21 Index Fossils: fossils that have a wide geographic distribution and a short period of existence (1 rock layer)
22 What is needed to make a fossil: Hard parts (soft parts decay quickly)Quick burial (slows decay)
23 Volcanic Ash: can be spread throughout the world and settle as a thin layer very quickly (very good time marker)
24 **Index Fossils are the best way to correlate rock layers
25 How can scientists determine the absolute age of rock layers? Radioactive Dating: determining the age of something by comparing the amount of radioactive isotope to the stable decay product
26 meaning…The INSTANT a rock is created, it contains 100% of some radioactive isotope. As a rock gets older, the radioactive isotope breaks down into a decay product….scientists can measure this breakdown.
27 But this can also be used to date the remains of once LIVING things… All living things on planet Earth are part of the CARBON CYCLE… (think Carbon Dioxide)…meaning over the course of our life, we build-up Carbon in our bodies…the instant we die, we have 100% Carbon
28 So, how do you do this…Half Life: amount of time it takes for a radioactive isotope to decay to ½ its original mass (occurs at a predictable rate)ESRT Cover
29 Carbon-14: isotope used to date the remains of once living things—people, shirts, mammoths—less than 50,000 years old (this is because its half life is only 5700 years—very short)
30 The box method…If 100 grams of pure carbon-14 (C-14) starts to decay, how much C-14 will be left after 11,400 years?
31 If 100 grams of pure carbon-14 starts to decay, how much nitrogen-14 (N-14) will be created in 11,400 years?
32 **half life never changes for a particular isotope no matter what happens! FireCrushing
33 Geologic Time ScaleGeologic Time Scale: time-line that shows the history of the Earth (created by correlating fossil evidence throughout the world)