Absolute Dating

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THE ABSOLUTE AGE OF A ROCK, FOSSIL, OR OTHER OBJECT TELLS US EACTLY HOW OLD THE OBJECT IS.  There are several methods for determining the absolute age of rocks and fossils.

Tree Rings

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The first method of finding the absolute age of an object is by examining tree rings.  If we looked at a cross-section ofa tree or log we would notice that all through it are concentric circles radiating out from the center to the bark. 

EACH TREE RING REPRESENTS ONE YEAR.   Each ring is also different, and the thickness of each ring is representative of the length of the growing season.  In a year with a long growing season the tree ring for that year will be thick.  If there wasa very long winter then there will be a thinner tree ring. 

 

Varves

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Another method of determining absolute age is by looking at varves. 

VARVES ARE LAYERS OF LIGHT AND DARK SEDIMENT THAT SHOW YEARLY CYCLES.  

They are most commonly found in glacial lake beds.  Each spring or summer when the glacier was melting the glacier deposits a ton of sediment it was carrying into strems of water that are melting off of it.  When the meltwater reaches a lake the heaiest sediments (sand and silt) sink to the bottom quickly and eventually formed thick layers of light colored rock.  Later in the year, by wintertime, the clay has had enough time to settle to the bottom and it forms a thin layer of dark colored rock. 

Because of the alternation between dark and light sediments we can use each dark band as a marker for one year.

Tree rings and varves are accurate methods for determining the absolute age of geological events, but neither one is useful for dating very far back in time.  

Radioactive Isotopes

For really old dates we need to use radioactive isotopes. 

RADIOACTIVE ISOTOPES ARE UNSTABLE VERSIONS OF NORMAL ELEMENTS.  BECAUSE THEY ARE UNSTABLE THEY GIVE OFF RADIATION TO TRY TO BECOME NORMAL.  

THIS PROCESS OF GIVING OFF RADIATION TO BECOEM STABLE IS CALLED RADIOACTIVE DECAY

 

There are 3 basic types of radiation:

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Alpha particles are made up of 2 protons and 2 neutrons which are shot out of the nucleus of the radioactive element. 

THEY ARE LOW ENERGY AND NOT VERY DANGEROUS

 

 

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Beta particles are made up of high speed electrons.  They are ejected from the atom at high speed during decay. 

THEY HAVE MORE ENERGY THAN ALPHA PARTICLES AND ARE THEREFORE MORE DANGEROUS. 

 

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GAMMA RAYS ARE SUPER HIGH FREQUENCY ELECTROMAGNETIC RADIATION.

THEY ARE EXTREMELY DANGEROUS.  

 

 

 

RADIOACTIVE DECAY

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EACH TIME AN ISOTOPE RELEASE RADIATION, IT CHANGES INTO A NEW MORE STABLE ELEMENT.

THE ORIGINAL RADIOACTIVE ISOTOPE IS CALLED THE PARENT ELEMENT.

THE MORE STABLE FORM IT TURNS INTO IS CALLED THE DAUGHTER ELEMENT.

THE DECAY PROCESS OCCURS AT A CONSTANT RATE FOR EACH ISOTOPE.  BECAUSE OF THIS WE CAN USE THEM TO FIND THE AGE OF ROCKS AND FOSSILS!