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**What pieces of evidence do scientists use to back up the theory of Evolution?**

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**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…

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**How do you think Scientist’s might use these tools to predict how old the earth is?**

Chat with the person next to you…

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Sedimentary Rocks 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

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If someone was going to look through your locker, could they estimate the date you put certain items in ? 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?

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**Law of super-position states**

If rock layers are undisturbed then younger rocks lie above older rocks. Youngest Oldest

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**Can you see the different layers?**

Which layer is the oldest? Youngest?

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Index Fossils 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!

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If different rocks found in different areas had similar rock patterns and similar index fossils scientists could conclude. They are probably the same age…

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**We are going to practice using the law of super-position and using index fossils.**

You will match up similar fossils found in 6 different locations in order to see how scientists determine the relative age of specific areas.

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Purpose: To use the law of super-postion and index fossil to determine the relative age of each and compare different samples in regard to age

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**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? If they can determine the amount the decayed element left in a rock they can predict the age of the rock.

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This is how this works… 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.

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More… 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.

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A scrap of paper taken from the Dead Sea Scrolls was found to have a 14C/12C ratio of 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 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 = / t1/2 so for this problem: k = / 5720 years = 1.21 x 10-4/year log X0 / X = [(1.21 x 10-4/year] x t] / 2.30 X = X0, so log X0 / X = log 1.000/0.795 = log 1.26 = 0.100 therefore, = [(1.21 x 10-4/year) x t] / 2.30 t = 1900 years

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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 = / t1/2

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so for this problem: k = / 5720 years = 1.21 x 10-4/year log X0 / X = [(1.21 x 10-4/year] x t] / 2.30 X = X0, so log X0 / X = log 1.000/0.795 = log 1.26 = 0.100 therefore, = [(1.21 x 10-4/year) x t] / 2.30 t = 1900 years

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**Try the problems on the packet…**

Pg and pg & 9

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