The Big Idea Geologic evolution occurs as Earth responds to geologic events Rocks, fossils, and other types of natural evidence reveal a timeline of Earth’s history

How old is old? Scientists use several strategies to determine age of rocks and fossils. Absolute Age: actual age of an object or how long ago an event occurred by using radioactive dating. Radioactive Dating: measures age by comparing the amount of radioactive element isotope present to how much is normally present in that material Relative Age: age of an object compared to another- “I am older than you”

Other Clues to Relative Age Clues From Igneous Rock Lava that cools at the surface is called an extrusion. Rock below an extrusion is always older. Magma that cools beneath the surface is called an intrusion. An intrusion is always younger than the rock layers around an beneath it.

Other Clues to Relative Age Faults (a break in the rock) are always younger than the rock it cuts through! Unconformities: An unconformity is a gap in the geological record that can occur when erosion wears away rock layers and other rock layers form on top of the eroded surface.

Using Fossils to Date Rocks! Scientists use index fossils to match rock layers. An index fossil must be widely distributed and represent a type of organism that existed only briefly. They are useful because they tell the relative ages of the rock layers they are found in.

The Trilobite One example of an index fossil is a trilobite. Trilobites were a group of hard-shelled animals whose bodies had three distinct parts. They evolved in shallow seas more than 500 million years ago.

Absolute Age Half life; the amount of time (years) it takes for 1/2 of the radioactive isotope to decay, indicates the age of the object.

Radiometric Dating

Now make up your own ½ life problem and solve it! If a fossil has ? of carbon left, how many ½ lives has…

Half Life* - The length of time required for half of the radioactive atoms in a sample to decay.

Half Life Lab 1. Place 100 atoms (pennies) into the plastic box heads up. 2. Shake for 10 seconds 3. Spill pennies on tray and remove the decayed atoms (represented by tails) 4. Count the number of un-decayed atoms (represented by any appearance heads) left on the tray 5. Enter the number of decayed and un-decayed atoms into the table. 6. Put the un-decayed pennies back in the box. 7. Repeat steps 2-5 until all the atoms have decayed 8. Create a line graph to represent your data in Table (# of un-decayed atoms vs. time)

What does it mean when we say an atom has “decayed”? At the end of each half-life, at approximately what percentage are the atoms “decaying”? 3. Does the amount of time you shake the box affect the outcome? 4. Do the number of atoms you start with affect the outcome? Explain. 5. How do scientists use radioactive decay to date fossils and artifacts?