Transcript Why do we care about the history of the Earth?

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What pieces of evidence do scientists use to
back up the theory of Evolution?
Why do we care about the history
of the Earth?
We’re curious…
 We can learn about things that have
happened in the past in order to predict
where we will be in the future?
 This may relate to resources we are able to
obtain (oil etc…)
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What scientists began to notice..
There were fossils (preserved traces of living
organisms trapped in sediment)
Some types of rocks formed as layers of matter
piled on top of each other.
Some radio active elements decay at a known
rate and these are present in some rocks…
How do you think Scientist’s
might use these tools to predict
how old the earth is?
Chat with the person next to you…
Sedimentary Rocks
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Sedimentary Rocks(Are rocks formed when rain,
heat, wind and cold break down existing rock into
smaller particles.)
Particles will collect in lakes and streams
Over time these layers will turn into rocks because
of being compressed, and other chemical reactions
that occur.
Organisms will often be trapped in these rocks and
will become fossils
If someone was going to look
through your locker, could they
estimate the date you put certain
items in ?
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How?
What is lying on top? Middle? Bottom?
Do you think some of the same papers might be in
some one else’s locker who is taking the same
classes you are?
Have you ever spilled food or coffee on a paper?
 Could you remember the date that happened?
Law of super-position states
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If rock layers are undisturbed then younger
rocks lie above older rocks.
Youngest
Oldest
Can you see the different layers?
Which layer is the oldest? Youngest?
Index Fossils
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Easily identified fossil that occurred over a
small period of time that scientists are
pretty sure of its age. Ex:ollenellus lived for
100 million year period.
Thus when scientists see this
fossil in a rock bed they
assume it is from the same
time period when this
creature existed!
If different rocks found in
different areas had similar rock
patterns and similar index fossils
scientists could conclude.
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They are probably the same age…
We are going to practice using
the law of super-position and
using index fossils.
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You will match up similar fossils found in
6 different locations in order to see how
scientists determine the relative age of
specific areas.
Purpose: To use the law of superpostion and index fossil to
determine the relative age of each
and compare different samples in
regard to age
What is Radioactive dating?
Some chemicals are more stable than others
 Less stable chemicals are called isotopes
 They will change into other chemicals at an
individual rate under specific conditions.
 These chemicals have a “half life”
 A half life is the amount of time that will
pass until half of the chemical has changed
into another form.
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Carbon 14 has a half life of 5730
years!
It changes into Nitrogen 14 as it decays.
 Thus if I have 10,000 Carbon 14 atoms,
after 5730 years I will have 5,000 Carbon
14 atoms
 After 5730 more years I will have 2,500
atoms
 After 5730 years I will have 1250 atoms
etc..
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Why do scientists care?
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If they can determine the amount the
decayed element left in a rock they can
predict the age of the rock.
This is how this works…
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scientists burn a small piece of the sample to convert
it into carbon dioxide gas.
Radiation counters are used to detect the electrons
given off by decaying Carbon-14 as it turns into
nitrogen.
In order to date the fossil, the amount of Carbon-14
is compared to the amount of Carbon-12 (the stable
form of carbon) to determine how much radiocarbon
has decayed.
The ratio of carbon-12 to carbon-14 is the same in all
living things. However, at the moment of death, the
amount of carbon-14 begins to decrease because it is
unstable, while the amount of carbon-12 remains
constant in the sample.
More…
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Half of the carbon-14 degrades every
5,730 years as indicated by its half-life.
By measuring the ratio of carbon-12 to
carbon-14 in the sample and comparing it to
the ratio in a living organism, it is possible
to determine the age of the fossil.
A scrap of paper taken from the Dead Sea
Scrolls was found to have a 14C/12C ratio of
0.795 times that found in plants living today.
Estimate the age of the scroll.
 The half-life of carbon-14 is known to be 5720
years. Radioactive decay is a first order rate
process, which means the reaction proceeds
according to the following equation:
 log10 X0/X = kt / 2.30
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log10 X0/X = kt / 2.30
 where X0 is the quantity of radioactive material
at time zero, X is the amount remaining after
time t, and k is the first order rate constant,
which is a characteristic of the isotope
undergoing decay. Decay rates are usually
expressed in terms of their half-life instead of
the first order rate constant, where
 k = 0.693 / t1/2
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so for this problem:
 k = 0.693 / 5720 years = 1.21 x 10-4/year
 log X0 / X = [(1.21 x 10-4/year] x t] / 2.30
 X = 0.795 X0, so log X0 / X = log 1.000/0.795
= log 1.26 = 0.100
 therefore, 0.100 = [(1.21 x 10-4/year) x t] /
2.30
 t = 1900 years
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Try the problems on the packet…
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Pg 742 27-30 and pg 738 8 & 9