How do we know that the earth is 4.6 billion years old? How Old is it Really? How do we know that the earth is 4.6 billion years old?

How do Scientists Determine the Age of Rocks and Fossils? 2 methods: Relative Dating and Absolute Dating - Relative dating estimates the time during which an organism lived. It compares the placement of fossils in layers of rock. Scientists infer the order in which species existed based on the Principle of Superposition - In undisturbed layers of rocks, the older rocks will be found beneath the younger rocks Relative dating doesn’t give an actual age, just indicates how old rocks and fossils are when compared to each other Youngest Rock Oldest Rock

Absolute Dating- Radiometric Dating Allows scientists to assign an actual age to rocks and fossils. - relies on the decay of unstable radioactive isotopes - as you will recall: (Yes believe it or not I do expect to remember) Isotopes are atoms of an element with different numbers of neutrons neutrons protrons

Radioactive isotopes Radioactive isotopes are unstable. They break down and become an atom of a stable element - they can release protons and neutrons (alpha emission) or - a proton can break down and turn into a neutron (beta emission) Alpha emission 2p and 2n are released beta emission a proton breaks down releasing an electron the proton becomes a neutron

How Does Radiometric Dating Work? Based on the half-life of the radioactive element - the half-life is the amount of time required for half of the radioactive isotopes to decay - comparing the amount of parent and daughter elements in a sample allows age to be determined

Elements and their Half-Lives There are many radioactive elements only some are commonly used for absolute dating Parent Daughter Half-Life - Carbon 14 Nitrogen 14 5730 yrs - Potassium 40 Argon 40 1.25 billion yrs - Uranium 238 Lead 206 4.5 billion yrs - Uranium 235 Lead 207 713 million yrs C and K are beta emitters – U is an alpha emitter Ar 40 is a relatively inert gas.

C14 is very accurate but can. only be used to date things C14 is very accurate but can only be used to date things 100-100,000 years old - is used to date things that were once alive

How was the age of the earth determined Potassium 40-Argon dating is good for dating rocks 100,000 – 4.6 billion yrs old The oldest rocks on earth are 3.9 billion years old - meteors and rocks brought from the moon are 4.5-4.6 billion years old - scientists believe that this is a good indicator of when the solar system and its planets formed