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This Week We Will Discuss…. : This Week We Will Discuss…. The difference between ‘Relative Time’ and ‘Absolute Time’ in Geology.
Learn the three scientific laws that support ‘Relative Time’.
Learn how ‘Relative Time’ is divided up.
Learn how fossils help in determining relative time.
Learn how ‘Absolute Time’ is determined.
Relative Time vs. Absolute Time : Relative Time vs. Absolute Time Geologists and Paleontologists make references to time using 2 scales: Absolute and Relative.
Absolute Time- The actual age of a rock or how long ago the rock was formed.
Absolute Time will refer to actual years and numbers of years for example: A rock may have formed 500 million years ago.
Relative Time- The age of a rock in relation to other rock layers and when they formed.
Relative time will refer to the fossils found in a rock or its layer in relation to other layers for example: This layer is three layers from the surface therefore it the third youngest layer compared to the layer closest to the surface which is the youngest.
Relative Time : Relative Time Relative Time is unique in that:
It’s time periods are not all equal number of years in length.
It’s time periods are determined by the number and types of fossils found in rock from that period.
Orders time by the sequence of events that determined how the planet and its living organisms changed.
Three Laws That Support Relative Time : Three Laws That Support Relative Time The Law of Superposition- Any rock layer that rests on top of another rock layer is younger than the rock layer it is lying on.
Law of Original Horizontality- Sediment rock layers form horizontally on top of older rock layers. If they are not horizontal a geologic event changed it and made it so. (For example: colliding plates making mountains)
Law of Faunal Succession- All fossils in rock were formed in a recognizable successive order all over the world. Meaning all types of life forms first appeared and/or died out at the same time around the world. (For example: dinosaurs first were found all over the Earth during the Jurassic Period but were extinct by the end of the Cretaceous Period all over the world.)
Relative Time and Superposition : Relative Time and Superposition We know the Law of Superposition is true because:
Because of erosion, new rock layers are deposited in the form of sediment on top of older rock layers.
Because of the forces that weather rocks, sediment cannot form new rock until it is buried by younger sediment that protects it from weathering. Therefore in this picture the shale
is younger than the granite.
Image from: higheredbcs.wiley.com
How It Fits Together! : How It Fits Together! Because Sedimentary Rock Forms In Horizontal Layers
Then…..
The Law of Superposition Must be True
And….
The Deeper We Dig And The Closer We Get To The Mantle, The Older The Rock.
Therefore…..
Fossils Found In Layers That Were Formed At The Same Time must Have Lived At The Same Time.
What’s The Oldest Layer Here? : What’s The Oldest Layer Here? If you said these layers,
you were not paying attention.
Remember the Law of Superposition! If you said these layers, you are getting
closer, but you are still a little off!
Try once more! Absolutely these are the oldest layers in
this diagram, with the bottom layer being
the oldest. Notice how the fossils change
too?
Image from: www.prehistoricplanet.com
Relative Time Is Divided Into Specific Periods : Relative Time Is Divided Into Specific Periods Eons are the largest periods spanning hundreds of millions of years. These are broken down into…
Eras are smaller periods in size spanning tens of millions of years each. These are divided into…
Periods These are divided into…
Epochs the smallest division.
What Makes These Divisions? : What Makes These Divisions? THE TYPES OF FOSSILS WE FIND IN THE ROCKS FROM THESE PERIODS!!!
FOR INSTANCE:
The Pre Cambrian Eon had only single celled animals, plants and bacteria fossils while the Phanerozoic Eon holds fossils from multiple celled animals and plants.
In the Mesozoic Era fossils show dinosaurs roamed the Earth but in the Cenozoic Era we have no fossil records of dinosaurs.
You Need To Know The Order : You Need To Know The Order Largest in size Smallest in size
Absolute Time…Finally : Absolute Time…Finally Measured in specific quantities of time and not bound by the fossil records.
For example: 500 million years ago, a billion years ago, etc. etc.
We use Carbon 14 testing and radioactive dating methods to determine the exact age of a rock.
C-14 or Carbon 14 Testing? : C-14 or Carbon 14 Testing? Carbon 14 found in rocks decays at a set rate. Half of the Carbon 14 minerals trapped in rocks will decay and no longer exist every 5, 730 years.
Therefore, if we know how much carbon 14 is deposited into sediment each year by our atmosphere, and learn how much is left in a rock, we can count back to where those levels in the rock would match what was initially deposited.
For Example… : For Example… If our atmosphere deposits 800 parts of Carbon 14 into sediment each year and we have a rock that has 100 parts, we can count back….
100 times 2= 200 (5, 730 years)
200 times 2= 400 (5, 730 years)
400 times 2= 800 (5, 730 years)
Total Age of Rock: 17, 190 years old.
Other Radioactive Testing : Other Radioactive Testing Works the same as Carbon 14 testing but we use different radioactive minerals to test for the exact age of a rock.