fossils.
• Organisms must be buried quickly after
they die to be preserved.
Principle of Fossil Succession:
– Fossil organisms succeed one
another in a definite and
determinable order, so any time
period can be recognized by its fossil
content
• Drawn from theory of evolution
• Fossil organisms become more similar
to modern organisms with geologic time
• Extinct fossils organisms never
reappear in the fossil record
How was the geologic time scale
constructed?
Figure 18.11
How have scientists determined
the age of Earth?
• Radiometric dating (radioactive decay)
provided a measure for the absolute ages of
rocks, and the 4.5-billion-year age of Earth.
(Image from http://rst.gsfc.nasa.gov/Sect16/full-20earth2.jpg)
How have scientists determined
the age of Earth?
• Radioactivity
– Isotopes – atoms of the same element with the
same number of protons, but a different number of
neutrons
– Radioactive decay – a change in the number of
protons, neutrons, or both that transforms an
unstable isotope towards a stable one
Fig 7.16
How have scientists determined the
age of Earth?
• Radioactive decay of specific elements
can be used to determine the Absolute
Age (the actual age of a rock or event)
– Some elements decay (change) into other
elements over a specific amount of time (halflife).
How is the absolute age of a rock
determined?
• Use Half-Life to measure the
isotopic abundance
– An unstable “parent” isotope
decays towards a stable
“daughter” isotope.
– Assuming each daughter comes
from a parent isotope, and that
we know the average time it
takes to convert, we can use the
ratio of the two to calculate the
age of the rock they are in.
– Half-life – the amount of time it
takes for ½ of the parent to turn
into the daughter isotope.
Fig 7.17
1st Half Life
After one half-life,
50% of the parent
isotope will have
become daughter
isotope, regardless
of the sample size.
2nd Half Life
After two half-lives
50% of the remaining
parent isotope will
have become
daughter isotope.
This means 75% of
the original parent
isotope will have
changed.
Figure 18.14
Half-Life Example
A sample with an original amount of 40g of
carbon-14 is found and now contains 2.5g
of carbon-14, how many years old is this
sample?
40g
20g
10g
5g
2.5g
This sample has undergone 4 half-lives, and
the half-life of carbon-14 is 5700 years:
5700 yrs. x 4 = 22,800 years!
Validating the method