# Relative vs. Numerical Dating and Geochronology with Beads

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Initial Publication Date: February 20, 2012

#### Summary

Students use relative dating principles to interpret the ages of rocks in a block diagram. They then "date" samples from these rocks to test their relative age hypotheses. Sample dating is done by counting beads that represent 235U and 207Pb atoms in a zircon.

## Context

#### Audience

The activity is designed for 8th/9th grade Earth Science students, but I have also used it with 5th grade students. It could easily be modified for use in 100-level geoscience courses.

#### Skills and concepts that students must have mastered

Students should be able to read x-y plots and divide two numbers.
Students should be familiar with relative dating principles, although instruction on these principles could be added to the beginning of the activity.

#### How the activity is situated in the course

It can be used as a stand-along exercise or as part of a sequence of exercises relating to geologic time.

## Goals

#### Content/concepts goals for this activity

Students compare relative and numerical dating techniques
students are introduced to radioactive decay, half-lives, and U-Pb geochronology

#### Higher order thinking skills goals for this activity

Students must evaluate whether or not their relative and numerical ages are consistent with each other.

## Description of the activity/assignment

Students review relative dating principles by interpreting a block diagram and are then introduced to radioactive decay and the concept of half-life to determine numerical ages. There is some ambiguity in the block diagram, so students must determine numerical ages for samples from the block diagram to test their relative age hypotheses. Students "date" samples from the block diagram by counting the number of 235U and 207Pb atoms (colored beads) in a zircon (Ziploc bag).

## Determining whether students have met the goals

Students should come up with the intended ages for their zircons and should be able to evaluate whether or not their relative age hypotheses are consistent with the numerical dates. This is evaluated through group discussion.