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NOTICE: If a student ever challenges you with criticisms of the reliability or validity of geological agedating methods, CLICK HERE 
SYNOPSIS 
Students get simulated "rock samples" which show a "highly magnified" selection of 128 atoms, each sample with a different proportion of the atoms of two different elements: a parent radioisotope, and its daughter product. By counting the parent radioactive atoms and knowing the "halflife" of those atoms, students can figure the number of halflives since the sample solidified, and therefore the "age" of the sample. 
PRINCIPAL CONCEPT 
Agedating rocks involves counting atoms and comparing the counts.  
ASSOCIATED CONCEPT 
Radioisotope halflives provide a reliable measure for agedating rocks.  
ASSESSABLE OBJECTIVES 
Students will.... 
MATERIALS 
 Ziplock bags containing various
ratios of two different items (e.g. beans and popcorn seeds)  Overhead showing those different ratios identified for each bag, and the halflife and age for each "bag"  Geological map (colored, on wall or overhead)  Some fossils, with known (or estimated) ages 

TIME 
One 45 minute class period  
STUDENT HANDOUTS  No printed handouts.  
TEACHING STRATEGY 
CONTEXT/BACKGROUND PREPARATION Set up a set of 9 bags, labeled with marker pen "A
I", and 9 more labeled the same (or "J R") Beanium > Cornium Half Life = 100 million years
Have available the following items: 
PROCEDURES 
PRESENTATION OUTLINE (See Script
of a sample dialogue for this lesson, in PDF format) 1. ENGAGE: Show geological maps, timelines, and/or fossils, pointing out ages in millions of years, asking "How do we know these ages? How sure are we about these ages? How do we measure the ages of rocks?" 2. EXPLORE: a. Count "atoms" in simulated rock samples of different "ages". Students relate halflives of radioisotopes to the application of dating rocks. b. Sequence all known radioisotopes with halflives greater than 1 million years; look for pattern, showing that all longerlived isotopes are still with us, and the shorterlived ones are not, indicating a finite age of our solar system, and that its age must be in the millions or billions of years. 3. EXPLAIN: Do the Deep Time lesson, which emphasizes why we are so confident of geological age dates. 4. ELABORATE: Do the Virtual Age Dating Tutorial (online), for further reinforcing of halflife concept and how this is applied to dating rocks. This is especially helpful in explaining how isochrones work, and why they are so compelling. 5. EVALUATE: Pre/Post test to sample understanding of material. 
ASSESSMENT 
Try the pre and posttests (a 10item multiple choice quiz provided), or create your own. 
EXTENSIONS 
& VARIATIONS 
1. Consider using other items to replace the beans and popcorn. Beans can break. The original version used peanuts in their shells (Peanutium128) and M&Ms (Emenemium128) in large Ziplock bags. Also consider permanently sealing each set in its plastic bag, if you have such a device.
2. If you have a fair number of mathliterate students, consider providing a number of bags ("rock samples") with ratios other than precise halflife proportions, and have class create a halflife curve and figure out the age of their particular samples according to where they fit on the curve.
3. Encourage (or even facilitate) students to do the online Virtual Age Dating Tutorial, using a high speed internet connection, with the prospect of earning their official "Certificates of Completion as a Virtual Geochronologist".
4. Consider taking your students on a simulated journey back in time in our Time Machine. It's an experience they'll never forget (well, maybe in time)
5. Be sure to display a scaled geological time in your room throughout the year, something you can use repeatedly, helping your students to internalize the relative timing for events occurring in geological history. See the Time Machine lesson for specifics.
ATTRIBUTIONSome of the ideas in this lesson may have been adapted from earlier, unacknowledged sources without our knowledge. If the reader believes this to be the case, please let us know, and appropriate corrections will be made. Thanks. 
1. Original Source: The original version (using peanuts and M&Ms) was done at the San Francisco Exploratorium by its Biology Education Director Karen Kalumuck in her Evolution Teaching Workshop, 11/23/2002. 2. Modified by: Larry Flammer, April, 2003, and successfully tested on classes of Earth Science students. 
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