# Relative Time.

## Presentation on theme: "Relative Time."— Presentation transcript:

Relative Time

Relative Dating Relative dating – events are placed in a sequence, but actual occurance dates are unknown. Who is older, Mr. Kopp or Justin Bieber? What are our actual ages? – Absolute Dating

Relative Dating Geologic Laws
Principle of Uniformitarianism – idea that the same geologic processes that are occuring today ALSO occurred in the past

Law of Original Horizontality
SEDIMENT IS DEPOSITED HORIZONTALLY

Law of Superposition If sedimentary rocks are undisturbed – THE OLDEST LAYERS ARE ON THE BOTTOM and YOUNGER LAYERS ARE ON TOP youngest C D B oldest A

Law of Crosscutting A cutting feature is younger than the layers it cuts through youngest B C E D A oldest

Law of Included Fragments
Pieces of rocks are older than the layers of rock in which they are found D youngest C A e e e B oldest

“Unconformity Rule” A place in the rock record where layers of rock are missing and gaps in geologic time exist. The layers were deposited and worn away OR never existed. The result is a large difference between rocks above and below the eroded surfaces. B C E D A

Law of Faunal Succession
Older rocks contain the remains of older and more simple creatures and younger rocks contain younger more complex creatures

Absolute Time

Radiometric Dating Calculation of the exact age of rocks using radioactive isotopes Remember Relative Dating is only a comparison of the ages of rock layers (older or younger)

With a partner- Start with 20 pieces of paper Give the partner 1 piece of paper every 10 seconds for 100 seconds. How many pieces of paper do you have? What is the half life?

Atomic # = ? Atomic Mass # = ? Isotope – an element with the same number of protons but a DIFFERENT NUMBER OF NEUTRONS = unstable nucleus Radioactivity: when nuclei are unstable (radioactive isotopes), they spontaneously break apart or decay

How Does Relative Dating Work?
PARENT = radioactive isotope which decays into a stable DAUGHTER product. Radioactive decay continues until a stable daughter product is formed….which happens within a predictable time period known as a half-life.

How does Relative Dating Work?
Half-life – rate of radioactive decay (the amount of time it takes for HALF of the nuclei of a radioactive isotope to decay into its stable daughter product) Uranium-238 decays into Lead-206 (4.5 by) Uranium-235 decays into Lead-207 (713 my) Thorium-232 decays into Lead-208 (14.1 by) Rubidium-87 decays into Strontium-87 (47 by) Potassium-40 decays into Argon-40 (1.3 by)

How does absolute dating work?
Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs. Question? Why have the oldest rocks on Earth been dated at 3.9 billion years old yet the Earth has been determined to be 4.6 billion years old?

Carbon-14 Radiocarbon dating – all organisms contain carbon-14 allowing scientists to determine the age of recent fossils Carbon -14 has a half life of 5,730 and can help scientists date recent geologic events up to 75,000 years ago making it extremely valuable for human history